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Or just compare seated cornering (COG up high) with cornering while standing, bike/body separation, weight on the pedals, COG low.
NotMeAtAll: "Its the same principle of supermotard bikes going circles around super sport motorbikes in a corner."
By that logic ADVs should be going circles around motards.
https://www.vitalmtb.com/videos/features/Matt-Walker-Charlie-Hatton-and…
Regarding the former turning radius i mentioned, it does not apply to this mixed wheel convo but of course wheel base/ turning radius effect tight switchbacks and slow tech riding. I havent found the change to make anything unrideable but it certainly does effect line choice.
My initial feeling of the change in behaviour on flat supported corners (Finale Ligure) was also similar to yours.
Thing is, you don't run them at the same pressure. You can run the 4" tyre at about half the pressure so you get double the contact patch.
Wheel diameter doesn't change the size of the contact patch either, but it changes the shape. A larger diameter will give you a longer, skinnier contact patch.
I'm sure that the casing also plays a role, as the tire isn't a uniform piece of rubber and may influence things somewhat
If you leave all the other variables alone, contact patch size is directly correlated to tyre pressure. Tyre pressure in pounds per square inch. Vertical load in pounds. contact area supporting this load in square inches.
Please don't try to argue against basic physics.
The difference in feel between 29 and 27.5" tyres is not well understood by most people and the reasons for the differences that most people state are wrong. It's not about angle of attack or a bigger contact patch.
The blog post by Joe from Starling linked earlier in this thread by SB14 explains this really well.
Back to the main topic, I would love to see some research into the gyro effects and turning arcs of different diameter wheels on cornering.
My anecdotal evidence is that mullets do corner better. I love my new mullet bike (thanks Joe, the Twist is fantastic!)
It would be awesome to have a full understanding of why.
I would have to say that the mullet does most things better compared to the full 29er setup based on my first impressions. Acceleration, general maneuverability, leaning the bike over, and it gives you a bit of extra clearance on jump landings. EXCEPT on the fast and chunky stuff it gets knocked around more which seems logical to me and potentially worth it in the end.
The mullet really rewards a bit more lean angle. Loads of grip and great transition between corners.
Tied together in a system like a bicycle they will be forced to carve a similar radius so if the smaller rear wheel follows the line of the front it will have to scrub a bit. This is one of the mechanisms put forward for the more lively cornering of a mullet.
The smaller wheel will turn more revolutions so have a higher angular velocity, but will still have less gyro effect than the bigger, heavier wheel and this might also help make it easier to lean over and turn.
A bit like the Yamaha R1 engine that spins reverse to conventional to counter the gyro effect of the wheel and reduce stability of the bike to make it more agile.
Lots of hypotheses getting around. It definitely needs a bit of scientific exploration to quantify all these effects.
I'm not sure you're looking at the whole picture when you're saying I shouldn't argue basic physics. I'm not. I'm just advising against going too basic. And don't worry, I'm well aware of what pressure does and the like. Don't believe me? Go through my post history, I think I don't need to type out my qualifications?
As for 'could vary construction and rim width', well... Yeah, you could. But the issue here is also that YOU vary it. By varying the tyre width, which by default varies the effective rim width, carcas shape, etc.
Don't get me wrong, all other variables staying the same does mean different pressure, different patch size. But like you said, all other variables staying the same. Different sized tyres is a changed variable, which is my point from the beginning, from the first mention of this (sub)topic.
Anywho, regarding wheel sizes, I've just been thinking a bit, a smaller wheel size is like a smaller side radius ona carving ski, X amount of lean angle will cause more 'turning' than on a larger wheel size, like you sorto of mentioned. But the thing is that a leaned over wheel rotating sort of 'falls down' as it turns - the smaller the diameter, the further it has to 'fall' to get into contact with the ground. Thus making a tighter radius.
Sure, diameter differences aren't that large between wheels when it comes to roll over differences, but this difference is increased (effectively) when leaned over.
Going from 26" to 27.5" I specifically remember rocky sections of trail where I braced for impact and never really felt the rocks, at least not what I was expecting to feel. Going back and forth from 29" to 26" was even more noticeable. I noticed how many fewer flat tires and dented rims I got after I switched to 29", despite running exactly the same tires and pressure. To hop in the way back machine, the old 24" rear wheels on Specialized Big Hits and Santa Cruz Super 8's were infamously bad for racing because they got sucked into every hole. Fact: smaller wheels hit bumps harder.
And yes, i agree completly with the feel of the ground and roots and all, but the reason being impact angle of the tires is up for debate. As Joe @ starling cycles explains, what you are feeling is the difference in gyroscopic stability. His theory is that the change in gyro-forces is greater then the angle of impact the wheel makes.
Though, it might be that a 1.4degree differance between 275 and 29 at a 2 inch root, is something that you feel and can attribute to this change. But it the mass in motion is greater, and makes a bigger variable between the two sizes.
I dont hold the answer, but it seems to me that we can not neglect these facts. And it is not the one over ther other that rocks the boat, but the combination. Though, one has a bigger change of factors then the other.
But I was thinking about the inertia effect of it all too. Have to put the XM1501/Aggressor combo back on the bike, it feels SO heavy and cumbersome since I've setup the 240s/EX511 32h Assegai combo last summer while I was without the bike and injured, so I got to properly ride it only in the fall and over winter. Now we're getting into summer conditions and conditions for more comparisons.
It would be cool if Vital did a quasi-scientific timed test like they did with tires a year or two ago. Ideally one bike capable of all 3 modes, same rider, same tires. Do timed runs down a straight smooth surface (like the road they used in the tire test) and average results. Do same thing but on a straight section of trail with a series of brake bumps.
In all seriousness though, I don't mean to disrespect you. I've read and enjoyed a lot of your thought provoking posts and analysis.
Doesn't mean I won't argue with you though! On the contrary. I'm more likely to argue with someone I respect.
From your reply above I don't think we are actually too far apart on the pressure vs contact patch topic.
I think your ski side radius analogy is a good one. A smaller wheel will behave like a smaller radius/deeper sidecut.
Teamrobot, I'm not arguing the difference in feel over trail bumps between 29 and 27.5. Just the reason for it.
The effect is real, but the mechanism probably isn't what the "conventional wisdom" about 29" wheels says it is.
Maybe there are some further benefits to be had by poking a bit further at it and understanding the reasons behind it a bit better.
This actually sounds like something a YouTuber would do, but I wonder if anyone at the big companies has ever tried this to see where they actually want the geometry to be instead of just following industry trends. Minaar mentioned a few years ago that he thought his bike should be longer but that people need time to adapt/accept the change, but do the bike companies know where the limit is? Or maybe the limit does not exist.
Not arguing with physics but are you sure we are taking all the variables in consideration ? For instance, common practice is to measure tire pressure unloaded right ? Additionally we can agree that for the same tire section, the total air volume will be bigger for a 29 wheel than for a 27.5 wheel. If my physics is not too rusty, air pressure increase exponentially in a given volume and the bigger the volume the more displacement will be needed to end-up at the same pressure. So considering that initial pressure, loading and tire section are kept constant, in order to reach the same loaded pressure the 29er will have to displace more volume (contact patch) more than the smaller wheels. Effect that would be exacerbated by the bigger tire area likely to stretch under pressure increasing even more the space available for the air to spread.
Remember the marketing stuff on wider rims and bigger tyres, how they give you more support. Now think about a car tyre in a corner, which will have more 'sideways stiffness', which will be more wallowy, a slightly stretched tyre (too narrow for the given rim) or a tyre that is just the right width or maybe a bit too wide even? And this is regardless of the pressures run (well, if they are the same actually), as the geometry itself will define a lot of that.
But like I said, tyre X, rim Y, more pressure, less contact patch, that part is clear and logical.
Post a reply to: BB-height, cornering and why mullet bikes might turn better.