Nice Technical analysis of the 26, 27.5 and 29er technology

[SpootNICK]

Asking a company to motivate its decisions is easy. Calling it a technical analysis is a little bit of a stretch. Clever, but still utter BS in terms of independent scientific research.

[Iwan Kemp]

Asking a company to motivate its decisions is easy. Calling it a technical analysis is a little bit of a stretch. Clever, but still utter BS in terms of independent scientific research.

Yip

[beanz]

I know this topic has been hammered to death, but most it's mostly been subjective opinion and conjecture. Giant have supposedly tried to do a scientific assessment and have been flamed for it, labeled as just pure marketing spin.

Their basic assertion is that although 650b (or 27.5 or whatever they want to call it) sits in between 26 and 29, it is closer to 26 where 26 is strong (weight, stiffness) and closer to 29 where 29 is strong (rolling resistance, contact patch).

It turns out that they actually do have a point.

 

Firstly, about the wheel sizes: 26, 650b and 29er are 559mm, 584mm and 622mm in diameter. Then add on 2 inches of tyre thickness (50.8mm) gives 26.01, 26.99 and 28.59 inches respectively. So 650b is just under 40% of the way between 26 and 29.

Really 27.5 and 29er should be called 27 and 28.5er respectively because that's what size they are, but that's just an aside that would just confuse things further.

 

Anyway, to get back to the Giant's claims.

Weight and stiffness:

I don't think that anyone would argue that 650b is closer to 26 than 29er in weight, and also in wheel stiffness for the same metal thickness in the spoke and rim. Weight in particular scales linearly with wheel size. I would argue that stiffness is an inverse square relationship, like strength of a magnetic field, so 650b would be much closer to 26 than 29er. I can motivate that but not now.

As for frame stiffness, that's hard to quantify other than experimentally, a 29er's is going to need a shorter headtube to keep the bars at the right height. That's probably going to make it less stiff. As for the bottom bracket I can't see what difference wheel size would make really. The only thing I can think of is this coming down to the longer chainstays on the 29er.

 

Rolling resistance:

Giant seem to have botched their calcs here.

Using simple high school trigonometry, a 26, 650b and 29er would hit a 6cm obstacle at an angle of 35.08, 34.41 and 33.47 degrees respectively. So none of the x - 4 and x - 6 degrees that they suggest, but rather x - 0.66 and x - 0.94. But the numbers do speak in their favour. E.g. for an obstacle of 12cm, the 26 will hit at an angle of x = 50.45 , a 650b at x - 1 degrees and a 29er at x - 1.4 degrees. Whatever size obstacle you choose, it does seem to back up their claims.

I haven't taken into account the effect of tyre deformation here, but I don't think this will change the outcome much.

 

Contact patch:

This is much harder to theoretically analyse, but they may still have a point.

On first thought, if you double the size of a wheel is that you double the contact patch. This is not the case. The size of the contact patch is determined by wheelsize, tyre volume, tyre pressure, and load (rider + bike weight). So to double the contact patch you'd actually need to double the wheelsize, quadruple the volume, double the load and keep the pressure the same.

By going to the bigger size wheels, you scale the volume linearly and keep the load the same. This means that the increasing the wheelsize give diminishing marginal returns in terms of contact patch size. This may well result in the gain in contact patch size got from going from 26 to 650b may be larger than that going from 65b to 29er.

 

Whether or not this "technical analysis" translates into a better, faster, more fun ride, we're back in the realm of the subjective personal experience. Sorry for the ramblings!

Edited August 16, 2013 by beanz