Who's good at maths: Brake effort after bigger tyres.

[Crispin]

I was wondering today, after having to do some hard braking on the M40, how much more work / how much less effective my brakes are after putting on bigger tyres.
Standard, I had 265 65 17s on. I now have 265 70 17.

According to this calculator, that is a 3.3% difference. So, I assume my brakes would either have to work 3.3% harder to stop me in the same distance or I would in fact stop 3.3% further assuming I was already braking as hard as I could without lock-up.

Let's assume with the normal tyres I would have tipped the car in front of me after braking from 70mph. How fast would I now hit the car seeing as everything is worse off.

I suspect it would all be exaggerated with brake fade and heat as everything is now taking longer but we could ignore that for the above.

 

[Crushers]

= drive 3.3% slower

seriously, if 3.3% tire size was an issue then you would not want passengers in your car. the weight of the vehicle will affect braking more than the size of tires.

the size increase is not enough to concern yourself with.
edit: stated size difference by text and actual size change can actually be smaller or bigger than anticipated depending on inflation, manufacturer, design or other factors. the only real way to figure the % change is to measure the old tires before removing and measuring the new tires after installation and a short road trip to flex the sidewall.

 

[Chris]

Agreed and remember Crispin - the 33% taller tyre is actually split in two. There is only 15 and a bit % from the centre of the hub to the edge of the tyre. Yes? A 33" tyre is only 1/2" bigger than a 32" where it counts. You don't get 1" of lift.

In your case changing the profile as you have, I doubt you'd notice if you had one wheel different to the others! It's almost within the confines of new tread v old worn tread.

Bigger tyres do take more stopping on the whole though. They usually weigh more to start with and essentially (insert Rob's explanation here) you are looking at a lever effect. The taller the tyre, the greater mechanical advantage it has against your brakes.

Chris

 

[Gary Stockton]

You'll still hit him moeruva hard irrespective. I've seen your driving ...

 

[Lars Bo Kjeldstrom]

I think we can talk a lot about calculations here...also 70 mph with your new tires might actually be a little faster in reality, than the older "smaller" tires !

So the important lesson is to know the car's ability to stop with the actual wheel setup......-and the tread pattern, -and the road condition, -and the actual load of your truck, -and the weater condition, -and condition of break system, -and -and -and....

These days I am testing my new 18" wheels with the 285/60-18 tires (3 different types !!! )...and they are 5,9% less than my normal setup (295/75-16). I feel a difference in driving of course (low profile vs high!) -but the breaking issue I never considered. I guess its only a problem if you take a lot of chances in the traffic, so actually the driving style is a more important factor here? I believe our most important responsability is to consider all the above factors -and to be familiar with the car ...more than thinking/calculating that if tires are 3,3% bigger we need to keep 6,6% more distance -or whatever the result of a rocket scientist calculation would bring

But thanks for bringing this to my attention as well....I admit I was not considering to test my various wheel setups for emergency breaking ability

greetings...

 

[chadr]

IMO, the small difference in circumference is one of the lesser things to consider.......assuming all other variables such as weight, speed etc. are equal and also assuming that you keep to the same make/model of tyre (so that co-ef of friction is roughly same) - I would imagine that the main factor would be the contact area of the 2 different sizes.

For the 2 sizes you have quoted, the difference in contact area would probably be negligible and hence same for the braking force.

I'm no maths expert or engineer, so would be happy to be educated (i.e. corrected) on this though...

 

[Dave Docwra]

Crispin, do not worry about the maths, once you reach over 33 years of age all braking takes much more effort

 

[Chas]

Crispin, do not worry about the maths, once you reach over 33 years of age all braking takes much more effort

But Crispin is only 21, leastways that's what he told me

 

[Rob]

Did a couple of calculations, both of them different ways and got 2 different answers so wont post the details. But I can tell you one thing, the torque needed to be applied by the brakes is 1.0165 times what it used to be so rather insignificant (its the radius not diameter that we are interested in). The work done by the brakes should be the same, assuming the total weight of the vehicle is the same, so the brakes should theoretically start to fade at the same time. Remember brake performance is limited by 2 things, the amount of torque they can apply and the ability to dissipate heat in order to avoid brake fade. In your situation you are theoretically only influencing one.

Might come back with

 

[Andrew Prince]

To add my 2c to the debate, I suspect the effect of bigger tyres on the brake system is under-estimated by most of us. Of course Mr T designed in some over-engineering in the braking system - obviously brakes are not an area where just enough is acceptable. So increasing tyre size is unlikely to push the braking system outside its design envelope. Adding bigger tyres for sure means that the brakes have to work harder to slow the vehicle as the angular momentum is increased with increased radius, as Rob mentioned.

Is the increase in brake effort significant for a single braking event? Probably not - emergency braking from 70 your stopping distance may be increased by a few percent. Even if your stopping distance has increased by 5 metres, this shouldn't be the difference between a crash or not (but might be ). In practice the road conditions (wet/standing/water/gravel/bone-dry tarmac), condition of tyres and load of vehicle will make more difference - not to mention reaction speed of the driver.
The most significant impact of bigger tyres would be long-term as brake pads and discs will wear faster, all else being equal. Worn pads and discs may make for poorer braking in time.
Driving style probably would account for more difference though.

 

[chriscolleman]

Unless you modify your brakes to bite harder and longer.

Then bigger tyres means longer stopping distances.
I've driven some horrible concepts with standard brakes and huge tyres.

An inch or 2 up from standard tyres is still pretty driveable.