Riser bars and 'pressuring the front wheel'

[2manyoranges]

...and then I found this...

https://www.mbr.co.uk/reviews/handlebar ... ars-review

useful....

 

[hamster]

The tyre contact area is a function of tyre pressure not rim or tyre dimensions.

Interesting, I'd argue that it's a combination of all three, as well as the actual construction of the tyre itself.

Here's why (apologies for crappy Imperial measurements, but it's easier as most people in the UK understand psi)

The tyre will deform under loading until the:

(area of the contact patch) = (the weight on the wheel) / (inflation pressure)

So a wheel with (say) a weight of 60lbs on it, inflated to 30psi will have a 2sq inch contact patch. Clearly if you deflate the tyre, it sinks and the contact patch enlarges. Overdo it and you break the equilibrium as when the rim hits the surface.
That's why lowering the tyre pressures increases traction, and how huge balloon tyres (like fat bikes) allows very low pressures.

You can weigh your car by driving it onto paper, drawing round the contact patches of the tyres, measuring the area and using the tyre pressures. If you work it out you discover that the contact patch for a car is about the size of two hand prints. :shock:

Rim width and wheel diameter change the shape of the contact patch - a more circular patch deforms the tyre less and as a result has lower drag. That's one reason why 29ers roll well.

It's basic physics but not widely understood.

 

[2manyoranges]

...nice linked thoughts.

I could argue that most basic physics is not understood...

…that's why we saw someone in the ferry queue in Calais last Summer in a fat 4X4 running the engine to keep the AC going to keep the car cool and sat there with the windows open to help the process...

sigh.

Look after Planet A. There is no planet B.

 

[d8mok]

Re:

But surely a wider rim would give a squarer shape to a tyre which would lead to more contact area? Assuming same pressures and tyres used?

I was running 27mm rims and 30mm rims on two bikes. Same tyres and same pressures and the wider rim had a squarer profile tyre in contact with the ground.

 

[hamster]

Re: Re:

But surely a wider rim would give a squarer shape to a tyre which would lead to more contact area? Assuming same pressures and tyres used?

I was running 27mm rims and 30mm rims on two bikes. Same tyres and same pressures and the wider rim had a squarer profile tyre in contact with the ground.

Right - wide rim gives a short contact patch, narrow rim is longer. The area will be the same, but the short contact patch also gives less tyre deformation (the bulge at the bottom).

 

[My_Teenage_Self]

The tyre contact area is a function of tyre pressure not rim or tyre dimensions.

Interesting, I'd argue that it's a combination of all three, as well as the actual construction of the tyre itself.

Here's why (apologies for crappy Imperial measurements, but it's easier as most people in the UK understand psi)

The tyre will deform under loading until the:

(area of the contact patch) = (the weight on the wheel) / (inflation pressure)

So a wheel with (say) a weight of 60lbs on it, inflated to 30psi will have a 2sq inch contact patch. Clearly if you deflate the tyre, it sinks and the contact patch enlarges. Overdo it and you break the equilibrium as when the rim hits the surface.
That's why lowering the tyre pressures increases traction, and how huge balloon tyres (like fat bikes) allows very low pressures.

You can weigh your car by driving it onto paper, drawing round the contact patches of the tyres, measuring the area and using the tyre pressures. If you work it out you discover that the contact patch for a car is about the size of two hand prints. :shock:

Rim width and wheel diameter change the shape of the contact patch - a more circular patch deforms the tyre less and as a result has lower drag. That's one reason why 29ers roll well.

It's basic physics but not widely understood.

I agree with what you're saying above, but still disagree with your original statement, and maintain it's a mixture of all three. a 2.6 tyre WILL have more contact patch than a 1.7 at useable pressures with the same loading. Equally, at the SAME pressures, a 2.8 will still have more contact patch. They may exert less ground pressure, but the contact patch will be different. By the time dynamic loads are taken into effect, all bets are off!

 

[My_Teenage_Self]

Typical pinkbike answer, at least mine had pictures

And probably said the same thing

*I forgot add a smiley from my phone*

**yeah, thats what HE says**

Moron.

 

[hamster]

I agree with what you're saying above, but still disagree with your original statement, and maintain it's a mixture of all three. a 2.6 tyre WILL have more contact patch than a 1.7 at useable pressures with the same loading. Equally, at the SAME pressures, a 2.8 will still have more contact patch. They may exert less ground pressure, but the contact patch will be different. By the time dynamic loads are taken into effect, all bets are off!

No it cannot.
If the tyre is pushing out at 30psi then it will reach an equilibrium, until the outward (downward) force is equal to the upward one. (The force analysis name is 'normal reaction'). Whne you stand on the ground the ground pushes up with a force equal to your weight.
How can the weight on the wheel push the tyre further in when exactly the same force is exerted per area?

If you don't believe me, try the contact patch trick, then halve the inflation pressure and repeat. It's easiest done with slicks.

Worth a read: https://kktse.github.io/jekyll/update/2 ... print.html

However it's also true that there can be supporting effects where the tyre structure is very uneven (run-flat tyres being the best example) and the sidewall actively takes weight instead of acting like the skin of a balloon. But that's not really like a loaded bike tyre.

 

[d8mok]

Re: Re:

But surely a wider rim would give a squarer shape to a tyre which would lead to more contact area? Assuming same pressures and tyres used?

I was running 27mm rims and 30mm rims on two bikes. Same tyres and same pressures and the wider rim had a squarer profile tyre in contact with the ground.

Right - wide rim gives a short contact patch, narrow rim is longer. The area will be the same, but the short contact patch also gives less tyre deformation (the bulge at the bottom).

I’ve read this 10 times and I don’t get it.

 

[hamster]

OK, a narrow tyre has a contact patch like 0 whereas a wide one is like O (looking from on top)

The wider rim puts the beads of the tyre further apart, reducing the thinner waist of the tyre at the top - so it's more of a shallow U rather than a tall one (for the same tyre). This in turn reduces the twisting squirm of the tyre. The tyre will twist in a corner, not only sideways but front to back (called shear). Again, a short contact patch reduces this twist / shear. On cars, that's why we use low-profile tyres.