# A Bicycle tire width vs. pressure for identical comfort

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1. Jun 9, 2017

### David VH

Here is what I want to know specifically: if I replace my 5cm wide rear tire with a 6.25cm wide tire, how much can I increase the inflation pressure to obtain the same comfort?

I realize "comfort" is not a quantifiable measurement, maybe shock absorption characteristics is better. Perhaps it can only be compared for one particular kind of shock? Representative for my use case would be riding over a +1cm step change in surface height at 10m/s. Wheel diameter is 723mm for the 5cm tire and 749mm for the 6cm tire.

Background facts that I am aware of:
- tire deforms so that load on the wheel equals contact patch area * inflation pressure (e.g. 15 cm^2 contact patch for 450N at 300 kPa),
- bumps cause the tire to widen locally so that the outer edge moves inwards while preserving volume.

Any ideas? Remarks?

2. Jun 9, 2017

### PeterO

I would run comparison tests to see what increase in pressure made it feel as comfortable, then try to reverse the calculation to see what was going on.

3. Jun 9, 2017

### rcgldr

Deformation is related to pressure and tire stiffness. Consider the case of run flat tires used on some cars. I don't know how stiff the sidewalls are for the tires you are comparing. Assuming the combined effect of pressure and stiffness are the same, then the shape of the contact patch changes (wider tire has wider contact patch), but not the total area of the contact patch.

4. Jun 9, 2017

### Staff: Mentor

I agree with @PeterO. You have hard to define criteria and analysis hard to do accurately. If ever there was a case calling for experiment instead of analysis, this is it.

Just ride over a test course with several different tire pressures and choose the most comfortable.

BTW, maximum comfort and maximum tire lifetime are probably not the same pressure.

5. Jun 9, 2017

### David VH

@anorlunda tires are expensive though, I was trying to get a better picture of what I would be getting for my outlay before I decide to do it. Maybe I should do it in the interest of science, no matter the cost.

I neglected stiffness but I'm aware that's how it works :)

What do you think of these propositions?
1. the amount of shock you feel over a bump is proportional to the volume of air that the bump must displace within the tire (call it air inertia),
2. a wider tire will place the same contact area against the bump, cfr. the static scenario but with the load now caused by the inertial effect of riding against the bump, but needs to deform less to do so,
3. the reduced deformation means reduced volume of air displacement,
4. the reduced deformation means the tire will "climb" the bump earlier (axle rises sooner).

This would mean less thud (reduced perceived sharpness) but more wallop (throws you up from the saddle harder) in entirely unscientific terms. Propositions 3 and 4 are a bit dodgy though, I can't immediately see what the effect of the tire width would be on the depth of the dent made by the bump, and therefore on the volume of the triangoloid-meets-torus indentation. See my assumed deformation as viewed side-on below.

6. Jun 9, 2017

### Staff: Mentor

I'm confused because all of that sounds backwards to me: wider tires run at LOWER pressure and provide MORE comfort than narrower tires. You seem to either be saying the opposite or trying to undo the advantage of the wider tire (Why would you do that?). Am I understanding right?

Either way, you may run into an issue that the wider tire is unable to handle the pressure increase you want to apply.

7. Jun 9, 2017

### David VH

@russ_watters you are correct, I intend to undo the comfort advantage of the wider tire in exchange for lower rolling resistance. If the increased width brings a comfort advantage at least... Right now it's not even confirmed that a wider tire at the same pressure is more comfortable, but I suspect it is.

8. Jun 9, 2017

### Khashishi

You should know that wider tires can't hold as much pressure as thinner tires (given the same wall strength). Those massive balloon tires need to operate at low pressure.

Thin tires are really bad for riding on gravel, since you are just hitting a few rocks at a time at random angles, making the traction bad and bouncing erratically. Wide tires let you average over more rocks, making it smoother, even if the pressure were the same, I think.

9. Jun 10, 2017

### rcgldr

Note that the larger wheel diameter will reduce the shock of going over bumps much more significantly than the width of the tire, as it will increase the duration of the periods of transitions over bumps. With a larger diameter wheel, the same displacement occurs over a longer period of time.