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Tire Pressure and Ride Quality

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Tire Pressure and Ride Quality

Old 06-09-16, 10:34 PM
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DarKris
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Tire Pressure and Ride Quality

I am a 350+lb rider, and one of the issues that I run into with my bike is proper tire pressure.

Currently I am running 700x40c tires at 80/85psi (which is the max recommended for these tires), and previously I ran anywhere from 50-75psi in my tires but never max. I experimented with lower pressures mainly because of the increased traction on loose surfaces and the increased compliance when riding on rough surfaces. Because of an event where I double pinch-flatted my tires on a curb running at 75 psi (again, max was 85psi) I thought I should be running a higher pressure.

For off-road riding, people generally recommend around 40psi for better traction and comfort. Those people are also more than 1/2 my body weight. That raises the question: given all other things equal would a bike running tires at 40psi feel the same as the same bike running tires at 80psi if the rider on the 80psi tires was twice the weight as the rider on the 40psi tires. Does the ride quality of a tire at a set pressure remain constant independent of rider weight?

Hopefully you guys can shed a bit of light on this topic for me. Let me know if I need to clarify anything that I stated here. Thanks.
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Old 06-09-16, 10:51 PM
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My advice would be to run the tires at their max pressure.

A tire is sort of like a spring, and the more air you give it, the stiffer it gets, but a stiffer spring sometimes does the job much better.

The physics probably get very complex around the multiple variables, but the short story is you're making a good choice with high volume 40c tires, and inflating them to where they don't get pinch flats is where everyone goes, so it makes sense that in terms of feel and rebound, the tires probably feel about the same for a lighter rider with lower pressure.
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Old 06-09-16, 11:54 PM
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You should generally get a similar ride with twice the weight at twice the pressure. But that is on a hard surface. Dirt gives way under higher pressures. You need tires about 50% larger to run the same pressure as someone half your weight, which would give you similar flotation and traction.

A good starting point for your tire pressure would be 75 rear, 65 front. So 80 seems quite reasonable if pinch flats have been an issue. A lighter rider probably will more readily unweight the saddle over sharp bumps, so might get away with proportionally lower pressures.

Your 40mm tires are a good choice for your weight. The load carrying capacity of tires increases exponentially with width, but in a linear fashion with pressure. 50% wider means half the pressure, but the pressure rating would be only a little lower for a similar tire.
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Old 06-10-16, 08:43 AM
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I think 40mm tires are too small for a rider that is 350+ that you have double pinch flatted sort of confirms this.
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Old 06-10-16, 09:10 AM
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Originally Posted by Spoonrobot View Post
I think 40mm tires are too small for a rider that is 350+ that you have double pinch flatted sort of confirms this.
To clarify: I didn't pinch flat the tires that I'm riding now. These are the tires that I double pinch flatted (38c model): https://www.serfas.com/products/view...-comfort-tires
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Old 06-10-16, 09:35 AM
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Ah, I stand corrected. However, I still feel 40mm tires are too narrow for your weight. Why did you pick that size? What kind of bike are you riding? I think 50mm should be your minimum and closer to 60mm; something like the Schwalbe Big One would be ideal.

But that's just me. I too know the pain of a weight-related double pinch flat and am much happier riding wider tires. In my case I went from 700cx25 to 650bx42; the change has been amazing.
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Old 06-10-16, 10:05 AM
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Originally Posted by Spoonrobot View Post
Ah, I stand corrected. However, I still feel 40mm tires are too narrow for your weight. Why did you pick that size? What kind of bike are you riding? I think 50mm should be your minimum and closer to 60mm; something like the Schwalbe Big One would be ideal.

But that's just me. I too know the pain of a weight-related double pinch flat and am much happier riding wider tires. In my case I went from 700cx25 to 650bx42; the change has been amazing.
Here is my bike:

The frame I believe has clearance for 700x45c, but the fork actually has a vertical clearance for a 38c. My 40c tires leaves about 5+mm of clearance on both sides, but the crown has about 2mm of space left. I wanted to entertain the idea of running 650b tires as I would probably be able to run up to a 2 in. tire. Unfortunately I cannot find a set of 650b wheels with 36 spokes for a quick release axle, and then comes the point of finding Slick/semi-slick 650b tires that don't cost an arm and a leg.

As for my current tires, honestly I haven't had a situation where I felt like I needed to size up even more except when I try going into single-track areas at max tire pressures. And I also haven't tried any stupid curb jumps that would cause another set of pinch flats.
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Old 06-10-16, 10:11 AM
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Originally Posted by DarKris View Post
Unfortunately I cannot find a set of 650b wheels with 36 spokes for a quick release axle,
This statement is a illustration of a sad trend in the contemporary bike market - people forget that wheels do not have to be purchased pre built! 650B rims are available with 36 holes (I actually had trouble finding 32 hole rims the last time I was looking), and your current hubs can easily be laced into said rims... unless you have some weird hub that uses straight pull or proprietary spokes, but then a nice shimano Deore or Tiagra hub is $30.
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Old 06-10-16, 10:29 AM
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Originally Posted by Wilfred Laurier View Post
This statement is a illustration of a sad trend in the contemporary bike market - people forget that wheels do not have to be purchased pre built! 650B rims are available with 36 holes (I actually had trouble finding 32 hole rims the last time I was looking), and your current hubs can easily be laced into said rims... unless you have some weird hub that uses straight pull or proprietary spokes, but then a nice shimano Deore or Tiagra hub is $30.
You're right, they don't. But it's a hell of a lot easier and more convenient than waiting to get parts and then waiting again to have the wheels built, not to mention all the guess work that goes into finding the correct spoke length etc. There's also the peace of mind knowing that if **** hits the fan there are replacements readily available when you need it.

This coming from someone who has never built or had a wheel built for them.
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