Road Racing: Different Tire Sizes in Front and Back

For road races (including crits), I ride a 2014 BMC Tachmachine SLR02. I love the frame, it’s snappy, it’s light, the bike climbs well, I like everything about it. Except this: the tire clearance in the back is so small that I can only run 25mm tires in very dry and clean conditions. (I actually raced a crit last year where the water from the road froze between the tire and the frame and was rubbing the entire race).

What I ended up doing was to run 25mm in the front and 23mm in the back. My thinking (based on pretty much nothing) was that running a slightly wider tire in the front with slightly less tire pressure would be beneficial in cornering even if the tire in the rear is thinner and harder. But I wonder if any of you have more insights into this? Does this have any disadvantages?

And no, I’m not buying a new bike right now. :slight_smile:

1 Like

That’s how BMX tire sizing is often done. Even some MTB riders like that for the better grip up front too. It’s a fine thing to try.

1 Like

I used to do that on my MTB, used to get “bigger tyre should be on the back” comments all the time, but my logic has always been, race cars need bigger tyres on the rear to put down power, not a issue for cyclists, who need more grip on the front

2 Likes

Right. I got that too once or twice. But obviously for me it’s not about “what tyre asymmetry should I go for” but “given that my rear tire clearance limits me to 23mm, will I get any benefit from running a 25mm up front.” Subjectively, I’d say that during crits I liked the handling better but that obviously doesn’t mean that it actually is better.

I had heard of MTB riders doing this, which is part of why I decided to. I wonder if there are other roadies out there (with the same constraints) who have done this.

I supposed some of it depends if you have wide rims, could be a little aero benefit, comfort wise I would have thought the rear would take most of this … handling wise, it won’t make it worse, and you will have a slight rolling resistance decrease

I have to confess that I am thinking about doing the same now, my aero’d came with 23/25 (front/rear) and I put the 25’s that were on my Madone onto my TCR, but it’s way to close and got away with it last year, so I had some 23’s delivered this morning, tempted to only change the rear now

1 Like

So I have narrow rims (as they’re the rims that came with the bike clearly designed for narrow tires). So there’s probably an aero cost. Handling-wise I supposed that when cornering in crits, the rear wheel is by far the most stable (I can take hits on the rear wheel and if it slips a bit, it doesn’t usually hurt) while the front wheel is the most vulnerable so if there’s a stability advantage to a wider tyre it should be on the front. But I might be wrong.

I have the same problem: rear tyre clearance. Running two different sizes makes no difference. Just run two sizes.

My Omega Alchemy Titanium/Carbon framed road/race bike from 2002/3 (no I am not buying a new bike either) has such a narrow rear tyre clearance between the tyre and the front mech band that I can only run 23mm on the back. Newer wider rim wheels also tend to exaggerate the problem.

Basically I run the same pressure front and back. I use Schwalbe Ones and they are fine at around 100 psi, either 25 or 23. Basically I would not worry. Just pick a tyre pressure you are comfortable with and go with it. (and ignore those clarion calls for a new shiny bike/frame…)

1 Like

Do you think there’s a disadvantage in running different pressures? I usually run them at 80/100 front/back.

Never tried it. Sounds like a large difference. I take the view that my weight is pretty evenly distributed front back. So the only reason is tyre size. Is 25 really that much different from 23? If you were saying 28/23 I might suspect the answer would be yes. I suspect if i was running only 80 in the front I would feel it was a bit flat. (But that is probably just me).

On the Schwalbe site the recommended pressure RANGE for 23 and 25 are slightly different in that the top end pressure is lower with the 25. https://www.schwalbe.com/en/road-reader/schwalbe-one.html . When they get to 28, the upper pressure is again lower. But in each case the lower (minimum) pressure is 85.

I have read that a wider tyres means a lower pressure. Not sure how much though.

I would do what i have just done. Look at the tyre manufacturer’s site for recommendations. Even drop then an email if unsure.

1 Like

So mininums for the Michelin Pro4 Service Course are 73 (25) and 87 (23). I guess I could run the rear at 90, but even then I probably would run the front at 80.

But yeah, I might email them.

I would ask, why run at the minimum? If you lose any pressure during a race (or even removing the pump) you might end up below it. …and then you are more prone to pinch flats if you hit something. Personally I would look for a “recommended” rather than “minimum”… and experiment near that.

…and I think you are sensible to email them…

(and frankly a watt or two of rolling resistance, in my book is minor compared with my body shape, of the effort required to stay on a wheel as the others up the gas. I do TTs and I suspect my body shape is more of a problem than a watt or two or tyre pressure - and I am running tubs at 140psi for that.)

I would carry on putting a 25mm on the front and 23mm on the back if that feels fine for you… no issues with that.

One other thing you could try is a wider rimmed rear wheel and see what that does to your clearance at the back? It could be that the tyre flattens enough to give you a bit more room?

1 Like

3 Likes

Yeah, I’ve considered that, though a big wheel upgrade isn’t really in the budget. I’ll just keep doing as I do, I suppose. :slight_smile: