# The Naked Scientists Forum

### Author Topic: Tyre pressure  (Read 7055 times)

#### paul.fr

• Guest
##### Tyre pressure
« on: 15/05/2007 09:22:50 »
why does my bicycle tyres have to be pumped up to the same pressure, as a car tyre? Surely it should be less. Afterall my bicycle carries less weight than a car!

#### daveshorts

• Moderator
• Neilep Level Member
• Posts: 2583
• Physics, Experiments
##### Tyre pressure
« Reply #1 on: 15/05/2007 11:25:07 »
Pressure is force per unit area, so
the pressure of your tyre x Area of contact with the ground = weight of vehicle

whilst the bike is lighter it has fewer narrower tyres so the area of contact is much less so the pressure still has to be high.

My racer wants it's tyres pumped up to 100psi where as my car wants 30psi because the tyres on the racer are very narrow so the pressure has to be higher.

#### another_someone

• Guest
##### Tyre pressure
« Reply #2 on: 15/05/2007 13:11:15 »
Also, rolling resistance is more important on a bicycle (given the lower availability of power).  Rolling resistance is also a byproduct of reduced tyre contact area, which is a desirable on a bicycle, and inflating a tyre to a higher pressure will cause it to flatten less under pressure, and so reduce contact area, and so reduce rolling resistance.

You can improve the fuel consumption on a car by increasing the tyre pressure; but if you push it too far, you will have too little tyre in contact with the road, and so it may have inadequate traction.

#### lightarrow

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##### Tyre pressure
« Reply #3 on: 15/05/2007 13:26:19 »
You can improve the fuel consumption on a car by increasing the tyre pressure; but if you push it too far, you will have too little tyre in contact with the road, and so it may have inadequate traction.
...Yes, and inadequate friction when you have to brake with a wet road...

#### paul.fr

• Guest
##### Tyre pressure
« Reply #4 on: 15/05/2007 16:53:02 »
My racer wants it's tyres pumped up to 100psi where as my car wants 30psi because the tyres on the racer are very narrow so the pressure has to be higher.

wow, never heard of a bike needing that much pressure.

You can improve the fuel consumption on a car by increasing the tyre pressure; but if you push it too far, you will have too little tyre in contact with the road, and so it may have inadequate traction.

would that not affect the mileomiter? i know putting oversize tyres on can.

#### another_someone

• Guest
##### Tyre pressure
« Reply #5 on: 15/05/2007 17:34:10 »
You can improve the fuel consumption on a car by increasing the tyre pressure; but if you push it too far, you will have too little tyre in contact with the road, and so it may have inadequate traction.

would that not affect the mileomiter? i know putting oversize tyres on can.

It will have a slight impact on the odometer and speedometer, giving the impression that the car is both travelling fractionally slower, and consequently covering a lower mileage than it actually is.  Then again, if the tyres are regularly losing traction because of the higher tyre pressure, this could have the opposite effect.

#### The Naked Scientists Forum

##### Tyre pressure
« Reply #5 on: 15/05/2007 17:34:10 »