26" vs 27.5" (650b) vs 29"

Andy Kessler - 23-May-2012
We're getting some questions about wheelsize, 26" vs 650b vs 29". Of course 650b is getting a lot of attention lately, and during this project we've looked at all three. The consensus in the industry at SeaOtter seemed to be that 650b is quite a popular wheel size for companies who missed the boat with 29ers and who now have a vested interest in convincing people that there is a better option. Of course, many of these people who don't really think 650b is that relevant are suppliers to the very companies pushing 650b, so nobody really wants to speak out and many are half-heartedly making some 650b product just to keep their customers happy. 

But that doesn't mean there is no case to be made for 650b! The problem is, 650b is touted as the best of both worlds when that's not really a fair assessment. Rather it's halfway in-between two worlds. It's a bit lighter than 29" and a bit heavier than 26". It rolls over obstacles better than 26" but not as good as a 29". That's simple physics, nothing anybody can do about that, no matter how much marketing you do or how many pros you sponsor with any of the three sizes. You can't have your cake and eat it too!

So in that sense, I'm not impressed with arguments that "rider X won race Y on size Z wheels, all that proves is who spends the most on sponsorships. In fact, there is a particular instance of a rider winning a big race on a 650b hardtail which is touted by proponents of that size as proof that 650b is superior. However, the rider's mechanic says the rider would actually prefer to ride a 29er but his sponsor doesn't make a good one so he is resigned to the 650b.

The weight difference between all three isn't really that large as a percentage of the overall bike+rider combo. A few hundred grams on a 70-100kg complete bike+rider weight is only 0.2-0.3% difference. The rolling difference is therefore usually bigger than the weight difference, although that's very dependent on the course. But it means that often, 29" is faster. 

However, when you get more suspension, a wheel gets over obstacles in a different way, not by lifting the bike over but by compressing the suspension. The more that happens, the less important the geometry of the wheel vs the obstacle becomes. So the 29" wheels provide the greatest advantage on frames with the least suspension, particularly hardtails.

On top of that, there are definitely situations where 29" won't work:
  1. Below a certain rider height, there isn't enough room to put the bigger wheels under the rider. So if 29" doesn't fit, then maybe 650b does, and if so that gives you better rolling than 26". Obviously, if you go further down in rider height you will even get to the point where 650b won't fit anymore, and you'll need to revert to 26".
  2. Above a certain amount of travel, there isn't space for the larger wheels. The bigger wheel also takes up more space as it travels up and down, and the more travel you have, the more difficult it becomes to construct a frame around it and position a rider on it. Making a frame with 160mm of travel using 29" wheels is very tough, and so a 650b wheel has a place there. It gives you a little bit better rolling than 26", and more space to construct your frame.
Bottomline, on a hardtail the 29" wheels give you the best rolling performance for a small weight penalty. Smaller wheels (650b or 26") will deteriorate the performance. When you add more and more travel, the rolling advantage becomes less due to the different way obstacles are crossed and on the flipside, the weight penalty gets bigger because efficient frame construction becomes a challenge.

Comments & Questions

So why not 26" in the suspended front and 29" on the rigid back?
Post #1 of 15. Posted by djconnel on 23-May-2012 12:43:36 GMT in reply to blog [0<--209]
I was thinking the exact same thing...plus, that also means that if you're using rear suspension, then you might as well stick with 26" wheels front and back.
Post #3 of 15. Posted by tanhalt on 23-May-2012 14:52:24 GMT in reply to post #1 [209<--211]
Hello Tanhalt, see my comment to Ben also. Rear suspension or not is not the decision maker. Its really the amount of travel that is important. Below 140mm 29ers have really some advantages over 650 or 26" as i have mentioned in the blog also.
Post #5 of 15. Posted by Andy Kessler on 24-May-2012 05:23:44 GMT in reply to post #3 [211<--213]
To add to what Andy said, it's all about the space you have. There is no problem putting a 29" wheel in the fornt with say 100mm travel, there is still enough space to put a headtube on there and have a decent bar height. It's also no problem to still draw a downtube from headtube to BB without it interfering with that wheel.

If you put a 160mm fork in there, your bars are going to be pretty high with a 29" wheel and it also requires a lot of room where normally you would put your downtube. So it becomes a problem. Definitely, for a shorter rider, this problem occurs a bit sooner than for a larger rider.

In the rear, a short travel suspension can make room for a 29" wheel without a problem, with larger travel the wheel swings so far into the frame that your tube-shapes become sub-optimal.

Luckily the rolling characteristics evolve the same way. With no suspension or short travel, the geometry of the wheel is important. With long travel this is less important. So you "sacrifice" less going to 26" from a rolling perspective, and you gain from the smaller space that the 26" wheel occupies. Does that make sense?
Post #7 of 15. Posted by @gerardvroomen on 24-May-2012 10:37:36 GMT in reply to post #5 [213<--215]
Thanks Gerard and Andy, that helps a lot in understanding your approach. Now then, on the subject of wheel sizes, and understanding that Gerard has long been a "data driven" sort of guy...do you guys have any actual data that supports the speculations about the effects of wheel size on performance? I'm not talking about subjective rider impressions, but actual data. There are many cases (especially in cycling) where what on the surface seems to be logical and "science based" ends up being not correct in the end when actually measured. A case in point is the current claim in road circles that wider rim beds on wheels naturally leads to lower Crr...when, in actuality, the interplay of ALL of the effects (tire shape, pressure, etc.) leads to there being no measurable difference. In the case of 26" vs. 29" wheels...sure, larger wheels theoretically will result in a lower Crr (but again, what's theoretical difference?...is it small?) but you're also adding weight and rotational inertia.

I guess I'm just looking for actual data to back up the claims that 29" is "better"...
Post #8 of 15. Posted by tanhalt on 24-May-2012 11:23:33 GMT in reply to post #7 [215<--216]
Hi Tanhalt,

I fully agree with you on that. There is some data but to be honest it is sketchy because it's never a double-blind test and even more basic, it's never apples to apples (there is no frame that is available in all 3 sizes with the same wheelset available in all sizes, etc. It's even in the example Andy used, with the manufacturer that offers a 27.5" frame with nice geometry and a 29er with lousy geometry. That doesn't mean 29" is bad, just that geometry trumps some other things. Of course when you get into the argument of what "the same geometry" really means, it gets very tricky. Same angles, or same trail, or same feeling?

BTW, the rotational inertia is tiny. Bigger diameter but lower rpm delta for the same acceleration on the bigger wheel, in the end the "tire speed" is the same and that's where all the weight is. so the only thing left is the actual tire and rim weight difference.

And so the question becomes, how real do you want the test set-up to be. If you're just looking at a wheel rolling over an object, that can be done and has been done. But you would probably rate that too theoretical (and rightly so). Yet the more realistic you want that, the harder it becomes to get the test right. And so you have time comparisons between two bikes with the same rider, same power output on the same course, but those rarely have the number of test subjects to be statistically sound.

I wish I had a more positive answer for you. I'm planning to do something in-between the two options I mention above, which I think would be a fairly solid way to test, but this will take a bit of time.
Post #9 of 15. Posted by @gerardvroomen on 24-May-2012 12:06:23 GMT in reply to post #8 [216<--217]
Hello djconnel, maybe i was not clear enough in my blog and it sounded that 29er is only good if there is no suspension. Thats of course not the case. You profit from the bigger wheels not only by having a fast and "good rolling" bike but there are also other advantages of the big wheel like traction, handling and safety. I have heard from a lot of people that ride a 29" bike that they decent stuff they would not have done before because the traction and the control is that much better. I personally changed my tire style completely. I ride now a Schwalbe Racing Ralph a tire that i never liked on a 26" bike because it did not have enough profile for me. The rolling resistance was great but the handling and safety limit. With a 29" wheel i love this tire because it gives me the same traction and control than a more wider and more profiled tire but of course the rolling resistance is much lower and the bike faster. So all in all a faster bike and better control and traction.
Post #6 of 15. Posted by Andy Kessler on 24-May-2012 05:34:54 GMT in reply to post #1 [209<--214]
Thanks for putting something in writing about this very talked about topic. What do you think is the right rider height for 29" and vice versa for a 650b and 26"?
Post #2 of 15. Posted by ben nott on 23-May-2012 13:45:55 GMT in reply to blog [0<--210]
Hello Ben,

rather than on body size i would make a differentiation on travel. Definitely on travel over 140mm 650 starts to make a lot of sense. On the lower limit it depends a lot on you body dimensions and the geometry of the bike also. So there is not a clear measurement where you can say below this size you can not ride 29er. I still think the best thing is to just go and ride a bike and you will feel if you are comfortable or not.
Post #4 of 15. Posted by Andy Kessler on 24-May-2012 05:19:18 GMT in reply to post #2 [210<--212]
Hey guys, remember the Trek 69er that had the 26" rear and 29" front wheel. I wonder if it is due for a revival???

Post #10 of 15. Posted by Shane on 19-Sep-2012 09:38:52 GMT in reply to blog [0<--362]
Yes, we talk about that from time to time when we're working on designs. I doubt it will make a comeback though.
Post #12 of 15. Posted by Gerard Vroomen on 19-Sep-2012 11:51:04 GMT in reply to post #10 [362<--365]
Another thing about the 69er idea; I rode a 69er a fair amount - a Singular Hummingbird with the rigid forks, as well as in 26er mode with suspension forks on the front. I'm pretty short - 5'7".
The first thing I noticed was how the big front wheel was easy to keep going, and made me feel like I was riding a monster truck - I was clearing lots of stuff, and while riding rigid was still as tricky and occasionally painful as I remembered from 15-odd years ago, I was still doing pretty well, andgoing very fast - it just hurt a bit more. The second thing I noticed was that I kept hooking the rear wheel up on roots and step-ups that I'd cleared easily with the big front wheel. Riding the same trails with the same bike, but a 120mm suspension fork and a 26" wheel, I was lifting the front and rear over these obstacles, because I had to pay attention to them or get stopped or knocked off.
That doesn't mean 69ers are bad - far from it - just that, to me, it brought out the benefits of a big wheel.
Post #14 of 15. Posted by bent_udder on 14-Dec-2012 05:57:11 GMT in reply to post #12 [365<--440]
Makes sense. And as you say, stuff you could roll over with the front 29 you sometimes can't with the (rear) 26. So making them both 29 makes sense (assuming you have space of course, given the suspension you may have in the back and in extreme cases, the rider height (though you don't need to be very tall to fit perfectly on a 29er hardtail).
Post #15 of 15. Posted by Gerard Vroomen on 17-Dec-2012 10:21:02 GMT in reply to post #14 [440<--446]
Maybe a 650B rear and 29" up front would allow for more travel in the rear shock while still having the the rolling over obstacle advantage up from because generally when riding you "pull" the back wheel over anyway by shifting your weight around. And from an asthetic perspective would not look as off balance as the original 69er....
Post #11 of 15. Posted by Shane on 19-Sep-2012 09:42:14 GMT in reply to blog [0<--363]
The difference between 650b and 26" is pretty small, a 26" with a large tire has a diameter that is tough to distinguish from a 650b with a smaller tire. So it would look pretty similar to the old Trek. And as always there is no free lunch, the bigger front wheel gives you the advantages but also the disadvantages (if your steering geometry was poor with two 29" wheels, you probably still don't know how to design it properly with one 29" wheel). And of course, if you want more travel in the rear, you also want it in the front and you end up with a pretty high headtube position after a while (not with the 100 or 120mm forks, but with a 140/160mm front travel, it gets tough in the smaller sizes).
Post #13 of 15. Posted by Gerard Vroomen on 19-Sep-2012 11:53:55 GMT in reply to post #11 [363<--366]