Changing rim/tire widths & pressure based on system weight

Something I’ve never seen discussed is how changing system weight on a road bike (rider + clothing + bike) would change recommendations for tire width. I can’t imagine that a junior racer and a Clydesdale would have the same rim/tire/pressure combo for optimal rolling resistance and aerodynamics.

I’m personally a fairly average height & weight so a lot of the normal recommendations work out well for me. This is more of an academic interest than anything I’ll put into practice, but I’m curious if there are any more-knowledgeable folks out there!

The online tyre pressure calculators I’ve seen take system weight into consideration:

https://axs.sram.com/guides/tire/pressure

Thanks! I have seen those calculators, but no discussion of whether a given rider (based on system weight) would be best served by a 23/25/28/32 tire. Maybe the answer is that it’s too marginal to matter? Just seems like you could get the same vibration-smoothing effect with a smaller air volume for a smaller rider.

1 Like

When it comes to hookless rims, the 5 bar/73psi limit means that larger road riders would have to use at least a 28mm tyre according to the calculator.

It wouldn’t… tire width is a matter of ride preference for things like grip and suspension whereas pressure is a function of system weight. A 32mm tire contacts the road like a 32mm tire whether the person riding it is a 50kg junior or a 100kg big boy. Generally speaking, larger riders have been told to ride larger tires and rims because there wasn’t much wiggle room in pumping up a slender 21-23mm tire on a 15mm rim so going to a bigger tire facilitated the volume of air necessary to support a larger rider while staying within max pressure ranges but with modern wider rims and tires it isn’t necessary anymore.

Assuming you’re not running a hookless, tubeless set up, then you’d adjust the pressure of different widths to equivalent sag as weight demands. That might be 70psi at 25mm if you’re fairly light, or it could be 100psi if you’re heavier.

Now if you want to run wider tyres (for comfort and grip) and pay some heed to aero considerations (the rule of 105 etc) you’re forced into running a wider rim. The calculators from Zipp and Silca take that into consideration.

It might be worth having a play around on the websites and seeing what numbers get generated; you might be able to work out what the algorithm is, with enough work!

More generally, if you’re not racing/TTing and your frame will take them, I don’t see why you’d run less than 28s, at least on crappy UK roads. If you live in Switzerland, I can see there could be a different argument. Personally, I will be running an experiment with Rene Herse 35mm later in the year when there’s a bit less debris on the roads.

This was the exact question I hoped to have answered in my earlier thread about considerations for skinny cyclists (spoiler alert- it wasn’t answered): Considerations for skinny cyclists

They say wider tires increase efficiency and comfort, but surely there’s an optimal maximum (we’re not riding bikes fitted with truck tires). And it makes sense that this optimal maximum would be lower for lighter riders.

(But I have no insight into what it would be. I’m going to try going from 28 to 30 on just my rear tire this year, assuming 30s ever come back in stock.)

Aerodynamics does not care at all how much you weigh.

Exactly. If aerodynamics were the only concern, you’d go with the skinniest possible tire. But obviously grip and cushioning matter, which is why we’re moving to wider tires. But those factors are affected by weight.

Those factors aren’t impacted by the system weight, they’re impacted by pressure which needs to be normalized for system weight… Grip is limited and a heavier rider may meet the limit more quickly than a lighter rider but being lighter doesn’t make your tires grip more, a rider at ‘x’ percentage of tire sag gets the same comfort and grip irrespective if that’s a 50kg rider at 40psi or a 100kg rider at 80psi.

This “optimal maximum” has nothing to do with rider weight, it’s a matter of the balance between personal preference for comfort vs aerodynamics. If you’re a casual rider who averages 15mph and doesn’t care about aero, then 32mm tires meet your “optimal maximum” better than 25mm irrespective of whether you’re 50kg or 100kg… if you’re a racer who averages 25mph and doesn’t prioritize comfort than your “optimal maximum” is going to push you to 25mm tires instead of 32mm tires.

All the pressure calculators seem to be built from a base of “you have these wheels and these tires, what’s the optimum pressure”. I’m thinking of a ground-up determination. There are upper limits to where tire width adversely impacts aerodynamics & eventually rolling resistance (holding all else equal) - that point must change based on system weight, right?

Based on comments, it seems like such a determination could come from a sensitivity analysis across multiple dimensions: input system weight, surface type, preferences for comfort, aerodynamics and weight; output rim width, tire size and pressure. Bounds would be set by min/max pressure limits as mentioned above. There may be multiple optimum points within that space - a junior rider may be able to optimize with a 20-25mm tire; a lighter adult with 25-28; a heavier adult with 28-30mm (all else equal).

1 Like

But (and I think other posters have pointed this out) the aero impact is what it is. It doesn’t shift with rider/system weight - so e.g. if you’re spending all your time in a race at 45+km/h you aren’t riding 30mm no matter your weight or comfort desires.

I think this is absolutely right on good roads and at pro speeds, though I’d be interested if this is/would still be the case at something like Paris-Roubaix. My gut tells me that 28s would be the weapon of choice here, as (generally) that’s about as wide as it’s possible to go without (at pro speeds) taking a big aero hit.

That said, I do wonder if a fast amateur (say average speed 35kph, riding solo over undulating terrain) would gain or lose speed on crappy roads if they ran 30s.

I inadvertently had a very unscientific, n=1, case in point this weekend. During a timed, semi competitive sportive, I got dropped by a rider on a long drag. I regained his wheel on the fast, bumpy, poorly surfaced descent that followed. I was running 28s, tubeless and hookless, at pressures in the 50s. I didn’t get a chance to look at his setup closely, but at a glance his tyres appeared knife-thin, and he was bouncing all over the road down this hill. Of course, there could have been lots more variables at play here, but it’s an interesting question.

Yeah I’m more interested in amateur speeds. Oh hey, there’s another variable to throw into the sensitivity analysis…

Pros are also much more closely grouped in weight too. I’m not 100% sold on the “pros ride what’s fastest” in this regard anyway given how many pros clung to 110psi 23mm tires long after they were proven to be slower. Maybe it’s a better picture in today’s marginal gains environment.

The aero benefit / cost remains a constant, but we all make some amount of aero tradeoffs for net speed (or comfort) gains given expected riding conditions. I’m just not sure if a 100lb system weight would warrant the same tradeoffs as a 200lb system weight (ie can you just adjust pressure to optimize those two riders, or do you need to change rim width / tire size).

In the end I’m not going to act on this, I’m a set-it-and-forget it rider who’s a big fan of underbiking - just pure curiosity here.

You bring up a good point regarding bumpy surfaces. While I’ve seen some testing that shows that lower tire pressures reduce rolling resistance on rough surfaces, a narrower tire will require more caution on rough surfaces and can be slower regardless of the actual resistance. I know that there are roads I can take at higher speeds on 32 mm tires than on 23-25 mm tires. As a 90+ kg rider, the fatter tires can absorb much more without feeling like something is going to break.

I am curious as to where the aero losses due to a wide tire will outweigh the benefits. I suspect you need rather high speeds on smooth roads, since going from 25 to 30 mm only adds 20% to your wheel drag (assuming rim profile is increased to preserve the ideal shape), and the wheels are a small part of the total drag.

That said, I do wonder if a fast amateur (say average speed 35kph, riding solo over undulating terrain) would gain or lose speed on crappy roads if they ran 30s.

Hmm, I think 28s versus 30s here probably comes down to the tires, and if tires are kept equal to how rough the road surface is?

Having raced at some weights that fit…

Junior weight 56-58kg - definitely can have a magic carpet, Cadillac ride with nice 23s or 22 (tubular)

Senior weight 60-63kg - can have a good ride with nice 23s or tubulars. Not magic carpet but definitely good

Commuting weight add maybe 13kg so call it equivalent to a 76kg rider weight (bike adds 10-15lbs depending on bike + another approx. 12-13 lb for assorted stuff like knapsack, laptop, folders, papers, basket, rack, fenders…) Definitely do not want to be on 23s!

I think lighter does allow someone to ride narrower for aero or whatever purposes and still be comfortable.

1 Like