Vorsprung Tuesday Tune: Advanced Bike Geometry II
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:
- Deviate Guide 2018 (size large): The best cornering bike we've tried
- 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
- 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
- Nicolai/Mojo GeoMetron G16 (size large): Even more extreme geometry than the Pole Machine
- 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.
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.