“Talking about that 'timing' or frequency point, it's an interesting point of circles geometry that the gain there was much greater when going from 584 rims to 622 (12% for a given smallish bump / a root size), than it is from 622 to 686 (4.5%). So not all gains are as proportional as we might think.”
I presume this is due to the relative sizes vs the bump size.
My youngest just moved up from 14” to 20” yesterday. She’s so much faster, obviously gearing (and gears) but on our local tiny kid bike training spot (derelict grit hockey pitch) the vast increase in smoothness is clear.
I totally agree that bigger wheels are a pain when it comes to fitting bikes in cars, houses, trains, etc. And once it’s steep I’d rather have a 27.5 behind me but I’ll live with a 29 rear on the hardtail because it’s better everywhere else (bar the odd pump track visit).
I presume this is due to the relative sizes vs the bump size.
I thought that with the wheel OD going up at about the same amount as we went from 26 to 29 to 32 (~65mm each time), and the angle of attack also being a proportional change, that the roll-over timing change would be in proportion - but drawing out the geometry shows it's different.
Yet the change or reduction in how the wheel drops into a hole at low speed, the axle drop, is also proportionally reduced as the wheel gets bigger.
Clearly my conceptual/mental maths isn't up to knowing why this is w/o further digging and I've not done that yet.. like there's a squared relationship in the formulas that would show what my 2D sketch does, something I'm not seeing. I just thought it was interesting since the reduction in bumpy trail chatter is something we associate with bigger wheels.
Most of the chat so far has only been considering a bike moving in a straight line. So I offer up two more avenues for discussion and perhaps smarter people truly explaining the science.
1. Rotational inertia of the wheels and the subsequent gyroscopic stability. Inertia is proportional to radius squared, so the bigger wheel with only a small increase in total weight with a like for like rim and tyre comparison will have more inertia. The bike may feel similar to pick up, but will have much more resistance to changing direction whether that is acceleration or leaning into a corner.
2. BB height. Chief did touch on this that the bb being slung” below the axle line gives a certain stability as the front hub moves laterally due to fork take and offset. However it also is affected by the gyroscopic resistance to movement of the wheels, which acts at hub level. If your feet* are significantly below that level it’s going to have a quite different feeling when you start moving the bike around.
*I specifically say feet, as really that’s the level at which you are interacting with the bike. Bunny hopping, cornering, even swaying the bike side to side as you sprint, a lot of the feel comes from how your feet interact with the gyroscopes. Whether that’s pedal level coasting or pedalling circles, the base of your feet is the key reference point, not the pedal spindle. On the last generation of 26er full susses, I think that point was round about level with the axles. So when you really pushed into a berm and compressed, you would get a real change in feel of being “in” the bike. A lot of 29ers you are well beyond that feeling just at sag point, so you don’t get that change. You feel in the bike all the time which is great for stability and safety and speed but possibly less engaging.
2.5… if the increased bb drop is found to be undesirable for some use cases, a slightly left field suggestion is just to raise it up. This would solve the stack issue for the low ape indexed of us, increase the ability to pedal over rough terrain for everyone like cx bikes, and reduce the arse contact problem for shorter riders. It does make stopping and starting more of a problem, but, unlike when 29ers first appeared, droppers, and long stroke ones at that, are the default. I think in some ways, this would be a good solution for an xco race bike, although may not be so ideal for the more casual rider or the long distance endurance rider.
So Sam has finally built a bike that fits him?
You’ve all ignored the fundamental aspect of riding bikes…rider comfort.
unless you address rider (dis)comfort, it’s going to impact performance.
im all for anything that can raise the front end of the bike, without recourse to stem riders, etc.
anything to take the weight off my wrists and redistribute the sack of potatoes that is my torso, away from my perineum.
ive even gone so far as to retrofit a 27.5” wheel to my e-bike. That made a huge improvement in comfort already.
i think someone could clean up by offering a rigid fork/ wheel/ tyre combo on eBay that could slot straight into a conventional bike.
the acid test are things like the great divide race.
So I offer up two more avenues for discussion and perhaps smarter people truly explaining the science.
**I should probably leave the chat at this point..**
Inertia is proportional to radius squared, so the bigger wheel with only a small increase in total weight with a like for like rim and tyre comparison will have more inertia.
I remember something about this from the 29er debate. This inertia point ^ is true if they were rotating at the same RPM but not for rolling wheels where a bigger wheel rotates slower at a given speed along the ground (let's ignore the point that 32" is faster cos brands say so etc .. ). The radius part of the inertia equation is cancelled by adding factor that in. So yes the mass further from the axle means they need a bit more input to get moving but once moving weight is all that matters, it's just kinetic energy changes. LFL the 32" will be ~10% heavier.
Gyro forces are a point I'd not thought much about in the past but it must affect how the bike feels when you small corrections in the steering or lean angle. Gyro force is linear for weight and distance from axle, again the wheel will be rotating slower. I think it's mostly about the weight gain but gyroscopics are more complex so I suspect there's more 'feel' difference than just the weight.
(worth mentioning again the Maxxis 32" is a very light tyre, about the same as my 29 x 2.3 thin casing old style Purgatory 2Bliss that I use in winter - interested to ride a T7 or EXO weight 32" trail tyre... )
Do you know, I’d probably need to ride one to comment on how it rides.
I agree with @BruceWee the longer narrower contact patch is marginal and I also agree that the contact patch area should be identical.
Sorry if what I said came across wrong - typing on a phone means I lose my thread sometimes.
Other than that, it’s a bike, hopefully it’s a bit different, probably not better or worse, but different is good.
Learning to ride with and around the differences is part of the fun.