discussion about brake wear

[AJI]

I was having a discussion about brake wear yesterday and there two trains of thought.

(1) Late hard braking causes less pad/disc wear, due to less time that pads are in contact with discs.

(2) Smooth gradual braking over longer distance causes less brake pad/disc wear due to lower magnitude of friction forces involved.

 

In both the above cases we were considering a car travelling at X speed slowing to speed Y for any particular corner. Therefore both cars do the same amount of 'work done' in terms of converting kinetic energy to heat via friction of pad-disc contact.

 

I was swaying for argument (2) but a few other guys were swaying for argument (1).

 

What are your thoughts?

[CardiffSupra]

Ummmm thats a hard one, Not sure tbh but id go with 2,

[Pig]

I would say 2. Think of sand paper. Press hard and you can cause much more damage than softly pressing just doing it for longer??

[AJI]

During my years of motorsport I've been coached by Tim Sugden who is a well established national racing driver and also Robin Lidell who is a well established international racing driver.

Tim liked the last second late braking technique and Robin liked the gradual longer braking zone technique.

Each one stated it was the best way to brake and by assumption due to the fact they both do endurnce racing it must mean that each thought it was the best for pad/disc wear rate.

 

If I were to trust either one's judgement on the above then it would be Robin Lidell's argument.... he seemd to know his car physics through to the very last detail.

[tbourner]

Both the same, as you say it's a case of 'work done', in the case of braking the pad material and disc material are equal throughout, and assuming the force transferal through the different materials doesn't change then the same ammount of effort will be required to stop the car from (for example) 100 to 10mph within any set distance - regardless of whether you do it all at the end, or throughout the entire distance, or indeed all at the beginning.

Differences will occur such as in the continuous braking example you will likely be off the throttle and therefore using engine braking - which will save some of your brake pad material, but in the late braking technique you are likely to be on throttle (or at least not off throttle) till the point of first braking, so your engine braking assistance is reduced.

 

Transferal of energy is set in stone, if it was purely the braking style in comparison there would be NO difference AT ALL, but add in wind resistance, engine braking, level of land and gravity etc. then you can't possibly compare which braking style gives best pad life as all these factors help (or hinder) your stopping power.

[merckx]

Applying the brakes earlier will always give you less wear, because if you do this you're dissipating the energy anyway through rolling resisistance and drag - you haven't got you're foot on the accelerator.

 

If you really wanted to you wouldn't have to use the brakes at all , just let it coast to a stop, then you would have zero brake wear.

 

You wouldn't do this in practice but it does demonstrate that braking earlier will always give you less wear.

 

It would be different if you were in a vaccuum and ignored rolling resistance/ bearing friction etc.

[Bobbeh]

2 for sure.

 

When you brake hard you're sometimes using more braking force than you need anyway, and just look at the state of your wheels with all the chunks of dust at the Ring!

[AJI]

yep all the above sounds accurate to me.

[AJI]

I suppose the crunch question is whether pad/disc wear rate is uniform with the rate of 'work done'.

ie. if you were to plot a graph of wear rate against the rate of deceleration as to whether it would be a straight line or not.

 

Taking into account the above mentioned resistant factors such as engine braking etc then I'm guessing it would be close but not straight.

 

The other argument was that of heat transfer and which method of braking best suited that, in terms of trensferral of heat to brake fluid etc.

In case (1) you'd get higher temperatures in both pad and disc which would tranfer to the fluid. In case (2) you'd get lower temps but for longer period of time... on another forum somebody mentioned that lower temps for longer period causes a higher rate of heat transfer than with higher temps for shorter period.

I don't have the technical knowledge on heat transfers of materials involved but I still think case (2) may be the better method for longevity of parts.

[tbourner]

In a perfect world the heat transfer would also be identical, ie: the total amount of heat transferred from pads to fluid to cooling would be the same whether it's transferred quickly in one go or slower over a longer period!

 

But we don't live in a perfect world And many other factors come into play (I couldn't name them TBH, but there would be LOTS of outside factors affecting heat transfer).

 

I suppose though that the pad material (whichever pads you have) would change it's wear rate with different temperatures, and so if (for example) a higher temperature produces a lower wear rate (more efficient) then (2) would be better again (carbon/ceramic discs etc. on supercars would have similar effect).

So in a lab scenario, we'd need to work out those types of perameters of the pad/disc combination, whether heat affects wear.

On the road it depends on your driving style; whether you drive hard and reach the next corner before the brakes have cooled enough, whether you use engine braking in (1) or just coast, cadence braking, current wind conditions, type of car used, mods, is the air con on, are the wing mirrors retracted, etc. etc.

[Getrag]

Ive no idea which is involved but the final equation will probably involve something like Energy = braking pressure x time braking (if at a constant brake pressure).

 

Another part of me is thinking that even if you use either method 1 or 2, both are bringing the same car from speed B down to speed A so will the total braking energy required for both be the same?

[merckx]

The energy level wouldn't be the same for both cases though.

 

Only if the drag and rolling resistance/friction losses are zero which they aren't.

 

If you have two identical cars they will have the same energy before they start braking, but the one who brakes late and maintains speed longer will carry this energy right into the braking zone.

 

The one who brakes earlier will lose energy through drag/ rolling resistance and friction losses in the bearings way before the late braker applies his brakes.

 

Like I said earlier you wouldn't have to use your brakes at all if didn't want to!