Anyone gone from 29 back to 26" ??

[Bodin]

OK, so raced a 1 hour dirt crit (B grade) at the You Yangs last Friday on my 26" Trek Fuel (my only bike at the moment). Gapped the 29ers on the steep, techy climbs and got caught on the flats and flowy, smooth descents.

Hard to know what this means, but I suspect it says more about the suspension, as I have 130mm and the 29ers were either short-travel or hardtails - my theory is that the extra suspension was giving me extra traction up the sketchy stuff and letting me pedal more, keeping my momentum higher.

Possible also that the smaller wheels were losing less momentum going uphill, but very difficult to prove it conclusively.

In terms of the flats and flowy descents, yeah, I can believe that bigger wheels would be faster on this terrain, having raced a few 100km events on HT 29er over the years. If I was still doing that kind of thing, I'd probably still have my HT 29er. But for truly technical riding and general fun, I still see plenty of signs that I'm faster and more capable on smaller wheels.

 

[Slowman]

OK, so raced a 1 hour dirt crit (B grade) at the You Yangs last Friday on my 26" Trek Fuel (my only bike at the moment). Gapped the 29ers on the steep, techy climbs and got caught on the flats and flowy, smooth descents.

Hard to know what this means, but I suspect it says more about the suspension, as I have 130mm and the 29ers were either short-travel or hardtails - my theory is that the extra suspension was giving me extra traction up the sketchy stuff and letting me pedal more, keeping my momentum higher.

Possible also that the smaller wheels were losing less momentum going uphill, but very difficult to prove it conclusively.

...

The techy climbs where you seemed to have an advantage was probably due in part, as you say, due to the extra suspension and I think also because the smaller wheels are easier to accelerate so after you slow down you can speed up again with less effort. Not so much a case of losing less momentum but when you do it matters less. With the bigger wheels it is more important to maintain momentum.

Even when you have a 26" wheel and a 29" wheel of the same weight, because the mass at the rim is further from the centre with 29ers they are harder to accelerate. You can see this clearly with the spinning figure skater slowing down their rotation every time they extend their arms and speed up again when they bring their arms in again. Remember rotational mechanics in HSC physics?

In rotational mechanics the moment of inertia (I) = MR**2 (that is, for a rim revolving around a hub see http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html#mi). Moment of inertia is like mass (M) in linear Newtonian mechanics. So what this tells us is that for the same mass rim (say 400g) and an ERD of 544mm for 26" and 606mm for 29" for Stans ZTR Crest rim we can clearly see the difference:

1. for 26": I = 400g * (272**2) = 0.0296 kg sq m
2. for 29": I = 400g * (303**2) = 0.0367 kg sq m

Clearly we can see it is higher for the 29" rim, the implication of this is that in order to get the same moment of inertia, and therefore the same acceleration behaviour as 400g 26" rim the 29" rim needs to be lighter. We need our 29" rim to weigh 322g to get the same acceleration properties as the 26" 400g rim. This of course is a simplified example and doesn't take into account the mass of the spokes and their length too but you get the picture; a lighter rim and reducing spoke count from say 32 to 28 and you'll get 29ers wheels that feel like 26" wheels. For lighter blokes this is possible now but for us fat old bastards it is pushing the strength limits of current materials, or our wallets!

 

[moorey]

Had my first ride on a 29" today that didn't totally suck balls. Mate is a trek dealer and brought out a carbon fuel ex.
No chance I'll be drinking the kool aid any time soon, but it's a hell of an improvement on others I have previously tried out.

 

[Bodin]

We need our 29" rim to weigh 322g to get the same acceleration properties as the 26" 400g rim. This of course is a simplified example and doesn't take into account the mass of the spokes and their length too but you get the picture; a lighter rim and reducing spoke count from say 32 to 28 and you'll get 29ers wheels that feel like 26" wheels. For lighter blokes this is possible now but for us fat old bastards it is pushing the strength limits of current materials, or our wallets!

...and there li-eth the problem. I'm 80+kg fully kitted out and don't exactly ride like the daintiest thing on 2 wheels. Especially in races where I exhibit my extreme lack of riding talent to the maximum. Light wheels just ain't gunna cut it under my heft and poor line choices.

My go-to 29er race wheels were DT470 rims (on Hope Pro 2s, can't remember the spokes). They were built well and did a pretty good job of staying straight, but they weren't competitive for weight against a 26" wheel. However, the biggest issue I had was with tyres - it wasn't hard to find ~500g tyres (e.g. S-Works Fast Trak front, S-Works Renegade rear), but they would squirm in corners under my weight to a disconcerting level, so I typically had to ride 600g+ tyres in order to not have the sh!t scared out of me every time I leaned the bike.

This is probably the main reason I'm back on 26" - for the same weight as a "racy" 29er wheel+tyre, I can get a bombproof 26" wheel that costs less and lets me ride anything without fear.

It's a bummer, because at my height (6'1" / 185cm), I reckon that 26" bikes look like little clown bikes under me, whereas 29" bikes seem to be better proportioned for my size. I'm gunna have to split the difference and go 650B one of these days, but the Fuel is such a good bike for pretty much everything that I can't justify a change right now.