Wheel size comparison, data based

[mtg]

So, this doesn't really seem to exist, and I had a couple spare minutes. With a sketch in CAD, I drew what is basically a 26" wheel, 27.5" and 29" hitting a square edge feature to put some numbers behind "rolling over rocks".

A quick tangent about load transfer: the neck of the woods that I was involved in with racecars used the terms of "elastic" and "geometric" load transfer to describe the difference between loads that came from the tires and either went into the springs & dampers, or directly through the chassis. In the mtb world, this would be equivalent to pivot height in the rear suspension. A BB-concentric pivot location has poor square edge performance because a significant portion of the tire load goes directly into the frame (high geometric load transfer) and feels harsh, or "gets hung up". A super high pivot, conversely, has higher elastic load transfer, and much less of the tire load goes directly into the frame without first going through the spring & damper.

Returning from the tangent, the below graphic shows the effective approach angle of different wheel sizes. With that, geometric vs elastic load transfer can be calculated. That will also vary directly with head angle and square edge height. I'll get to those calcs soon.

Discuss.

[Sandwich]

Did you take into account actual rim sizes? I know 650b is not 27.5", but I was kind of under the impression that neither is 26 actually 26" nor 29er exactly 29"...just for the sake of accurate comparison.

Norco presented some interesting stuff here: http://reviews.mtbr.com/2013-norco-r...ght-650b-bikes but some of the mathification is more "Rolls gooder" vs. "17% increase in tactical rollover"

I think head angle will have to be "re-figured out" in that the holy grail 63* HA may not work for bigger wheels...but you bring up another very interesting point. If you have better rollover due to bigger wheels, could you make the pivot lower, giving better cornering response but not dealing with the harshness of a lower pivot?

[mtg]

Those diameters in the sketch are nominal sizes. If we can get measured diameters of equivalent setups in different wheel sizes, I can update the sketch.

That video was interesting, but...
Quote from that video that sums it up nicely: "No real facts, here". Scroll to 6:41, it's where there's a graph where the Y axis values are "pretty good", "noticeably better" and "mega awesome". Lol. That Y axis should have labels of Watts.

And, the whole "bigger wheels have moar inertia, and that's good for rolling" is complete BS. If that were true, everybody would run DH tires on their 26" wheels and be winning at everything. Also, I'd like to see a detailed speed trace while riding at "steady speed". I highly doubt that even the TdF road riders can spin a smooth enough circle to keep the speed completely constant. There's bound to be a cyclical nature in speed, which means that you are constantly providing pulses of power to the wheels, even when riding at a "constant speed". Thus, you're constantly accelerating the wheels while pedaling.
It's been a bit since I've done this calc (I'll have to redo it to make sure it's correct), but I recall calculating the rotational inertia difference between a 26" and 29" setup with the same wheel and tire combo. If memory serves me correctly, the 29" setup had 25% more rotational inertia, which is a big deal.

[kidwoo]

And, the whole "bigger wheels have moar inertia, and that's good for rolling" is complete BS. If that were true, everybody would run DH tires on their 26" wheels and be winning at everything. Also, I'd like to see a detailed speed trace while riding at "steady speed". I highly doubt that even the TdF road riders can spin a smooth enough circle to keep the speed completely constant. There's bound to be a cyclical nature in speed, which means that you are constantly providing pulses of power to the wheels, even when riding at a "constant speed". Thus, you're constantly accelerating the wheels while pedaling.
It's been a bit since I've done this calc (I'll have to redo it to make sure it's correct), but I recall calculating the rotational inertia difference between a 26" and 29" setup with the same wheel and tire combo. If memory serves me correctly, the 29" setup had 25% more rotational inertia, which is a big deal.

That sounds kind of contradictory to me. If there's 25% more rotational inertia, that's where the moar gooder rolling comes from. Talking coasting here, not acceleration, where obviously it's the exact opposite.

Neither of which are news really though. That's kind of the conventional wisdom anyway. Continuity is great, any kind of acceleration/deceleration suffers the bigger the wheel.

[OGRipper]

That sounds kind of contradictory to me. If there's 25% more rotational inertia, that's where the moar gooder rolling comes from. Talking coasting here, not acceleration, where obviously it's the exact opposite.

Inertia plays a part but so does the physical properties of the bigger wheels relative to obstacles. That's how it's been explained to me anyway.

[Sandwich]

I always thought it was approach angle and had nothing to do with inertia. I didn't sense a large increase in rolling mass with lightweight tires and 29s vs lightweight tires and 26. There seemed to be a small decrease in acceleration but I attributed it to moar leverage on the wagon wheels. That's why I think 29ers COULD have merit on a DH bike. I don't think they've been done right yet though. It's all about geometry and 29ers have their challenges. 650b solves most of em. I also wonder about contact patch and what that could do on a big wheel. As has been discussed, it gets longer on niners, so I wonder if you could have a more specific contact patch which could provide for lower rolling resistance due to lack of contact of shoulder knobs when vertical, or something like that.

[buckoW]

I always thought it was approach angle and had nothing to do with inertia. I didn't sense a large increase in rolling mass with lightweight tires and 29s vs lightweight tires and 26. There seemed to be a small decrease in acceleration but I attributed it to moar leverage on the wagon wheels. That's why I think 29ers COULD have merit on a DH bike. I don't think they've been done right yet though. It's all about geometry and 29ers have their challenges. 650b solves most of em. I also wonder about contact patch and what that could do on a big wheel. As has been discussed, it gets longer on niners, so I wonder if you could have a more specific contact patch which could provide for lower rolling resistance due to lack of contact of shoulder knobs when vertical, or something like that.

My favorite part about the middle sized wheels is the increased directionality when drifting. They seem to hold a better line and almost feel like little "rails" (similar to skiing). I think the difference in roll over is slightly compromised by the new relation of the main pivot to rear axle. The bigger the wheel size the more forward the axle path for all systems without idler pulleys. There is an increase in rollover but if all things were equal (only wheel size changes) it would be even greater.

[Sandwich]

Interesting about the tires/drifting!

I hadn't thought about the effect on wheelpath via the increase in axle height....I don't have the CAD experience to try and determine pivot height vs. approach angle but I think it would be interesting. Is there enough of an improvement in approach angle to lower the pivot, and therefore get better handling? Is there really a good way to calculate that?

[joeg]

I am going to build a 100% rearward axle path bike. If only one person never says rearward axle path again, it will have been worth it.

[Sandwich]

I'll keep saying it just to annoy you. Also because I think the bikes ride better.

[Huck Banzai]

bunghole.

[rosenamedpoop]

You should try a V10... super rearward

[joeg]

drops to flat would suck

[rosenamedpoop]

Not if you're pointed straight at the ground.

[dilzy]

Rotational inertia of a wheel will not really effect the it's ability to 'roll betterer' over things, especially since an increase in rotational inertia usually comes with an increase in unsprung weight. The linear inertia of the bike+rider however sure does.

You want to roll over things better, eat some pies/build your main frame (not your swingarm) out of lead.

Personal opinion after damaging many 26" wheels, f-29". It's just silly, so's 650b. The force vector difference when encountering an obstacle with these different size wheels is bugger all (as noted by the drawing on the op) and the whole thing smells of i-phone.