Washboard: 650B high volume vs 700c medium volume

[Wayne77]

I know 650B road plus is all the rage lately. I've done a lot of riding and competitive gravel racing on 650B wheels with either WTB Byway tires, Schawalbe G-ones, and even a Schwalbe 2.1" Thunder Burt. Yet I still experience the mad-painful shudder when I hit washboard at speed. I know its painful not matter what combo, but it has me wondering if a 700c wheel with a 45mm tire would be better due to the larger overall diameter. I keep hearing that 650B with a larger volume tire has a very similar diameter to a road tire, but when I lined up my 650B wheel with a WTB Byway next to one of my road wheels with 25mm tires, the 700c wheel appeared to be a bit taller still. A 45mm tire on a 700c wheel has a significantly larger net diameter than a 650B wheel with a 40mm plus wheel.

I ask the question because this year I was still dropped by a lot of guys with 700c cross tires descending a very steep gravel road (with A LOT of washboard and tight very loose switch back corners) in the Crusher race a couple weeks ago. I chose a 2.1" Schwalbe Thunder Burt front / Schwalbe 40mm G-one in the back, thinking the fatter tire would give me an advantage in the rougher gravel sections of that very competitive race. So either I am a horrible descender, 650B high volume tires are not really a differentiating factor, 700c 32-35mm cx tires are better than people give them credit for in rough conditions, or these guys who dropped my on that descent with their CX bikes are amazing descenders.

I am half considering selling my 650B Enve disc wheels for an equivalent 700c set for competitive gravel racing. Maybe I'm crazy. Or maybe its the classic grass-is-greener fallacy. Or maybe its just my "change for change sake" personality.

While my overall time at the 'Crusher in the Tushar' race this year was quite a bit faster than last years' time because my FTP is 20 watts higher, my time descending the Col de Crush was slower than last year when I had 700c wheels with 45mm WTB Riddler tires. The whole reason I chose a fat 650B wheel upfront was to improve the descending confidence and speed on that section of the course. That theory doesn't seem to have been correct in hindsight.

Thoughts?

[Lovetoclimb]

While I do not race gravel by any means, I can easily be dropped by people on 700x40mm setups while I am using a 29x2.1 ... Hell I have been dropped by my friend on some WTB Exposure tires (700 x 30) while I was using the same 2.1 Nanos. I definitely think descending ability, especially on rough and loose terrain plays the biggest role. Beyond skill and risk taking, you’re limiting losses IMO.

All that being said, I am switching to a completely different bike, trying thru-axles, and going to the 650 x 2.1” setup shortly.

[pbarry]

Are you freewheeling the decents or pedaling? Unweighting the saddle to counter the washboard surface? Keeping your elbows bent and avoiding a death grip on the bars? Fast descending has more to due with confidence and skills than equipment: Hence the cx guys on relatively skinny tires crushing it. ;) The fatter tires may not be an advantage as they deflect more when cornering, and you might be subconsciously slowing down in a turn to not feel that perceived breaking point.

[Ken Robb]

You're a racer so try some time trials with each tire and see where you are faster on which tire?

[doomridesout]

I haven't put in enough time on the 650b v. 700 in this situation, but a guy whose opinion I trust (who is like 5'4" and well in the market for 650, size-wise) claims the 700c format is best for washboard gravel because of the rollover effect. Intermittent bumps- high volume 650. High frequency bumps- 700c. Braking bumps and washboard, the lower angle of attack is better, just like everybody hyped when 29ers came out.

[CMiller]

I say try tubeless and lower the pressure a buuuunch, before investing in a new set.

[spoonrobot]

For washboard, the biggest overall diameter is the most important thing - after having a suspension fork as well as a dropper seatpost.

Fat 650b at low pressure tend to g-out when hitting washboarding at speed. The larger diameter and higher pressure of something like 700cx40 handles it better and the tires doesn't load up and start bucking the frame/fork quite so bad. Higher pressure also seems to hit a max frequency that is more within the realm of going fast and being in control.

IMO if you want to be competitive you will need to be riding closer to what your competition is riding. For all the bluster about fatter tires nowadays there's a reason the people winning are often on more conservative choices.

Is this the segment? https://www.strava.com/segments/7636359?filter=overall

[Wayne77]

Are you freewheeling the decents or pedaling? Unweighting the saddle to counter the washboard surface? Keeping your elbows bent and avoiding a death grip on the bars? Fast descending has more to due with confidence and skills than equipment: Hence the cx guys on relatively skinny tires crushing it. ;) The fatter tires may not be an advantage as they deflect more when cornering, and you might be subconsciously slowing down in a turn to not feel that perceived breaking point.

Thanks. All of the above. Good points, especially about tire deflection. That said, I’m pretty familiar with the basics of off road bike handling but off course there are other nuances I still need to improve on.

[Wayne77]

You're a racer so try some time trials with each tire and see where you are faster on which tire?

Perhaps I wasn’t very clear but this isn’t a question about rolling resistance...I have done quite a bit of research with real-world experiences...so I understand the trade offs. My question is purely about 700c larger diameter/narrower vs 650B smaller diameter/fatter on washboard dirt roads.

[Wayne77]

I haven't put in enough time on the 650b v. 700 in this situation, but a guy whose opinion I trust (who is like 5'4" and well in the market for 650, size-wise) claims the 700c format is best for washboard gravel because of the rollover effect. Intermittent bumps- high volume 650. High frequency bumps- 700c. Braking bumps and washboard, the lower angle of attack is better, just like everybody hyped when 29ers came out.

Thanks...intuitively that makes sense. Very helpful explanation.

[Wayne77]

That’s the exact segment I’m talking about. It’s very steep and there are some extremely bone-jarring washboarded sections. The switchback turns are pretty rough and rutted out. All that said, I wouldn’t say this is segment would typically be pivotal in the race. There is a lot of pavement right after and riders typically regroup. This is extremely helpful information! Thanks

For washboard, the biggest overall diameter is the most important thing - after having a suspension fork as well as a dropper seatpost.

Fat 650b at low pressure tend to g-out when hitting washboarding at speed. The larger diameter and higher pressure of something like 700cx40 handles it better and the tires doesn't load up and start bucking the frame/fork quite so bad. Higher pressure also seems to hit a max frequency that is more within the realm of going fast and being in control.

IMO if you want to be competitive you will need to be riding closer to what your competition is riding. For all the bluster about fatter tires nowadays there's a reason the people winning are often on more conservative choices.

Is this the segment? https://www.strava.com/segments/7636359?filter=overall