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I received a new tire for my bike. I believe this is a "Hybrid bike" in English and looks more or less like that

Giant hybrid image courtesy of Wikimedia

I use it to commute, 50% on bike paths and 50% on easy forest tracks. I have a normal pace (say, 15 to 20 km/h) without any special acrobatic maneuvers.

The problem is that this tire, mounted on the front, is thinner than the one on the back (significantly, I would say it is at about 2/3 of the width, leaning towards 1/2). The size of the tires are:

  • front: 28x1.4 (ETRTO 37-622)
  • back: 622 (28x1.60-700x40C)

Sorry for the difference, one is from a photo of my tire, the other one is from the command details. I can have a look at the tires tomorrow if the measurements above are not sufficient.

Is this an issue? Specifically: can this be dangerous? As I mentioned, I drive carefully but some unexpected moves (hard break, sudden change of direction) are always possible and these are these I fear most.

I mostly know the consequences of having a narrow tire vs. a wide one - I am happy with both widths and their features. The question is about having two that are different.

I already carefully did ~50 km and did not see anything special in the way the bike behaves, but the problem may be in some specific situations (wildly guessing)

  • Can you add info on your riding style? Going a relaxed 10 km/h is different than 30 km/h with aggressive cornering and braking – L.Dutch - Reinstate Monica Jun 14 '18 at 18:44
  • Minor differences are common, but this is a big difference. Perhaps you could read the sizes on the sidewalls as appearances can be deceptive – Chris H Jun 14 '18 at 18:51
  • @L.Dutch: relaxed to normal "going back home" speed. i updated the question. – WoJ Jun 14 '18 at 19:10
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    700x40C is just 40-622? That's hardly a different from 37-622. – gschenk Jun 14 '18 at 21:54
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    There is absolutely nothing wrong with this, so long as both tire widths fit your riding needs. (And two tenths of an inch is hardly measurable.) – Daniel R Hicks Jun 15 '18 at 1:53
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I started doing this by chance: I got different width tyres on special, and had to use the narrower than usual one

So I run 32mm front, 37mm rear at present. As the rear load is 60-70% of gross weight, it makes perfect sense. It is what motorbikes do.

There is one drawback: you can't swap front and rear tyres to even out the wear**

Width does not directly affect friction (grip). The friction coefficient of the compound is what directly affects grip. You can choose soft, short-life, high grip, or you can choose long-life low-grip compounds.(Well you might be able to - you take what you can get here most of the time)

The second most important factor is how well the tyre surface conforms to the road surface during heavy braking and cornering. It is quite likely that some thinner, ligher, narrower tyres actually do this better, but I have never seen any information about this.

Given that on-road tyres are perfectly usable in widths from 22mm to 40mm, and all brake just fine, it is obviously not that important.

** or move the front tyre to the rear, when the rear is worn out, putting the new one on the front.

  • "There is one drawback: you can't swap front and rear tyres to even out the wear." That is an advantage. It is not advisable to use one's worn rear tyres at the front. You may still abandon your worn out rear tyre and ship the not yet worn out front tyre on the rear, buying a new front tyre. – gschenk Jun 14 '18 at 21:51
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    There is no reason why you can't swap the tires. Normally you'd want the wider tire on back, but the world won't end of the wider one is on the front. – Daniel R Hicks Jun 15 '18 at 1:54
  • @gschenk that only makes sense if you're running slicks. In all other cases swapping at the early sign of wear can get double the distance out of a set. Or do you suggest throwing away a perfectly good front tyre at the first signs of wear? – Chris H Jun 15 '18 at 8:08
  • I ran my hybrid on 32 front 28 rear for a year, only changing when I put the winter tyres on – Chris H Jun 15 '18 at 8:10
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37mm / 1.4" vs 40mm / 1.6" is not a very big difference. There should be no problem for casual riding.

I'd keep the larger tire on the back as the rear wheel supports more rider weight.

2

Having two different sizes of tires for common use, is not an issue. Nevertheless there are some concerns that needs to be addressed.

Having a thinner tire in the front is frowned upon. You need your best grip in the front wheel, the one that gives you direction and takes weight load on braking due to momentum transfer. Also it gives you less of an attack angle between your frame and fork. Again, going at reasonable speed this is not a problem.

What's recommended is to sport to tires of the same width, or the front tire slightly wider than the back.

The only scenario I'd use a bigger tire, is when I need more "air" in it, to haul stuff, or when the back is loaded. Like in a postman's, delivery, etc; bike.

In your case, I'd just swap them.

Cheers.

  • Good point about the wider tire in front for breaking. But what do you think about the pressure difference? Wider tires usually have a smaller pressure rating than thinner ones. – cmaster Jun 14 '18 at 19:19
  • @cmaster I'd not worry. That's because you have more air. When you a "fat" tire you have a taller profile, that's give you some suspension travel between the ground and rim, that's a softer ride, wide grip and less pressure on the ground. Whereas a "slim" tire has a harder ride, are prone to puntures and apply so much pressure to the ground that it cuts the terrain(you can't go on sand). On my BMX 20"x2,5" I use 80psi and have to control absorve a lot force with my body. Also never had a tube exploding on me by too much psi in it. – dmb Jun 14 '18 at 19:27
  • I understand the relation between air pressure and street contact area. What I'm wondering about is, whether the difference in shock absorbtion can have negative effects. I guess, any such effect is negligible with the OPs ration of 2:3, but I'm still curious :-) – cmaster Jun 14 '18 at 19:44
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    I don't really believe this is valid. Motorbikes have had thinner front tyres for ever, and they brake hard. Tyre width doesn't directly affect braking grip, it is the rubber compound (stickiness) that sets grip. A narrower tyre has a longer tread patch, which equals out. It has to - the weight on the tyre is constant regardless of width – Henry Crun Jun 14 '18 at 20:53
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    Agree with @HenryCrun. Friction is proportional to pressure * surface area, and the surface area of any tire is inversely proportional to its pressure. This cancels out. The "higher grip" of a skinny high-pressure tire is cancelled out by its smaller contact patch, and the "larger contact area" of a fat low-pressure tire is cancelled out by its low pressure against the ground. Two tires of the same material will brake equally hard regardless of width or inflation. – Ryan Cavanaugh Jun 14 '18 at 21:19
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The wider tyre on the back will be for comfort: more of your weight is on the rear and a wider tyre at lower pressure is better at absorbing the bumps. There's no reason to expect any handling issues or any other problems.

If you're worried about it, you can always ask the shop.

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There is no problem with having different size tires on the front and back. In fact Continental have a product call Attack that has a 22 on the front and a 24 at the back, the idea being that the 22 on the front gave better aero performance (whatev's). When 25's first became popular with 'roadies' it was quite common to rock 23 on the front and 25 on the back. The inner tube will be the same for both.

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If you need to carry two different spares, eg inner tubes or tyre, then that is obviously an issue.

Beyond that, I think you should try it and see how it feels.

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