Wider tyres and the rule of 105

Ok, so this comes from a bit of an accident.

On the way back from a gravel ride, I was riding the really terrible section of road close to my house. I hardly noticed the rutted surface, and felt much more confident on the sketchy corner before the roundabout. Of course, I was on my 40c gravel tyres.

This got me thinking. I know many people have been banging on for a long time that wider tyres are more comfy, grippier, and no slower, and I’m tempted to try some 35s on my all road bike.

But I’ve worked pretty hard for what speed I have, and have paid good money for my 45mm deep carbon wheels (which the all-road bike wears in its road guise). So thinking of the famous rule of 105, how much speed would I lose (on road) from switching from my current 28s to something much wider?

I’ve seen this question asked elsewhere but not seen much of a consensus.

FWIW, this would be on road only.

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It’s pretty impossible to quantify how much speed you’ll lose without listing out a whole array of variables and doing a bunch of math but, the short answer is it really depends upon how fast you’re riding. If you’re averaging < 20mph, you really won’t lose much. If you’re averaging > 25mph then you’re going to be giving up a noticeable amount of speed. For most riders, going from a 28 to a 35 is going to feel like you have to work a bit more to stay on top of your gears but ultimately not be a huge sacrifice.

There’s also something to be said for a middle ground, going to a 32 will give you a tire that’s a bit less than 15% wider rather than the 25% wider 35 but strikes a pretty happy balance between width and aerodynamic drag. I run 30’s on 25mm/32mm wide rims so they plump out to about a true 32 and they don’t feel any slower than running a 28 on the same wheels.

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If you’re running 28s that’s already probably too wide for most 45mm aero rims out there so 32s ​wouldn’t be much of a loss, at all.

I have two 45mm deep wheelsets and GP5000 TL in 25mm are still a hair too wide for optimal efficiency. Fortunately new GP5000s tires are narrower (I heard by 1.5mm which makes them near current 23s) and I believe this is no accident. New standard for TT and Tri, certainly - will be racing on those next year.

Thanks for this. I’d love to be averaging >25mph on the all-road bike; that’s a hard effort on the TT bike!

Somewhere between 18-21 mph, depending on solo vs group, terrain, and distance would cover 90% of my standard road riding.

I’m not sure what calculations you’re trying to do that makes you think rim depth has any impact on tire width but, it doesn’t… the rule of 105 is an aerodynamic principle, I believe from Zipp, that states the width of the rim at its widest point to recapture the airflow once it’s been disrupted by the tire. So, a 28mm tire would need a rim with an external width of 29.4mm, irrespective of how deep that rim is. In my case, I’m running 30mm wide tires on 32mm wide, 45mm deep rims… with an internal width of 25mm, my 30mm tires plump up a bit to just shy of 32mm which puts me short of a the 105% mark… 28mm tires on those same rims, plump up to about 30mm which comes in at about 107%

In that 18-20mph range, you’re right at the point where aero really matters but it doesn’t matter enough for the fractional aero gains on top of aero equipment to be a huge factor. So, you’ll get good aero gains from your 45mm deep wheels but you’re not going to notice a huge swing one way or the other adhering to the rule of 105. That said, the frontal area difference of going from 28-35 would still be pretty significant so I’d suggest trying the intermediate approach with something like a 32 first.

My point is that most 45mm rims on sale right now aren’t very wide; the latest generation Mavic SLR45 I have are 28mm at widest point, the LB R45 wheelset is narrower still, only 27.5 at widest point. However, both are narrower than that at the rim-wheel interface, though, and the tire bulging outward can’t be ideal even if “rule of 105” is nominally followed. I have no doubt that the new, narrower GP5000s will test faster.

The relationship between width and depth is certainly going to make a difference, it just can’t be distilled into something as simple as “105% width”. Increasing width more and more for a given rim depth gives you something increasingly resembling a round tube, a shape known to be aerodynamically horrible.

As for aerodynamics, doing 200W solo - a target power for many triathletes in a long event, it matters - if it’s hilly the average speed might make it seem like it doesn’t matter that much, but that 30-33 km/hr average is composed of periods of climbing at 15-20 km/hr where it just doesn’t matter, a fair deal of riding on the flat at 35 km/hr where it does, and descending at 50+ km/hr which is where even the details matter.

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I would probably be more concerned about the effect a wider (than rim) tire has on the handling of the front wheel in crosswinds. What about going wider on the rear wheel only, where air is pretty turbulent anyway?

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There are a few aero wheelsets out there designed explicitly for wider tires. Hunt Limitless (34mm external) and the 3T Discus (40 mm external) spring to mind.

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I can say that, as someone who has a setup that roughly obeys the rule of 105, it might be faster but the juice may not be worth the squeeze.

  • wider rim = more cornering stiffness. Too much cornering stiffness will destroy the handling of the tire. I can’t use a 28mm panaracer race d evo4 on my 23i rims. It ends up squeaking in corners, which is horrifying

  • wider rim = wider contact patch. Sounds good right? But what if your tread is too narrow? Corsa speeds, hutchinsons, the first gen pirellis etc all have this problem. Turn down the pressure and try to lean, and you’re gonna be hitting sidewall. It feels like the tire is screaming at you.

  • exposed rim sidewalls means chipping. I had a friend who chipped his rims like a week into owning them and decided he wanted to replace them. Wonder if anyone with the new Rapide wheels or the KNOT wheels can comment on whether more expensive wheels are less prone to this. If you live in an area with gravel strewn across the road, maybe not great

  • straighter sidewalls means more sidewall tears. Again, mainly an issue if you’re hitting gravel.

So those are the cons. Also, the pros may not be as big as people think. I believe wind tunnel testing shows that a deeper rim is still pretty darn fast even when you go with a crazy wide tire. The rule of 105 might matter more for stability than outright speed (even if the two are closely related). I can say that despite being a light rider using 56mm rims, I don’t get blown around much at all. Perhaps that wouldn’t be true if I were using 32mm tires that blow up to 34.

That being said, tires are relatively cheap. I would say just try replacing the front tire and see if you notice a difference on a straight descent. If you can subjectively feel any difference at all, then feel free to chase those aero gains. But you might like the feel of a wider tire so much that you’re willing to give up that aero.

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The confidence on the different road could just be down just to the amount of pressure in your tyre. I bet on your gravel bike you have probably close to half the pressure compared to your road bike?

I let my tyres down once to see what it was like riding 70psi instead of my usual 110psi and my bike felt much nicer in corners when turning more confidence inspiring but I didn’t keep it because I don’t like the sponge feel. You could lower your road bike 20 psi and get a completely different feel you don’t necessarily need a wider tyre.

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I recently read an article on this topic. Here it is: Schwalbe Marathon 32, 37, 40, 47 Comparison (bicyclerollingresistance.com)

Should give you a very good idea on how much slower it makes you. If you want to convert the watts to speed, use this calculator: Bicycle Speed (Velocity) And Power Calculator (kreuzotter.de)

Hope this helps.

A test which compares road tires, and doesn’t make the egregious error of not adjusting pressure;

Basically, no real RR difference on a drum test when pressure is adjusted to offer similar tire drop. On a road test where impendance losses also occur, it is not entirely clear if larger sizes have a rolling resistance advantage or no, possible that they do.

However, the question is about aerodynamic implications of wider tires, especially paired with rims which aren’t 105% as wide or more. There there is much less data. Silca blog has some interesting measurements on a pair of old ZIPPs where 25mm goes over the limit when inflated fully - the wheel goes to behaving almost as badly as a much shallower one.

Something to consider for timetrials and triathlon at least.

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yup, that’s how I look at it and accordingly run a narrower tire on the front. faster front wheel as well as being less grabby in the wind.
for road: 26F/28R

I think more important than this data is how often we’re seeing clean air hit the front (this barely matters if the air is already turbulent) and how often we’re seeing large yaw angles. I totally believe that you can measure the difference in speed, even on the open road, between a good tire fit and a bad one. But in order to make an intelligent decision about the (very real) tradeoffs associated with narrow tires on wide rims, we need more data.

That being said, yes, for TTs and triathlon, there is very little debate. Deep rims, TT bars, very little cornering and constantly being exposed to wind means if the the rule of 105 is worth following anywhere, it’s worth following here.

It’s also worth mentioning that tubular setups have been following this “rule” for a long time now. For some reason, until recently, clincher aero wheels had the same profiles as tubular wheels and thus were optimized for perfect 25mm tires and had 26mm outer widths. But then people would put “25c” tires that measured closer to 27mm on them, ruining the aero. For some reason nobody making these wheels thought to clarify that they were talking about measured width, not labeled width. Probably because nobody wanted to come out and say “you need to use absurdly narrow tires for our rims to behave optimally”.

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This is interesting. There seems universal agreement that you will lose speed by violating the rule of 105. That would lead one to speculate that the data for rim/tyre interfaces of less than 105% must exist, yet it seems very difficult to find.

I’m not at all sufficiently fast to worry about a few watts here or there, but there is definitely part of me that is quite irritated by not being able to find out an answer (which I’m sure exists).

The discussion surrounding rolling resistance lacks a bit of context when done on a drum. Sure, the tire itself can be compared, but is this the metric you should look at for choosing your proper tire pressure or tire size?
Taking a laboratory setup like this neglects a lot of factors when actually riding your bike. Foremost damping losses of vibrations that get transferred to the rider and are slowing you down and add to fatigue over time.
This is described here:

Sure, adventure cycling is not foremost about racing. But the power figures in comparison on smooth tarmac raise a few questions if it wouldn’t be more efficient for the whole bike including cyclist when going for smoothest ride. I remember Josh Poertner of Silca saying something in line with this on some podcast regarding the best tire pressure - if only I could remember the podcast…

Obviously on real road 120 psi in 25mm tires isn’t optimal as it is on a drum test, rather depending on inflated size for 25s it is something in the region of 85-90 psi on good tarmac because it offers the lowest combination of impedance losses and rolling resistance losses.

The unsolved question is whether larger tires on the road, at their optimum pressure, which is necessarily lower for wider tires, offer lower rolling resistance or not on the road. Drum test suggests that at a similar comfort level, wider or narrower tires have the same rolling resistance, bit it is possible that the scaling of 15% tire drop isn’t accurate.

That’s totally unrelated to air drag, where what scarce data there is suggests that going as wide or wider than the rim significantly degrades the performance od aero rims. For racing disciplines where you spend most of power pushing the wind out of the way solo, this makes a far greater difference than any feasible rolling resistance difference could.

Smoothness wise, top shelf narrow road tires do it for me on the road, as long as they aren’t overinflated. Of course offroad or on cobbles, wider is going to be better and faster, you couldn’t possibly inflate a 25c or even more 28c tire low enough to be smooth there. However, 99% of the time on my road bike I’m on tarmac, most of it pretty decent quality.

That’s a point I think I misunderstood. I always thought that with wider tires you CAN have lower pressures…although you say it has to be that way.
Maybe you can help me understand better

A wider tire at the same pressure is a stiffer spring. You can try it, if you have a road bike with 25s, pump it up to, say, 75 psi. It is probably going to be pretty comfortable, if a bit under ideal pressure. Now, you take a bike with 35+mm tires and pump those to 75 psi. It’s going to deliver a much harsher ride over any road irregularities.

There’s the Silca pressure calculator derives from their road testing which agrees pretty well with what I find to work well (after I calibrated my pump using a digital gauge, anyway). For racing on worn pavement with a 26mm measured tire (that’s what inflated 25mm tires end up as) it suggests 88 psi is about optimal. For 32mm tires it suggests 63 psi as optimal.

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