Shock Rates V 1.0
The original subtitle to this article was Learn how your Santa Cruz Suspension Works, but we thought owners of other bike brands might like some schooling on bounce as well. This series of articles was started by Joe Graney when he was the Director of Quality and Engineering at Santa Cruz Bikes, but now that he’s the COO, he asked Nick Anderson to update this article for us (complete with new diagrams!) which was originally posted in 2012.
This piece, dealing with shock rates, is not gonna be a walk in the park but hopefully it’ll give some of you a little more insight into the how and why of suspension workings, and how Santa Cruz designs in particular behave.
Levers
We have to start with this. Levers are one of the simple machines, of which there are five (or six, depending on how nerdy you are). Levers hold company with the inclined plane, wheel and axle, wedge, pulley and screw. In a nutshell, levers operate on the principal of the Conservation of Work. Work is the amount of force exerted times the distance over which the force is exerted. If you push on one end of a long lever arm you can exert a high force on the smaller end but the small end moves over a shorter distance (work is conserved).
Got all that? Diagram – Nick Anderson
With suspension bikes, Wheel Travel divided by Shock Stroke = Average Leverage Rate (eg, a 5 inch travel bike with a 2.5″ travel shock has an average leverage rate of 2.0).
Shock Rate is the inverse (opposite relationship) of Leverage Rate. Shock Stroke divided by Wheel Travel = Average Shock Rate. The aforementioned bike would have an average shock rate of 0.5. Why use shock rate instead of leverage rate? The hell if we know, but it’s common for bike people to use the terms falling rate and rising rate, both of which refer to shock rate. So at some point in the past we decided to use shock rate so it fits that nomenclature. It’s easy to convert – just remember that a high leverage is a low rate, and that rising rate means falling leverage. Ready?
Bike Rate
A rising rate on a bike means that at the beginning of the travel, the rear axle has a higher mechanical advantage or leverage than at the end of the travel. This typically implies that the bike won’t “bottom out” easily. Falling rate means the opposite; the bike may bottom out easily, as the mechanical advantage increases through the travel – it ‘uses’ the travel easily. A bike with a constant leverage, or shock rate, has the same leverage throughout the entire travel. A rate of +/- 3% can be considered constant.
Rate
Spring rates are a measure of how much the spring pushes back at you as you push into it. A constant rate spring is assigned a value, typically referred to as k. The k value of a spring is in units of force per unit length, or lbs per inch. Practically speaking, if you have a 500 lb. spring, 500 is the k value, meaning 500 lbs per inch compressed. At zero inches compressed, it pushes back zero pounds, which is handy because then it just sits there on your desk holding your pencils. At one inch compressed, it pushes back 500 pounds. At two inches compressed, it pushes back 1000 lbs, and so on. Coil springs on bike shocks aren’t perfectly linear, but they are damn close. Air shocks are different, though. Air shocks work by compressing air, which doesn’t push back the same way as a metal spring does. The air’s force pushing back has to do with volume, which is tough to make a linear. So you set the air pressure (the volume is typically constant, and set by the shock manufacturer), and when the volume is cut in half, the pressure is doubled (remember PV=nRT from chemistry?). So half way through the shock stroke, the force pushing back is doubled. Then, 3/4 of the way thru the stroke, the pressure doubles again. And so on. Hence, that ‘rampy’ feeling, since the spring rate skyrockets toward the end of the stroke as the volume gets cut in half over and over and the force goes way up.
If you increase the volume a lot, the spring rate doesn’t do a linear progression like a coil, and has what is sometimes called a cavitation, or a place where the spring rate drops below a constant k value.
The beginning of the stroke on an air shock also doesn’t start at zero force, due to negative springs and all sorts of trickiness that air shock designers could write a book on. Preload doesn’t change your k value, it just makes the start point go higher since you start compressing the spring, but you haven’t compressed your bike yet.
Wheel Rate
Wheel rate could be defined as the combination of Bike Rate and Spring Rate. In practice, it’s really how your bike is going to behave since it defines the amount of force that the shock exerts on the rear wheel at any given point in travel. Damping is of course also a big factor but this will be covered later on.
VPP Rates
The VPP platform is very flexible and can allow for nearly any shock rate to be designed into the frame. A typical linkage moves in a linear fashion, and the shock can be positioned to have a falling or rising rate, but not both. The VPP system has two links that rotate in opposite directions. The top link moves counter clockwise, the bottom link clockwise (from the drive side viewpoint). This is unique to VPP bikes, and is specifically protected by US Patent 6488301.
As the suspension compresses, the two links rotate, but not at constant rates. The upper link starts rotating quickly, then slows down mid-stroke, and then speeds up again. The lower link does the opposite, starting slowly, then faster, and then slows down later in the travel. Exactly how much they speed up and slow down can be manipulated by changing pivot points, link angles and lengths. Attaching a shock to one of the links makes it compress at different rates through the travel. On a VPP bike with the shock attached to the upper link, the shock starts with a high rate, decreases through the middle of the travel, and then increases again. This gives the rider a feeling of great bump absorption on small and medium size bumps, but then ramps up so the suspension doesn’t bottom out on larger impacts.
The V10 shock rate starts very low (it means a very high leverage at the beginning). This allows the rear wheel to move very easily when it drops away into a hole, and then gets hit hard and fast by an obstacle. Instead of ‘kicking’ the bike, the wheel easily moves back to sag which is where it should be when a rider is aboard.
As always, there is a balance here. Too much ‘falling’ in the middle in the shock rate can yield a bike that feels like it wallows in the mid-stroke, or mushes down in corners, without a nice snappy feeling on the rebound stroke. Too much ‘rising’ from mid-stroke to bottom out can give a bike that doesn’t use full travel under normal circumstances. Complicating the matter is that different shocks have different spring rates and damping characteristics and can change the way a bike feels.
Damping
So far we have only talked about spring rate and shock rate. Both of these properties are position sensitive. What this means is that if you push the bike into the travel and hold the rear wheel there it will exert a force. If you continue to hold the wheel there it will continue to exert the same force. Damping is speed sensitive. The faster you move the rear wheel the more damping force the shock exerts on the wheel. So if you move the wheel quickly into the travel the shock will exert more force on the wheel than if you move it slowly into the travel. When the wheel is not moving the damping forces are zero.
A damper is a complex set of valves that allows oil to flow from one part of a shock to another.
The faster you try and force oil through these valves the harder it is to compress the shock and therefore the rear wheel.
Developing good bike suspension requires an understanding of the shock rate of the bike, the spring curve of the shock and the damping characteristics of the shock.
Check out the dozen articles on the Santa Cruz web site in Joe’s Corner
This is not sponsored content; the presence of this article isn’t the result of cash flowing in either direction between NSMB.com and Santa Cruz. So thanks to Joe and Nick for their work on this. We often brag that we have the smartest audience in mountain biking (a self-serving opinion bolstered by anecdotal evidence), and we knew a portion of you would appreciate this nerdery. So congratulations; if you made it this far you are a genuine bike geek. CM
Any questions about – or do you have this all figured out?
Comments
wysiwyg
9 years, 2 months ago
I like turtles
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Anthony Trujillo
9 years, 2 months ago
amazing. you guys added another 10 comments on this article since yesterday!
what a beautiful disaster this all is.
my comment would be if you are going to talk tech AND use graphs to reference your points, it ALL needs to be accurate and consistent.
some great points made in the comments though!!
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james
9 years, 2 months ago
that's it, gettin a hardtail…
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Bavaria 2.0
9 years, 2 months ago
All of that and they could have just said, "Yes, air shocks still suck".
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james
9 years, 2 months ago
am i to understand correctly that a metal coil spring shock is still superior in performance to air shocks? is it worth the weight penalty for the performance gain?
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Udi
9 years, 2 months ago
To your first question - yes indeed, but to the second question - it depends. If you have a bike like for example a Nomad that doesn't cater well for the air sprung shock (that it comes with) then there's substantial benefit to be had in switching to a coil, particularly along with something like the PUSH link which helps iron out some of the linkage's flaws.
It's also not a small weight gain if comparing an AM air shock to a DH coil shock - eg. Fox Float / DB inline to an RC4 / DB coil. Even using a Ti spring, you'd be adding 350-400g by doing this swap. If that's something you and your personal bike setup can handle while still not being too heavy for climbs, then it's a great option.
If however you have a frame that is well-optimised for an air shock, the performance gain in switching to coil will be smaller, thus the value for money (or weight, rather) will be smaller. I still think it's worth it, but there's definitely less of a need.
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Matt
9 years, 2 months ago
Would a more tuneable air shock like a BOS Kirk or the CCDBAir improve the ride? Thinking that running more high speed compression may stop it blowing through the mid-travel. Obviously it still doesn't solve the harshness off the top unless running bugger-all low speed compression would aid this?
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Udi
9 years, 2 months ago
Unfortunately not, in my experience, as the problem stems from the spring curve rather than the damping curve. Keep in mind that damping is primarily speed sensitive, so it's very hard to even target support in terms of position sensitivity (specific to the midstroke) the way you describe. So even ignoring the initial harshness, the issues at the other point in the travel are pretty hard to accurately solve with damping too.
I haven't found that the DBA or DBI are better than a Fox (for example) in this particular aspect, they just allow better control over the damping, but as I said that's a different layer than where our issue lies.
The problem needs to be addressed at the spring level. I believe Vorsprung Suspension may have something in the works to target this issue at its source rather than blanketing it, if you are interested it'll be worth keeping your eye out on their website or facebook page over the next couple months.
Keep in mind that a frame that better "balances" the problem areas of existing spring curves will also improve the ride, so while it won't quite mimic a coil, there's no reason you can't have a reasonably decent air sprung bike in this day and age. It just requires understanding the behavior of both air springs, and the frames they are being installed in.
Hope this is useful.
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Clark Lewis
9 years, 2 months ago
i can confirm that there's some secret sauce coming from vorsprung, spent a lot of time on it last summer, and its very very impressive.
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Matt
9 years, 2 months ago
Very! Thanks Udi
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Kenneth Perras
9 years, 2 months ago
This.
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Gord SB B
9 years, 2 months ago
Thanks for making me smile this morning!
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Udi
9 years, 2 months ago
This article is misleading at best, and incorrect at worst. I have posted some clarifications here for anyone interested:
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Dirk
9 years, 2 months ago
I think their focus is to show that the "bike rate" is the sum of multiple components. They're not trying to show detailed air spring curves, the effects of negative springs or anything about mass transfer.
You should also consider that this article was probably written before the new Heckler, V10 or Nomad existed, so the curves might be a bit different than you expect.
The falling rate at the beginning of travel could be an interesting discussion. But this would also counteract some of the early travel rise inherent to an air shock shown in your graph, no? I understand their theory - that this allows the bike to settle into its sag point easily - but I can't really speak to its validity. No matter what you think of the theory, it's a pretty subtle effect that probably doesn't have much impact one way or another.
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Udi
9 years, 2 months ago
Well what you suggest they're trying to show only makes up half the article, so I think you're giving them too much credit. If you want to provide a simple article, write a simple article. Instead they've made a few correct statements, made a roughly equal amount of incorrect statements, and posted incorrect graphs which as I said is highly misleading at best.
The fact is, to understand a lot of intimate concepts in vehicle dynamics, you unfortunately need to simultaneously have a reasonably thorough understanding of the whole picture. If you don't, it is VERY easy to negatively impact one performance criteria (substantially more than required) by trying to improve a different criteria. Furthermore, many concepts (like the air spring for example) just cannot be simplified as grossly as they've tried to do here - because it ends up being too far from correct to be useful.
To answer your question (without getting into huge detail), no - you've got it the wrong way around. A falling rate indicates low leverage initially, which means a higher wheel rate (vs. linear) initially. This does not help, and actually compounds the problem with the air spring because that also has a higher rate initially (for a given average spring rate, compared to a linear coil spring).
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Dirk
9 years, 2 months ago
I might be giving them too much credit, but I think you're overly harsh and missing the point. To deal with your issues:
1) pv=nrt - This was given as a passing reference and used to make a specific point about air shock volume.
2) Spring Rates Graph - Okay. Incorrect. They've continued to perpetuate incorrect information used by the entire industry. Fair enough. Still, not an article about detailed air spring rate curves.
3) VPP Rates - I'd have to see the detailed comparisons of all the bikes, but I'm pretty sure a Bronson has a curve similar to this…Falls then rises. As well, what other bikes exhibit this property? Not being snarky, asking sincerely.
4) Cavitation - No idea, fluid dynamics was never my best subject.
5) Falling Rate - Like I said, I understand the theory, but the benefits/drawbacks are arguable.
With regards to your final point above, I think their terminology is confusing the point. But I'm pretty sure I have this right.
Air Spring Curve - Rises, flattens, rises
VPP Leverage Curve - Falls, flattens, rises
The falling VPP leverage curves indicates the suspension is putting more leverage on the shock, not less. These curves add together to give the "bike rate".
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Cam McRae
9 years, 2 months ago
Since you seem to be a pedantic type, here's a heads up. The singular of criteria is criterion. You have managed to use it incorrectly twice though - so that's something.
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Udi
9 years, 2 months ago
I appreciate your level-headed comments, but you are actually wrong, the VPP frames (other than the v10, as I explained in my forum post) are putting LESS leverage on the shock at the beginning of travel. I know I'm being a little harsh, but once you realise that the opposite of what you are saying is true, you'll suddenly realise the reason for my harshness.
Your confusion is stemming from terminology issues (my apologies if I have contributed to this, I may have), what you need to see are numerical values. The SCB graph does not actually show leverage vs. travel, it shows shock rate vs. travel (so the graph is the opposite shape to the one below, but in the case of this particular graph, both the article AND my graph are correct in general shape, they're just using a different y-axis). You need to distinguish these two from each other.
See the attached graphs - these are copied from the site linked by another user above, they are only as accurate as the input data so I don't claim they are numerically perfect, however they do very correctly illustrate the general shape of the curves.
You can see that the Nomad curve starts around 2.45:1 leverage, increases to 2.8:1, and then decreases to 2.0:1. So the Nomad is initially putting less leverage on the shock (contrary to what you hoped - and with all due respect, you were just giving them the benefit of the doubt). Unfortunately, it's the opposite of what you want ideally.
I hope this helps you understand. I've attached graphs of some other bikes that have LR curves of opposite shape that WILL actually do what you think the SCB bikes should do.
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Udi
9 years, 2 months ago
If you're going to "correct" me on something Cam, you might want to do it with something a little more substantial than this. Maybe even something relevant to the topic of the website you moderate for - I think it's about bikes or something?
"The plural criteria has been used as a singular for over half a century. Singular criteria is not uncommon in edited prose, and its use
both in speech and writing seems to be increasing. Only time will tell
whether it will reach the unquestioned acceptability of agenda."
Language changes over time. The laws of physics (at least for the purposes of this discussion) do not.
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Dirk
9 years, 2 months ago
It depends on how you look at it, from the wheel, or from the shock. The Spanish Linkage curves are backwards. What we consider to be a "rising rate bike" - i.e. the suspension is firmer at the end of travel, shows up as a descending line.
"%2B2015_LevRatio.gif
Rising rate means that it is progressively more difficult to compress. It takes more force to compress the shock a specific distance. For a rising rate bike, the leverage of the shock is increasing, the leverage on the shock is decreasing. This is what is shown in the graph, the leverage on the shock. And logically, falling rate means the opposite.
So. What does that mean?
V10 - Progressively harder to compress the shock throughout the travel.
Nomad - Easier, then progressively harder to compress the shock throughout the travel.
The way the curves are shown, everything is magnified. If you plot out a shock vs. wheel travel graph for the Nomad, you will see a straight line with minor deviations at the start and end of travel. The slope of that line will be slightly shallower at the start of travel (easier to compress) and slightly steeper (harder to compress) at the end.
Anyhow. I'm not the guy accusing Santa Cruz of not being able to design bikes.
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Udi
9 years, 2 months ago
Everything you said there is correct except your description of the Nomad (and thus every other non-v10/driver VPP bike). Using words at this point is not helping, use numbers and you'll see I'm right.
I attached graphs to my last post but they seem to have disappeared, please look at these and get back to me:
Nomad III:
A bunch of other bikes:
Please tell me you can see the issue here. To reiterate from what I said earlier:
"You can see that the Nomad curve starts around 2.45:1 leverage,
increases to 2.8:1, and then decreases to 2.0:1. So the Nomad is
initially putting less leverage on the shock (contrary to what you hoped
- and with all due respect, you were just giving them the benefit of
the doubt). Unfortunately, it's the opposite of what you want ideally."
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Udi
9 years, 2 months ago
PS. Just so I can't possibly make it any clearer:
If you use the v10 curve on an air shock, it would work much better, at least in the first half of travel. The v10 for the most part does the right things. The high initial leverage VALUE would counteract the initially high spring rate on an air shock (vs. a linear coil spring).
However, the the v10 curve is the OPPOSITE to the curve of the short travel VPP bikes. The Nomad has a low initial leverage value. Thus the Nomad does not do well, in fact it does the opposite of what is ideal.
This isn't a matter of opinion, there's a right and a wrong answer. I have attached the v10c curve below so you can compare it to the Nomad, and visualise how these will act upon an air shock. The v10's high initial leverage will counteract (and thus mellow out) the high initial spring rate of an air spring. This = good. Every other VPP bike I mentioned has low initial leverage, which will amplify the high initial spring rate of an air spring. This = bad.
V10C leverage ratio curve:
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Dirk
9 years, 2 months ago
Okay. I think we agree on what we are saying, but we are not agreeing on what the effect of that is.
Starting leverage - 2.45:1
60mm leverage - 2.8:1
This is from the perspective of the wheel. So, at 60mm of travel, there is more leverage on the shock. So yes, at the start of travel, it requires more force to compress the shock a specific distance than it does at 60mm. But at 5mm of travel, it takes a bit less, and it 10mm of travel, it takes a bit less, all the way to 60-70mm of travel, where it starts to turn around. What that means is that it takes less force to get the Nomad to 60mm of travel than it would to do on a bike with a straight or rising rate, all other things being equal.
And that is the Santa Cruz theory. It takes less force to settle into the sag point.
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Dirk
9 years, 2 months ago
Let's break this down. We can agree on two things, right? An air shock has a rising rate at the start of travel and the V10 is progressive throughout, right?
Start of travel
It takes a specific force to compress the shock a specific distance
10mm of travel
Shock - Rate increasing - It takes slightly more force to compress the shock the same distance than it did at the beginning of travel
Suspension Design - Rate increasing - It takes slightly more force to compress the shock the same distance than it did at the beginning of travel
20mm of travel
Shock - Rate increasing - It takes slightly more force to compress the shock the same distance than it did at 10mm of travel
Suspension Design - Rate increasing - It takes slightly more force to compress the shock the same distance than it did at 10mm of travel
This keeps going until you get to 25% travel or whatever. The rising rate of the shock and the rising rate of the suspension design are both making it more difficult to compress the shock a specific distance. They both require progressively more force to do the same thing.
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Udi
9 years, 2 months ago
Your understanding of the function is absolutely correct.
Where you are incorrect is the effect this function has on the bike.
Perhaps what might help is considering ONLY the relevant portion of travel we are interested in here. Air spring ill effects are experienced within roughly the first 15% of travel, so your consideration of 60mm worth is really blurring the data for you.
I have a graph of two of these curves together to try and illustrate the point a bit better:
Imagine (if you like) that these curves were moved over each other so that the average value for both curves was identical, for the sake of a simpler comparison.
Which frame will take less force to use 15% of travel? The Pivot will. The VPP frame on the other hand will be harsher in the first 15% of travel, blow through the midstroke easier (highest leverage ratio is in mid travel), and then to top it all off, become harder to use full travel on because the decreasing leverage ratio towards the end exacerbates the high ending spring rate on the air spring. This is not just "average", it's the opposite to ideal.
Like I said, there is a right and a wrong answer here, while some variation is acceptable, we are talking about polar opposites here. It's not really a matter of opinion.
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Udi
9 years, 2 months ago
No, this description of yours is wrong.
You're trying to equate rates that are increasING to rates that are high. A rate that is increasing must start low, so for a leverage ratio to increase (which 2.45:1 going to 2.8:1 is doing) it must be relatively low at the start. An air spring curve's initial harshness which is present from zero travel requires a high leverage ratio straight off the bat, but instead (on something like the Nomad) you've got a low leverage value.
If you started at 2.8 at zero travel and went to 2.45 on the other hand, you'd have a high leverage over the part of the stroke you needed it (it might be going from 2.8 to 2.7 or something, for an average of 2.75) to combat the high air spring rate, and as the harshness disappears, you would reduce your leverage (towards 2.45) to avoid blowing through the travel when the air spring rate becomes linear and ceases to be harsh.
I unfortunately do not have time to explain this any further. I can't explain this anymore clearly. If you scroll up a bit I posted a graph with both types of curves on the same graph which might help you understand.
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Dirk
9 years, 2 months ago
This is the end for me. Please don't take my abandonment of this conversation as conceding that you are correct. You are precisely correct about my interpretation - the actual numbers don't matter at all, it is entirely whether or not they're increasing, decreasing or staying the same.
I've included a handy (exaggerated) graph below showing a constant rate bike in red, and a Nomad style bike that starts off falling, levels and then goes rising rate. Compared to the constant rate bike it takes less force to get through the initial part of travel and more force to get through the last part of the travel. It takes less force to get through the first part of travel.
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Udi
9 years, 2 months ago
Well if "the actual numbers don't matter at all" then we may as well all give up and write poetry.
Your graph is the inverse of reality when it comes to the Nomad. Perhaps you own one and wish this is what it did, but it doesn't.
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Dirk
9 years, 2 months ago
So rising rate doesn't go up and falling rate doesn't go down? Huh. Okay.
And yes. The actual physical number doesn't matter. Does the fact that a Nomad hast a starting leverage ratio of 2.45 and a V10 has a starting ratio of 4.2 say anything about how the first bit of travel will feel on those bikes? No. Different shocks. The actual number has absolutely no real meaning.
What you're saying is "the bike has a falling rate at the beginning, which makes it harder to compress and it has a rising rate at the end which makes it harder to compress." These concepts are mutually exclusive.
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sandwich
9 years, 2 months ago
the nomad will be harder to compress off the top and very firm at the bottom. this makes shock tuning difficult and compounds the "stickiness" you feel with an air shock. That's the 2.45 or whatever to 2.85, at the start, aka "falling" rate", then 2.85 to 2.45 on the end, or "rising rate" makes end of travel usage difficult.
If you want to see all the glory of "falling rate" bikes in action, look to the mid 2000s Cannondales.
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Dirk
9 years, 2 months ago
No. I'm sorry. This is wrong. Imagine two bikes, both start with a leverage ratio of 2.0:1 (from the wheel perspective) or 0.5 (from the shock perspective). One has a rising rate all the way through travel, one has a falling rate all the way through travel. Which bike would take more force to bottom?
Answer: The Rising Rate bike.
So a bike with a falling rate at the beginning of travel takes less force to get to the same point than a similarly leveraged bike with a straight or rising rate. It doesn't matter that the number is bigger to start. It matters that the number is getting smaller as you move through the travel.
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reformed roadie
9 years, 2 months ago
Udi, you must be a blast at parties…
What bike company do you work for?
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Cam McRae
9 years, 2 months ago
When lots of people are wrong it starts to become right. Lots of people say 'real good' but it isn't any more right than it used to be. Anyway, it was more about pointing out what a dbag you have been in your approach to this. Your arrogance isn't exactly a barrel of laughs. But maybe it plays well on the site you normally hang out at?
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Gord SB B
9 years, 2 months ago
Did you need more graphs? ; )
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Gord SB B
9 years, 2 months ago
Of everything just said, that makes the most sense to my simple brain.
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John Rodriguez
9 years, 2 months ago
^This.
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smaertin
9 years, 2 months ago
And if you inflate both shocks to give you the same sag, one of the bikes would feel harsher at the beginning of the stroke. My understanding is that it would be the one with falling rate.
I think this is the point that Udi tries to convey.
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Dirk
9 years, 2 months ago
I have no idea what Udi is trying to convey.
To get to the same sag point, the falling rate bike would need a tiny bit more air pressure than the rising rate bike. I don't know how this relates to "harshness". That's not really a scientific term.
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smaertin
9 years, 2 months ago
Yea, I think that is exactly the thing. If we now disregard the rest of the travel, from sag to bottom out, and just look at what happens from top out to sag, we can conclude that falling rate in this part of the travel should be "harsher" than a rising rate. If the effect is big enough to notice is of course hard to say and depends on specific bike/shock/rider, but the fact that a falling rate in the first part of the stroke is less ideal than the other way around seems clear, right?
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Udi
9 years, 2 months ago
I was not comparing numerical values between bikes, I am comparing them over the curve of the same bike. The numbers tell you everything you need to know.
If you have a leverage of 2.45:1 initially and then end up at 2.8:1 (i.e. Nomad), then it is the equivalent of using a short wrench to undo a nut and increasingly extending the lever length to be longer. This means that it STARTS OFF BEING HARD TO TURN.
This means that your shock rate curve is FIRM at the start of the stroke and digresses - i.e. gets softer - i.e. the rate FALLS. A low leverage value (2.45:1) in this case means LESS leverage = firmer, and a high value (2.8:1) means MORE leverage = softer. Going from firmer to softer = digression = falling rate.
Smaertin and sandwich are both 100% correct. You clearly have a very
poor understanding of simple leverage ratios and need to go back and
learn how they work before reading my posts again.
The resultant behavior is exactly as sandwich and smaertin both describe, the bike will feel harsher at the start of the stroke (in this case compounding negative air shock behaviors).
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Inclag
9 years, 2 months ago
Holy crap. Can't believe it hasn't been put down succinctly yet. Dirk, the point you're missing and the point udi has failed to convey is that in order to tune each shock they would need vastly different air pressures. The falling rate needs more pressure so it actually doesn't feel wallowey and to have better dynamic stability as to not have crappy midstroke performance. Factor in the lower leverage ratio at the start of travel and the non linearity at the beginning of the air spring curve and voila you have a very less than ideal scenario.
Sooooo now that we have that cleared up lets play a little game called describe the curve that best fits an air shock. Hint, I'd be ok with slightly regressive at end of travel.
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Steve Mathews
9 years, 2 months ago
Dirk, you are correctly describing the way the leverage rate changes, but not the effect that it actually has on the firmness, because bikes are set up to have a certain amount of sag, not a certain initial spring rate. Where you say "it takes less force to reach the sag point", the reality is that there is a set amount of force at the wheel under static sag - which means there isn't "less force" to reach the sag point (because that force is actually a constant), it actually just means there's more force required to start the thing moving early in the travel. Falling rates necessitate smaller amounts of sag due to the stiffer initial stroke, and this reduces small bump absorption and traction, especially in conditions where the rear wheel is leaving the ground momentarily.
On that note, air springs aren't rising rate at the start of the stroke, they are MASSIVELY falling-rate. For example, at 200psi, the initial spring rate in a particular size of Fox Float starts at over 2000lbs/in (for the first mm or so)! Yet within 15mm of its stroke, it's dropped to about 280lbs/in - almost a 90% reduction! This is why they feel so stiff initially, then are so soft in the middle. Running a falling-to-linear-to-rising rate spring in a falling-to- linear-to-rising rate linkage (ie putting a standard air shock in a Nomad) exaggerates the effect, and you end up with something that's fairly stiff in the initial travel, very easy to blow through the middle of the travel, and ramps up quite hard at the end. Other bikes like the Enduro, Rune etc use leverage rate curves that are actually the inverse of what the Nomad uses, in order to help iron out the actual spring rate at the wheel.
-Steve
Vorsprung Suspension
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Dirk
9 years, 2 months ago
You are correct. I phrased it poorly with the sag point.
And I was never trying to comment on the performance of the Nomad. Simply trying to explain what a falling rate means.
Thanks for the info on the air springs. That is interesting.
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Dirk
9 years, 2 months ago
Actually, your nut analogy is spot on, but I still don't understand your interpretation.
"Rising Rate Nut" - The first millimeter is the easiest to turn, but then each millimeter requires more and more effort to turn.
"Falling Rate Nut" - Each millimeter is progressively easier and easier to turn.
It doesn't really matter how easy or how hard that first millimeter is, because that's all relative. It matters whether or not it's getting easier or harder to turn after that.
The only point I have been trying to make is that your statement "a rising rate bike would make a rising rate shock easier to compress" is incorrect. Both of these require more force to compress as they move through their travel and would be stiffening at the same time.
Anyhow, as Stephen Matthews points out below, your graph showing the rising rate nature of an air shock in the first bit of travel isn't necessarily correct.
And please don't tell me what I need to learn, thanks. I'm not necessarily a "suspension expert" but I understand the shit out of Statics and Dynamics.
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Udi
9 years, 2 months ago
"Phrased it poorly" is a funny way of admitting you were wrong, but I "digress". 🙂
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Udi
9 years, 2 months ago
The point you failed to understand was nothing to do with the shape of
the curve on the graph that I posted, in fact you went on to post a
graph of your own that was the COMPLETE OPPOSITE of the actual behaviour
of a typical upper-link driven VPP bike.
If you read my post you'll notice I point out that the graph in my post is purely for the sake of clarifying a behaviour the exists (and is not shown AT ALL on the SCB graphs). I did at no point state that the shape of the initial peak on the graph I linked (an old Specialized item) was a perfectly accurate depiction of reality - it just (crudely) portrays a behaviour that exists on virtually all current air shocks (higher spring force than on a linear spring at start of travel), needs to be addressed, and ISN'T ADDRESSED by SCB.
If you actually "understood the shit" out of statics and dynamics, then we wouldn't have had a 3000 word essay worth of posts to explain to you that a lower leverage VALUE does in fact result in a harsher feel at the beginning of the stroke, compounding the ill effects of an air shock.
I stated this in my original post on RM but you seemed to be the only person unable to comprehend it. Perhaps work on simple static leverage ratio values before venturing into telling us what you "understand the shit out of".
FYI I discuss this stuff on an almost daily basis with with the Steve character you're quoting.
Nice try though, points for effort.
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Dirk
9 years, 2 months ago
Oh dude. You're precious.
I'm not the one claiming that a rising rate linkage counteracts the effects of a rising rate shock.
I don't think I've said a thing about harshness, or feel of the Nomad. But that's cool that you want to throw words in my mouth.
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Steve Mathews
9 years, 2 months ago
That graph, while exaggerated to drive home the point, does actually show the correct nature of an air spring (stiff/soft/stiff) - but going from stiff to soft is actually falling rate, not rising rate. The first part of an air spring's travel, particularly with those springs currently on the market, is falling rate. I think you may be confusing the steep gradient with a rising rate - the change in that gradient is actually what determines the rate, not how steep or flat it is at any one time, in other words the 2nd derivative of the force/displacement curve is what tells you whether the rate is rising/linear/falling (ie whether the 2nd derivative is >0, 0 or <0 respectively).
Vorsprung Suspension
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Dirk
9 years, 2 months ago
Now that makes a lot of sense. Thanks for helping out and discussing it like a human.
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Nat Brown
9 years, 2 months ago
Petty as it may be to be called on this, if you're going to address it you might as well do it seriously. You seem like a serious character here. Merriam-Webster has been extensively criticized for its permissiveness, and this is just another example of that. So, it hardly serves your point. Who knows where this will lead. Maybe 'digress' will start to be used as an antonym for 'progress,' or 'loam' will be used to describe the organic top layer of soil instead of a specific mixture of mineral soil particle sizes. But I regress…
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Udi
9 years, 2 months ago
As I said, the graph you drew is just flat out wrong and the opposite of correct. Keep trying to play this off as a semantics issue though, as I said no one else seemed to have trouble understanding this but you.
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Ray
9 years, 2 months ago
No, it's a red herring.
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Gord SB B
9 years, 2 months ago
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Gord SB B
9 years, 2 months ago
Select ENGLISH in drop down at top of page for translation if your browser is set up for translation. Use as an additional resource to the initial conversation.
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Don't believe the hype
9 years, 2 months ago
Physics is physics… thanks for the information and good to see thru the marketing mumbo jumbo.
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Jitensha Kun
9 years, 2 months ago
Do NOT try and read this before you've hand your morning coffee.
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Brock Fisher
9 years, 2 months ago
I have had 3 cups and I am still not understanding everything..
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Henry the afe
9 years, 2 months ago
santa cruz are the last people that should be lecturing on shock rates.
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Derp
9 years, 2 months ago
where does the 'mid stroke harshness' graph get explained?
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