Tubeless Road Safety and Relative Descending and Cornering

Do bigger riders have an advantage on downhills and/or corners?

Dear Lennard,

I told you about the ole speed whumptiddy whumps at 55mph that nearly ended me. I put the road machine on the rollers and this Silca sealant had been sitting for a month. Unbearable whumping.

On the road it’s not as noticeable until you get to higher speeds. I wholeheartedly don’t believe in tubeless, and this is the reason: these clumps.

When I was coming down Highway 285 from Deer Creek taking corners at 50+ with bad road conditions, my handlebars started vibrating my grip loose. That was scary. Never again.

Caleb

Dear Caleb,

Yikes. That is a big clump. No wonder it threw off the balance of your wheel!

Your experience at a minimum makes the case for clearing the clumps of solidified sealant out of road tubeless tires after any time that they go unridden for a few weeks or more.

― Lennard

More on safety of tubeless road tires:

Dear Lennard,

Which rim material is safer for tubeless road tires?

I truly don't know.

Carbon is probably stronger and better in every way (assuming no rim brakes) up until the point when a carbon rim takes a sharp hit that induces shattering.

At that point it would have been better to be on aluminum.

An aluminum rim taking a catastrophic hit will deform permanently (bend, etc.) but will stay intact. When carbon takes a hit like that it will nearly explode, I believe.

So I guess my answer is that for catastrophic hits, aluminum is safer. The rim will be ruined but better that than an exploding rim that can result in a horrible and potentially deadly crash.

Alan

Dear Alan,

You answered your own question. I think my answer would be the same.

― Lennard

Dear Lennard,

Do small riders corner better than big guys? Does their center of gravity lie higher/lower closer in or further out from the wheels/frame?
At a given cornering speed, does body weight of a heavier rider make the tires traction grippier than a smaller lighter rider?
Do bigger riders descend faster than small riders?
Paul

Dear Paul,

I’ll answer the last question first. If you are talking about just coasting without braking and not on a curvy road where a rider’s skill comes into play, then yes, bigger riders descend faster than small riders.

You can see this by getting on a tandem. If you roll down the same hill under the same conditions on your single bike as on a tandem along with another rider aboard, you will see how much faster you pick up speed on the tandem. Anyone who has ridden down steep hills on a tandem and wondered if their brakes will be able to slow them down can identify with this.

On the face of it, this phenomenon seems to agree with Aristotle’s disproved theory of gravity that held that objects fall at a rate proportional to their mass and to disagree with Galileo’s result of two stones of unequal mass dropped from the Leaning Tower of Pisa landing simultaneously. Rather, it demonstrates that the wind resistance of a heavier rider relative to a lighter rider will not be higher by the same proportion as their relative masses.

In other words, the force due to gravity on the heavier rider will be higher than the gravitational force on the smaller rider by the ratio of their masses. However, the resistive force on the heavier rider due aerodynamic drag will not be higher than the force of air resistance on the lighter rider by as big a ratio.

There are many ways to explain pieces of this phenomenon. One is that the aerodynamic drag on their bicycles will be close to the same, assuming similar components, since the drag on the wheels, fork, drivetrain, etc. will be independent of rider weight. So, the increased propulsive force due to gravity on the heavier rider and their bike relative to the other rider and their bike will not be balanced by an equal increase in resistive force on the heavier rider simply because the wind drag on their bikes will not be significantly different.

Also, the aerodynamic drag on their bodies will differ approximately by the ratio of their frontal areas, and the frontal area of a heavier rider will not tend to be as much greater than the frontal area of a lighter rider as the difference in their masses. Their masses are proportional to their volumes, whereas their aerodynamic drags are proportional to their frontal area. If their bodies were approximated by a simple geometrical shape like a cube, their mass, and hence the force on them due to gravity, would be proportional to the cube of the length of one of their sides, whereas the force of the air pushing back on them would be proportional only to the square of one of their sides.

Next, I’ll answer about the grip of their tires. Assuming identical tires and tire pressures, the heavier rider’s tires would have more grip than the lighter rider’s tires. However, the force trying to break that frictional grip on the road will also be higher for the heavier rider. Both of these forces will be proportional to their mass, so this will be a wash.

Now for your first question, “Do small riders corner better than big guys?” This depends so much on rider skill that it cannot be answered, IMHO. We have seen in Grand Tour stages small, light riders like Tom Pidcock go much faster on technical descents than the pursuing riders, just as we have seen big, heavy riders like Wout Van Aert do the same. We have also seen the same thing with these same two riders on technical cyclocross courses. Rider cornering skill is obviously more important here than rider size.

Finally, to, “Does their center of gravity lie higher/lower closer in or further out from the wheels/frame?”, the answer is maybe. Their center of gravity depends on weight distribution. Two riders of equal height and weight may have different heights of their center of gravity (COG) off the ground if one has a big butt and heavy legs and the other has more of their weight in their shoulders and arms. If a heavier rider is also taller than a lighter rider and their weight is distributed similarly over their bodies, then the heavier rider’s center of gravity will indeed be higher than the lighter rider’s COG.

― Lennard

Send brief technical questions to Lennard at: veloqna@comcast.net.

As a frame builder, Lennard Zinn has been designing and building custom bicycles for over 42 years; he founded Zinn Cycles in 1982 and co-founded Clydesdale Bicycles in 2017. His Tech Q&A column on Substack follows his 35-year stint as a technical writer for VeloNews (from 1987 through 2022). He is a former U.S. National Cycling Team member and author of many bicycle books including Zinn and the Art of Mountain Bike Maintenance, Zinn and the Art of Road Bike Maintenance, and The Haywire Heart. He holds a bachelor’s degree in physics from Colorado College.

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