Video

Vorsprung Tuesday Tune: Advanced Bike Geometry II

Video Vorsprung
Date Jan 3, 2019


Steve Matthews of Vorsprung is back with the Tuesday Tune but rather than delve into suspension, Steve has tackled geometry. The second episode has some interesting discussion as he focuses on the ratio of Front Centre to Rear Centre (FC:RC) and why he feels it's an important factor in bike handling. 

What's perhaps more interesting are the comments where Leo Kokkonen of Pole Bicycles offered his opinions. A snippet from that below:

Pole Bicycles (Leo Kokkonen): "Hi Steve! It's Leo here. I think you were asking me to answer this one :) Cool, crushing numbers! Kudos for that but you fell into the "static vehicle design fallacy". We did similar graphs a few years back before we started to change the geometry. One of the assumptions was that if we make the reach longer, the rider might get more tired if your reach goes further in front. The numbers don't really take in count the reality where the rider position of riding is not static. The optimal rider moves up and down on the bike rather back and forth. The riding stance is not very static, and the rider kinematics, the bike kinematics, and braking need to take into account. So, just trying to calculate the numbers on a sheet for a bike is not worth it because you start jumping into conclusions without thinking about the rider. Many bike designers seem to follow motorcycle and car design principles and that is a big mistake on bike design. What you need to do is start to stand on a scale and lean to a desk and bend your knees. ;)

Braking effects quite a bit to your weight distribution because riders don't go through corners very often just by coasting without braking. Fe. Matti Lehikoinen drags his front brake throughout the corner. When we looked at the cornering by video, we saw that all riders on old school bikes, tend to reach back on their bikes rather than in front. The reason is that on an old school bike there is not enough room (your body mechanics limits it) to move down to reduce the weight on the front. The riders control the front wheel weight by moving up and down. You can test this by standing on a scale next to a table and lean to it. Play around with the reach and the height of the table and bend your knees while leaning to the table and you'll see a dramatic change on the weight. This is why your calculation is not very valid and the reason you did not have grip is more about the suspension tune and probably more about your riding style (For the record: Steve is fast). I filmed Joe Nation and Matti Lehikoinen riding our Stamina in Madeira. You can see from the video clearly that the guys are not trying to reach forward to gain more grip https://www.youtube.com/watch?v=O35iidRWKEw Later this year we will have Joe's and Matti's cornering tips on our vlog (supported and a flat corner). 

A longer wheelbase is quite different when you ride downhill as you are not in a steep angle on drops. The longer wheelbase front wheel reaches the drop bottom before the rear. This means that the rider does not have that much flexibility and therefore he saves energy by moving less. With all designs, there are drawbacks though. On a longer wheelbase, the front end is not that easy to manual on a road. You need more effort on that. But on the other hand, on the trail, you can get your front wheel up anytime you want and you can also plow through stuff and save energy like that. Also, the longer wheelbase makes the ride calmer because there is less movement on both ends of the bike. Our conclusion is that with a longer bike you save more energy and you are 2-3% faster on an Enduro track. 

I think comparing Sam Hill and the reach number is a bit far-fetched because if you look at his riding on any bike, he hardly seems to do anything. The reason he looks like he's not doing anything is just that he has mad skills and he is strong as a horse. On EWS, the pedaling is a very crucial part of the winning ;) On the grip issue you had on the Machine: If you remember, we had to use the same rear shock spring rate that I did, and the reason why you did not have enough grip on the Pole you rode was mere that your shock did not have the right spring rate. I weight roughly 73kg, and you are somewhat 90kg? I think it's fair to say the bike setup was far off for you. 

Your tech vlog is cool! Actually, I was just about to email you about the hysteresis on shocks. I'm still thinking if the hysteresis on shocks is useful or not because we can create a similar effect with the leverage ratio and a low tune as well. I would love to chat again soon and change thoughts because I think that you are one of the smartest guys on the bike industry!"

Vorsprung (Steve Matthews): "Greetings again Leo! This is obviously a static, flat-ground comparison simply for the sake of eliminating variables in order to maintain consistency and only really looking at cornering grip. FC:RC is not really relevant in a straight line or in any situation where you aren't exceeding lateral tyre grip. It's easy to say something is a static fallacy but short of coming up with a 6DOF simulation incorporating the rider (which btw I have actually tried to model in the past) everything will have some simplifications or situations where it isn't applicable. This is part of the reason why I said that my opinion was that optimum front tyre load (as calculated here) was actually LESS than 50%, because obviously descending sloped terrain will always increase front tyre load for any given body position when compared to flat ground. At any rate, trying to quantify these things in a dynamic environment is hugely problematic in no small part because the rider can move so much to compensate for the bike. 

You may be interested to know that this was a topic I started examining in more detail after riding the Machine to try to explain what I was feeling on the bike because after riding with you, I got back on the Deviate and went for a ride to see what differences I would notice. The improvement in front-end grip on the Deviate—specifically, how centred I could stay on the bike whilst maintaining front wheel weight in both smooth berms and rough flatter corners—was extremely noticeable. Having spent a large amount of time riding various suspension setups, I'm very confident that the somewhat too soft rear end wasn't to blame (although on that note, I'd recommend on future models making the centre of curvature higher and the linkage less progressive. That'll help reduce the harshness on square edges, improve rolling speed and get rid of that tendency to kick on jumps), and I even went as far as testing that by underspringing the rear of the Deviate too. The effect it had was the opposite of what you suggested though—running the rear end disproportionately soft led to a reduction of grip at the rear wheel rather than the front. 

I'd agree that riding styles affect it a lot—shorter riders and riders with stronger upper bodies will find it easier to keep weight on the front wheel in most cases. As you say, riders on more conventional-geometry bikes tend to be further back on the bike when cornering - but my opinion seems to differ from yours here, because I don't believe it's that the bike is in any way forcing them backwards, I believe that they're simply able to keep enough weight on the front wheel without being forced to lean far forward to achieve that. Essentially, they can support their body weight more through their legs and less through their arms whilst keeping weight distribution appropriate, and even the strongest guys out there are still stronger in their legs than their arms. 

I agree that long chainstays are little hindrance on the trail—they suck a bit for skatepark riding but I don't think I'd really care about the extra difficulty manualling. I'd really interested to ride a Machine again, except one with a ~490mm chainstay instead of 455. 

Hill is a beast, no doubt about it, and we can all agree that his success is a lot more to do with his skills and fitness than the bikes he was riding. But you should go back and look at the bikes he's had the most success on over the years and compare the FC:RC ratios. The Sunday he was destroying everyone on in 2007/8 had a ratio of around 1.68. Oddly similar to that Mega. 

Shock hysteresis can't be replicated with the leverage rate of the frame as such. I don't think it's a big deal really, having zero hysteresis is almost certainly worse for bump absorption than having some moderate amount (an ideal amount of which I have not yet quantified - having a ton of it isn't great either). 

Cheers for the discussion!"


Beyond the comments above the discussion becomes more heated, but you can head to the YouTube video to follow that.




In part two of Advanced Geometry we look at Front Centre to Rear Centre (FC:RC) ratio and why it's a seriously important factor in bike handling that's rarely discussed. We compare numbers between a few different bikes, all of which have notable geometry for one reason or another: 

  1. Deviate Guide 2018 (size large): The best cornering bike we've tried 
  2. Transition Patrol 2016/17, (size L and XL): Pinkbike's Bike of the Year in 2016 (same geometry in 2017), and an all-around great bike 
  3. Pole Machine 2018 (size large): In the vanguard of "long/low/slack with a steep seat tube angle" and geometrically an absolute weapon in a straight line, with some caveats 
  4. Nicolai/Mojo GeoMetron G16 (size large): Even more extreme geometry than the Pole Machine
  5. Nukeproof Mega 2018/19 (size medium): Radically "conventional" geometry compared to the rest... and the overall winner of the Enduro World Series in 2018 under Sam Hill. 
Phenomenal skills aside, why is Sam Hill winning on seemingly old-school geometry? 

Note: there's an error in our spreadsheet, where the RC for the Mega is shown at 446mm instead of 437mm. The FC:RC ratio there should be 1.64 rather than 1.61.

Comments

xy9ine
+2 Steve Mathews fartymarty
Perry Schebel  - Jan. 3, 2019, 10:56 a.m.

good stuff. i have a crush on steve's big brain.

Reply

geraldooka
0
Michael  - Jan. 10, 2019, 4:53 p.m.

Same.

Reply

morgan-heater
0
Morgan Heater  - Jan. 3, 2019, 12:50 p.m.

It seems like bigger ratios actually allow more rider movement, which would make it easier to control for less expert riders. Similar to wider bars and shorter stems.

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xy9ine
0
Perry Schebel  - Jan. 3, 2019, 1:22 p.m.

for sure. a larger sweet spot, if you were. makes it easier (i've found) to modulate front / rear grip.

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morgan-heater
0
Morgan Heater  - Jan. 3, 2019, 2:41 p.m.

Sam Hill needs a finely balanced scalpel, whereas punters like me are just fine with a chainsaw.

Reply

AJ_Barlas
0
AJ Barlas  - Jan. 3, 2019, 4:39 p.m.

Well put and so true!

Reply

Vorsprung-Suspension
+4 BkrAdam Metacomet James Vasilyev Morgan Heater
Steve Mathews  - Jan. 3, 2019, 2:41 p.m.

Bigger wheelbases (not specifically FC/RC ratios as such) allow more rider motion, but they also require it. One thing I have discussed with a few others is how longer wheelbases effectively increase the resolution of your movements, ie if you have a 1000mm WB, a CoM motion of 30mm constitutes 3% of the WB that you've moved. If that WB was 1500mm, that 30mm CoM shift is only 2% of the WB - so coarser motions give you finer control. That's good for precision and stability, but in turn also means you need to move further to achieve any given CPL shift desired. However, even the difference between the medium Mega and the large Machine is only 12% in wheelbase, more like 10% in the same size, so the rider has to be 10% more precise (10mm motion becomes 9mm motion) in order to accurately weight the wheels as he wants to. That's not an insignificant percentage, but we (or I at least) don't have any data to show how precise the rider can actually be. Like if you have to be accurate to within +/-5mm of CoM position to get the load on the front tyre within the necessary range, and that becomes +/-4.5mm instead, but the rider's sense of balance allows him to be easily precise within +/-2mm then it's a bit of a moot point there. Or maybe we're already the wrong side of that and most riders can only be precise within +/-20mm to begin with and they need to be within +/-10mm? I don't know the answer to that.

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morgan-heater
0
Morgan Heater  - Jan. 3, 2019, 2:44 p.m.

I think you've probably got a point for optimal cornering, but it's also important for less skillfull riders to be able to move around on the bike more without being in danger of going over the bars in rough terrain. (as one of those less skillful riders, the new longer bikes definitely feel significantly easier to ride).

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Vorsprung-Suspension
+1 Morgan Heater
Steve Mathews  - Jan. 3, 2019, 4:52 p.m.

I'd agree with you there, and that was largely based around the endo/looping angles discussed in part 1 of this video. Link here if you haven't seen it - https://youtu.be/P18SutYYL5I

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AJ_Barlas
+1 Steve Mathews
AJ Barlas  - Jan. 3, 2019, 4:41 p.m.

More great information. Thanks for getting, and keeping, the gears ticking Steve!

Reply

g_k
+1 IslandLife
Gernot Kvas  - Jan. 3, 2019, 1:04 p.m.

> Phenomenal skills aside, why is Sam Hill winning on seemingly old-school geometry? 

Because people win races, not bikes.

Reply

Vorsprung-Suspension
+2 Metacomet AJ Barlas
Steve Mathews  - Jan. 3, 2019, 2:38 p.m.

The rider is obviously the dominant factor by far. Hill is going to smash most of us even if he's on a hardtail with a 100mm fork, but do you think he'd be winning WC DH races or EWS races on that? I think we'd all agree that that seems unlikely, so then the question becomes "how much of a bike handicap would he need to have before he's unable to win"? IE at the top level, between say Hill/Maes/Rude/Bailly etc, is it plausible that a better/worse bike could be the difference between winning and 2nd or 3rd place when the gap is 3 seconds after an hour of racing?

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AJ_Barlas
+1 Steve Mathews
AJ Barlas  - Jan. 3, 2019, 4:45 p.m.

I often ponder that if he were on something with more wheelbase and slacker angles, would he be even stronger? Similar to what you're saying but coming at it from the positive side. Sort of like when Stevie (and others) were moving from 26-inch wheels to 27.5. They didn't like it but (I've heard) the clock consistently showed he was faster when testing. Would it benefit Sam? Possibly. Would he enjoy riding it as much? Maybe not.

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Vorsprung-Suspension
+2 Metacomet AJ Barlas
Steve Mathews  - Jan. 3, 2019, 4:53 p.m.

Entirely possible - hopefully we'll find out once Leo signs him to Pole next year! :)

In all seriousness though it's hard to know. The super long/slack bikes are still mostly coming from relatively small manufacturers who aren't really able to afford to put the guys who are winning races on them. Mondraker is obviously the historical exception to that (although compared to Pole/Mondraker, the geometry is not as extreme), and they have pulled a few wins on the DH bike under several different riders (Barel, Hart and Brook IIRC?). Interestingly though, the Summum's FC:RC ratio @50% travel is about 1.61 in size large (if their figure of 1250mm WB is correct at the longest chainstay setting of 460mm?) which is actually quite low compared to many bikes.

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mike-wallace
0
Mike Wallace  - Jan. 3, 2019, 10:36 p.m.

This is great stuff.  Anyone else riding a new geo bike and finding the their upper body getting more tired relative to their old bike?  Anyone willing to admit it after spending 9K.  Early feedback will likely need to come from bike testers who are not as invested and ride many different bikes.  If Steve is correct (and he likely is) then there should be an upper body fatigue trend that follows the longer reach trend?

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AJ_Barlas
0
AJ Barlas  - Jan. 4, 2019, 11:22 a.m.

I have to admit, that hasn't been the case for me. I find that I've been able to relax more on the bike because the angles allow it to pass over terrain with less effort from me. That and improved suspension. The greater picture outlines other variables, like the fact that I'm tall and am finally able to fit in a bike, allowing me to relax. Things like that need to be considered and blanket statements will never be true for all. It's super interesting though and I look forward to seeing where it all goes.

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Vorsprung-Suspension
0
Steve Mathews  - Jan. 4, 2019, 11:28 a.m.

How tall are you AJ, 6'5" or so? I agree that proper fitment trumps pretty well everything else really - having a bike you're comfortable on physically has a huge impact on how it feels to ride. 

Also worth noting that this comparison was only relevant when cornering (and maybe off camber straights). Generally though, in a straightish line, the longer FCs should be less tiring not more (unless for some reason you're struggling for grip on the front tyre, like it's off camber), we went into straight line performance a bit in the part 1 video.

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AJ_Barlas
0
AJ Barlas  - Jan. 4, 2019, 9:42 p.m.

191cm mate (so ~6’3”). Proportionally my torso is longer too. Totally agree. Fit and physical comfort are huge players.

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jaydubmah
0
jaydubmah  - Jan. 5, 2019, 9:17 a.m.

Hi there. I spent all of last summer and fall riding a new geo bike and didn't find my upper body getting more fatigued relative to my old bike. (From a size large Ibis HD3 to a size medium Pole Machine). If anything, I had the opposite. As an overall package, the bike just fit better. It felt better for the climbs, and preserved energy for the downs.

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fartymarty
0
fartymarty  - Jan. 4, 2019, 5:13 a.m.

This needs a whole lot more testing, a stop watch and some fatigue testing at the end of multiple runs.

Reply

lukey
-1 holmesslice
lukey  - Jan. 4, 2019, 4:59 p.m.

You can't analyze a suspended vehicle this way.  How can you say anything clear about a dynamic system like a bike and rider by analyzing it as if it's at rest?

As Archimedes pointed out a couple thousand years ago, adding input at multiple points along a mechanically joined thing acts exactly the same as if all the input happens at one point right through the center of gravity.  Common sense states that if you take your hands off the bike, the pedals will still put a bunch of pressure right on the front wheel, and the way you unweight a wheel isn't via your hands on/off or your feet on/off, but by adding torques to the bike and also by shifting your body's weight to deliver momentum, unweight, etc. by moving the momentum of your own center of mass.

The way that this video's analysis works, it commits a fundamental mistake where it assumes that the hands put the weight on the front wheel and the legs manage the back wheel, but there's no overlap and perfect division of duties.  It's further messed up because this analysis treats the bike as if it's a 2D structure, which it isn't.  Bikes have a left and right side, and the hands and pedals are not on the mid-line.

Bottom line, you cannot analyze a dynamic engineering system as if it's a static engineering system or you start making nonsense.

Imagine if someone analyzed a suspension linkage at full rest or maximum extension, without taking into account that the system moves, there are valves, there are fluids, there are pressures and that its work is delivered through a bunch of moving parts like linkages and frames.  Makes sense that that analysis would be super flawed, right?  This is why we talk about the kinematics of the whole moving system.

If you don't think I'm making any sense, here's an experiment for you: put a bathroom scale under your front wheel and stand still.  Now see what you have to do to make the weight go away.  If your can't unweight your front wheel by any static resting position, how valid is this whole method of analysis?

Reply

Vorsprung-Suspension
+3 holmesslice BkrAdam dave_f
Steve Mathews  - Jan. 4, 2019, 6:57 p.m.

Unfortunately pretty well everything you just said that I said is simply flat out wrong. It's essentially a 2D planar free body diagram (missing force vectors at the wheels, to be fair, so not a true FBD of the whole vehicle, only the rider). The rider pivots around the BB unless there's chain tension to prevent it, and the load on your hands is what balances the moment generated by the horizontal delta between your centre of mass and your BB. You've made multiple incorrect assumptions here, including the notion we're analysing it at rest - steady state planar acceleration in the Z direction (ie being leaned into a corner during steady-state conditions) is where this is relevant. If the resultant resolved force vectors from the tyres aren't in plane with the rider, the bike/rider mass rolls one way or the other, but that isn't the point of the explanation, so the 2D analysis is entirely reasonable. You also didn't pay attention to the video whatsoever, to be frank, and clearly didn't watch the previous one, since it's been made very clear that if your CoM is directly above the BB, the weight on the front wheel is not zero as you say was claimed, but apportioned directly by the front centre and rear centre ratios. IE if your front centre and your rear centre were the same length and you were balanced over the BB with your hands off the bars, 50% of your weight would be on the front wheels. There are numerous valid critiques that could be made of this 15 minute concept explanation, that was not one of them.

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