Technical FAQ: Narrow tires on gravel bikes, through axles, rotor tub, clamping carbon, leg length with age

Published July 28, 2022 05:39AM

Have a question for Lennard? Please email us veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
Curious what your opinion is regarding narrow tires on a gravel bike. I recently purchased a Cervelo Aspero with the intention of running two wheel sets, one for gravel and one for more road-biased rides and for centuries. The bike shop was pretty adamant about not going narrower then 32mm because of “handling changes”. Will there really be that noticeable a difference or danger running say 28mm tires instead? For the record I’d like to split the difference and go 30mm tires for “road mode”.

Dear Mark,
Go ahead and put the 30mm (or 28mm) tires on. I would not worry about these feared handling changes.

You will be slightly lowering the bottom bracket, which improves stability at the expense of cornering clearance while pedaling.

The smaller tire diameter will slightly decrease the fork trail and decrease the wheel flop, explained here . This will make an unnoticeable difference in steering—less trail and less wheel flop means quicker steering and less stability. That said, dropping from 32mm tires to 28mm tires will reduce trail by less than a millimeter (0.8mm out of around 61mm, or around 1.5%) and wheel flop by 0.3mm (out of around 18mm, also around 1.5%). There will be a noticeable difference in the feel of the tires on the road; I would be very surprised if you could notice the handling difference.

If you have two sets of wheels, then you also won’t have to worry about the other possible caution about putting narrower tires on a gravel bike, namely the rim width. If you put on too narrow of a tire for the rim, you can reduce tire retention to the point of being dangerous. Narrower rims on your road wheels than on your gravel wheels eliminates this consideration.

I have three sets of wheels for my gravel bike—a road set, a gravel set, and a cyclocross set. The bike handles great with any of them. Don’t sweat trying different tire sizes on your gravel bike.
― Lennard

Dear Lennard,
I’ve been looking ahead to a future bike and reading about through axles. It seems to me that it’s just a big, beefy bolt that holds the fork tips together in a rigid frame sort of way.

Since it’s just a bolt, are there torque specifications? I haven’t seen any published, but I could have easily missed it.

Second, what keeps it from vibrating loose, particularly if not tightened properly? In critical non-bike uses, there may be a lock washer, or even a castellated nut and cotter. Is thread lock appropriate?

Dear Richard,
Yes, there are torque specs. For instance, search “Maxle” on this page , and you will find all sorts of different torque specs depending on the particular Maxle (RockShox through axle) and the particular fork. In general, most people are not going to have a torque wrench with them when fixing a flat on the road. You’re not reefing on the bolt like it’s a big crank bolt, and you’re not going easy on it like it’s a stem bolt or bottle-cage bolt. You’re tightening it pretty tightly—12-15 N-m.

I’ve never seen one loosen up. With the weight of the rider on the wheels, I have a hard time imagining the bolt being able to unscrew. If it were to unscrew enough that the axle ends are no longer tightly pinched between the dropouts (which never seems to happen in practice), the weight bearing down on the axle would prevent it from being able to advance its way out. It’s hard enough to push one in with no weight on it if the hub is not lined up perfectly! I see no reason for threadlock compound.
― Lennard

Dear Lennard,
When I switch wheelsets on my disc brake bikes, some minor intervention is typically required to eliminate rotor rub, despite using identical hubs and identical rotors. On some wheelsets I’ve used shims to minimize the differences, but also accept that some additional caliper repositioning is occasionally required for rub-free rotors. As I watched a wheel change from Shimano neutral support during the Tour today, I wondered if the professional mechanics and manufacturers have found a way to address this issue across all of the different wheelsets used by the various teams. Or do pros simply have to accept a little bit of rotor rub if they accept a neutral wheel?
Tim

Dear Tim,
Team mechanics at the team’s service course typically go through all of their wheels and do the minor truing required so that all of the team’s wheels center the same way in the same caliper. This is generally done with a dial indicator and bending the rotor with a slotted metal “tuning fork.” That way, whenever the team’s riders accept wheels from the team car, the rotors will not rub. Of course, when they accept wheels from neutral support, which happens a lot less at the Tour than, for instance, at Paris-Roubaix, the chance of having some rotor rub is high. There is no coordination between teams and neutral support to check every wheel for identical rotor alignment.

That said, you’ve also probably noticed that when a team’s GC rider or sprinter gets a flat, his team car will (a) tend to be with him and (b) tend to give him a replacement bike rather than change wheels. And a replacement bike, rather than a wheel change, is almost always the case for all riders in time trials.
― Lennard

Dear Lennard,
Some months ago, you recommended some type of tube to which you clamp the bike stand, rather than clamping it to the top tube of a carbon fiber bike. I looked through the Q&A on the website but could not locate your suggestion. Can you please provide me with the name of that product?

Love your column, even when I don’t understand all the physics and science!
Phillip

Dear Lennard,
The June 28, 2022 writer about shorter legs with age https://velo.outsideonline.com/gear/tech-wearables/technical-faq-shorter-legs-with-age-lighter-cranks-10s-11s-crank-compatibility-and-decreasing-drivetrain-wear-with-chain-waxing/ rang a bell for me. I’m now 83 and have been riding racing bikes since I was 14. In the last few years, I haven’t felt stable on my bike. I took this as a message that it was time to stop riding because I no longer felt secure on the bike. I could no longer put my toe on the ground while sitting on the saddle when I stopped. (I’ve never had knee problems.)  So I remeasured my leg length and compared it to my records of many years. To my amazement, I had lost a little over an inch!! I’ve since readjusted my saddle height and presently playing around with my bar height and reach. My comfort, security and joy of riding has returned. Michael

Dear Michael,
That’s a great discovery, to find out that you can keep riding joyfully 70 years into your cycling career!
― Lennard