7/30/14 10:13 PM

I am thinking this may be the real place possibly for 650B.


Not to mention more tubeless application. The high pressure for road tubeless make no sense to me.

8/2/14 4:00 AM

I've done a fair bit of unsealed road riding on my 650b machines. The fat tyres (I use 38mm and 42mm) certainly help to smooth out the uneven surfaces, and, except on very loose surfaces, the minimal tread on them copes ok. I haven't tried tubeless with them, though I do have A23 650b rims, which Velocity claim as "tubeless ready". I don't think there are yet any tubeless 650b road tyres, as opposed to MTB rubber. Some people on the 650b mailing last have reported success with running non-tubeless tyres such as the 42mm Hetre as tubeless, other have reported problems with the sealant migrating through the thin casings and causing the tread to start delaminating from the casing.

8/2/14 7:46 AM

With all of the 'cross, gravel-specific and 29er tires available in 700c, what possible advantage would 650b have over 700c on a gravel grinder? Just as with a 29er MTB, the larger wheel rolls over rough surfaces better and provides better traction. I don't think that there are any 650b 'cross tires available, so you'd be stuck with heavy MTB rubber. If you're tall enough to ride a 700c road bike, I don't see where 650b offers any advantage whatsoever.

As for tubeless, on a 700c gravel bike, you would only need to run 30-50 psi, depending on the tire size, your weight and the road surface. MTBs - including 29ers - have been doing that for years, so it's obviously not a problem.

I don't have any problem with road tubeless as a concept, but I'm not sure that it's fully ready for prime-time. I plan to give it a bit more time before diving in. I don't need any new wheels at the moment, anyway.

8/2/14 8:18 AM

"what possible advantage would 650b have over 700c on a gravel grinder?"

Higher volume solution of size/fit challenged 700C frame/forks not of the 29er clan?

8/2/14 1:38 PM

I think not. Most folks don't want to deal with yet another tube, tire, rim size. Things have gotten out of hand in some ways. Rim sizes is one of things that is silly, no advantage to anyone but the makers.

650b to me is a fancy name for Walmart size wheels.

8/2/14 6:00 PM

As a thought experiment, I see surprisingly little wrong with all adult bicycle wheels becoming 650b (584mm rim diameter), with 135mm rear spacing and disc brakes.

On the road side of the wheel-size debate, the slightly-reduced torque at the hub from braking could only help with disc-brake fork flex and weight concerns.
Also, since good, modern aero rims seem to need added width for aero reasons, the appropriately-fatter tires would make up for the minor contact-patch reduction of the slightly smaller-diameter tires while making the wheels yet stronger, lighter and more-aero.
As a way to remove the 1-lb weight penalty that disc brakes seem to be imposing, this seems like a way to even things up, even as racers would then need perhaps 3 extra teeth on each chainring after taking into account a slightly taller tire cross-section. Masters riders could kick the can down the road for 10 more years before needing that compact crank!
And, since wider tires take better to tubeless air loss concerns, tubeless practicality could only be improved by the switch to 650b wheels.
Plus, wider road tires would be no heavier, yet would be more resistant to tracking any longitudinal pavement "step" or "groove" defects.

So, with 650b wheels freely interchangeable between all adult-size mtb, cross and road bikes, riders would be better able to tailor their bikes by more-freely interchanging various wheels and tires, with only tire width concerns getting in the way.

Lastly, with fewer rim and tire standards likely allowing dealers to hold smaller inventory levels, parts costs relating to wheel/tire items would likely be reduced at the retail level.

8/3/14 7:35 PM

All three types of bikes are currently available with disk brakes and 135mm rear ends, so if it make so much sense, just do it. I like the idea personally, but not enough to spend the money required.

There is one error in your thought experiment; contact patch size does not vary with wheel diameter. It is solely a function of load and tire pressure. What does change is the contact patch shape . Smaller diameter tires have a shorter, wider contact patch than larger diameter tires, given the same tire pressure and load. It's this shape that gives larger diameters a traction and rolling resistance advantage. The lower angle of contact of the larger diameter provides an advantage when rolling over obstacles.

8/4/14 1:45 AM

If we're balancing a ~5% decrease in rolling diameter with a commensurate increase in tire width, won't the traction gains/losses be a wash?

Of course you're right about contact patch size vs. pressure, and the wider tire is also a bit taller up from the rim, so the lower threshold of pinch-flat-resisting air pressure might be a bit lower?

As far as rolling resistance goes, might the increase in longitudinal contact angle at the front/rear edges of the contact patch be offset by the wider tire's lower contact angle along the sides of the contact patch?

And would the aero advantage of smaller wheel + shorter fork be greater or less than the rolling advantage of larger wheel + longer fork?
At what speed conditions might these two advantages reverse?

Just thinking here, but might the smaller-wheeled bike also be less affected by crosswinds for an equivalent degree of aerodynamic drag?

It sure would be cool if this smaller road-wheel size could offset both the weight and aero penalties of good hydraulic disc brakes and increased tire clearances.

And it suddenly occurs to me that virtually all of my road, CX and MTB racing over the last 17+ years has been on 700c wheels (and on the same bike for that matter).

8/4/14 5:46 AM

The longer, narrower contact patch increases traction, but since it results in less tire deflection,it also decreases rolling resistance. There are numerous tests that have confirmed this, if you feel inclined to do some Web surfing for them.

Aero advantages would be minimal considering that the the biggest source of drag - the rider - is still going to be in the same position and the same height above the ground, regardless of the wheel size. The rider is positioned in relation to the bottom bracket, not the wheels.

The fork will be shorter, but the head tube will be longer, so there's not much gained there.

The smaller wheel also spins faster, which means more rolling friction, hub friction (admittedly, this is minuscule) and aero drag at the top of the wheel.

Frankly, I just don't see 650b making inroads beyond MTB use. We already have 650c as a smaller road standard with reasonable manufacturer support and it would work fine for CX as well, if there were CX tires available (there aren't any that I know of).

8/4/14 10:07 AM

On balance, maybe not better, but again, there are several other advantages, as I mentioned, other than those you addressed.

And as I suggested, the inclusion of disc brakes further affects the overall balance of relative wheel size, and whether a different wheel size might be optimal.

Given that a great deal of re-design, (including fork, frame and wheel hub standards) is occurring, this might be the only practical time to re-examine optimal road bike wheel size (unless it can perhaps grow legs later as a marketing drive aimed at flat road-bike sales).

It does seem like 700c has the momentum however, and "optimal" and "practical from a marketing standpoint" are of course two different things.

Not saying I'd want to see this happen, as I think hydraulic rim brakes have yet to show their stuff in 700c format, and that carbon-rim braking can only improve from it's current level.