Can differing brake pads affect disc tempartures
10-26-2009, 06:59 AM
#1
Thread Starter
Can differing brake pads affect disc tempartures
To avoid spamming Steve's two piece brake disc thread i've started this one to discuss whether the coefficient of friction of the brake pad can affect the temperature that the discs reach during braking.
Hypersonik's view is that in slowing the car the same amount of kinetic energy is converted to heat and therefore the disc temperature will be the same irrelevant of the length of time it takes to slow the car.
My view is that there are other heat generating mechanisms at work than just the kinetic energy being changed to heat and therefore changing pads can reduce disc temperature.
I posted this in the other thread:
Ron, i had/have the same problem understanding the concept as you. But there are more heat generating mechanisms than just the kinetic energy being converted into heat.
The higher the coefficient of friction of the pad the shorter the period of time the pad is in contact with the disc. Therefore the disc has a longer period of cooling between braking efforts. Pedal pressure also has an effect, as does the shearing of the brake pad material on the pad and the transfer layer on the disc.
Now i wouldn't think that this would have a huge effect on disc temperature, but it does.
I thought the same as you and had some correspondance with DBA over it. Their response is below
You don't want your pads and discs operating at as high a temperature as possible because there are temperature limitations to the materials they are made of. Eg once you get above 630 degree cast iron is prone to cracking. Once the pads get above a certain temperature (dependant on the pad material) the resin binders that hold the pad material together break down, turn to gas and blow the pad material apart.
That's why its so important to choose the appropriate pad for your use.
Hypersonik's view is that in slowing the car the same amount of kinetic energy is converted to heat and therefore the disc temperature will be the same irrelevant of the length of time it takes to slow the car.
My view is that there are other heat generating mechanisms at work than just the kinetic energy being changed to heat and therefore changing pads can reduce disc temperature.
I posted this in the other thread:
Ron, i had/have the same problem understanding the concept as you. But there are more heat generating mechanisms than just the kinetic energy being converted into heat.
The higher the coefficient of friction of the pad the shorter the period of time the pad is in contact with the disc. Therefore the disc has a longer period of cooling between braking efforts. Pedal pressure also has an effect, as does the shearing of the brake pad material on the pad and the transfer layer on the disc.
Now i wouldn't think that this would have a huge effect on disc temperature, but it does.
I thought the same as you and had some correspondance with DBA over it. Their response is below
Originally Posted by DBA technical manager
Now to answer you question in relation to the temperature the rotor gets to regardless of what pad is used; The transfer layer plays a very big part in this due to the fact that the pad uses this transfer layer to maximise the coefficient of friction through shearing between the pad and transfer layer. The duration the brakes are applied will also directly affect rotor temperature as the longer the brakes are applied the more heat is generated. The other factor is pedal pressure as the more pressure that is applied to make the pad work the more temperature is also generated. The best possible brake setup for any car will have a pad that has a high enough coefficient of friction to minimise the braking time and pedal pressure required to slow the vehicle as quickly as possible.
The best way to explain this is probably though what I see at the track with cars that I help develop the brake setup on. We have a production series class here down under that requires the vehicle to run rotors of the same dimensions as the EOM rotor and must run the factory calipers. One of the cars I help is a Holden Commodore which is a 4 door version of your Vauxhall Monaro 2002 model. This vehicle was running Hawk pads front and rear and at the end of each race when we checked the rotors, the rotors were running above 630 deg. C and the driver was saying there was no problem with the brake performance. I was asked to find a solution to rectify this high rotor temp problem so we fitted a different Hawk pad compound that had a higher coefficient of friction. The results after the next race meet were very positive. The driver commented that he had extended his braking point on the straight by almost 20 meters and on inspection the rotors were now only running at around 550 deg. C. The driver also commented that the brake feel was much better and more controllable. The driver of the car has raced in many different categories over here and has raced on numerous different brands and compounds of pads including DS2500's. He was very impressed with the original Hawk setup that was in the car but now loves the new setup even more.
Sometimes it is a case of trial and error to get the optimal compound for the vehicle and drivers driving style and occasionally it requires changing the drivers braking habits too.
The best way to explain this is probably though what I see at the track with cars that I help develop the brake setup on. We have a production series class here down under that requires the vehicle to run rotors of the same dimensions as the EOM rotor and must run the factory calipers. One of the cars I help is a Holden Commodore which is a 4 door version of your Vauxhall Monaro 2002 model. This vehicle was running Hawk pads front and rear and at the end of each race when we checked the rotors, the rotors were running above 630 deg. C and the driver was saying there was no problem with the brake performance. I was asked to find a solution to rectify this high rotor temp problem so we fitted a different Hawk pad compound that had a higher coefficient of friction. The results after the next race meet were very positive. The driver commented that he had extended his braking point on the straight by almost 20 meters and on inspection the rotors were now only running at around 550 deg. C. The driver also commented that the brake feel was much better and more controllable. The driver of the car has raced in many different categories over here and has raced on numerous different brands and compounds of pads including DS2500's. He was very impressed with the original Hawk setup that was in the car but now loves the new setup even more.
Sometimes it is a case of trial and error to get the optimal compound for the vehicle and drivers driving style and occasionally it requires changing the drivers braking habits too.
That's why its so important to choose the appropriate pad for your use.
10-26-2009, 07:01 AM
#2
Registered User
And I PM'ed Rich this response:
The reason higher friction pads are moot in our case is because you are limited by tyre grip.
If you have enough power to force the ABS to kick in and you ride the ABS, you are stopping at the limit of the tyres, thus you will be on the brakes the same amount of time
Lets ignore tyre grip for a moment and lets say it is infinite - pad friction is limiting factor.
Remember what I said about kinetic energy.
The same amount of energy would be transferred to the disc REGARDLESS of how long you were actually on the brakes for.
You brake for 10m you brake for 100m, the energy transfer, assuming initial and final velocity is the same in both instances, which means that the heat in the disc would be the same. Pure theory.
Real world there are other factors.
Air/tyre/bearing/drvetrain etc etc resistance. This resistance means the longer you take to brake, the more air resistance time you have so less energy is being required by the brakes.
Despite the brakes being on, they are still subject to being cooled by the surrounding airflow so the statement about offering the brakes longer to cool the less time you are on them is incorrect as they are constantly being cooled regardless of whether you are using them or not
As for the Auzzie guy - pinch of salt.
20m is a MASSIVE distance to shed.
I put it to you - do you really think a racing team wouldn't be able to work out the maximum friction of the tyre and whether or not their braking distance was closely approximating the friction limit of the tyre?
We also don't know what the weather was like - Temperature/wind/humidity - I know you know they play a massive part.
As for as high a temperature as possible - of course you want them to run as hot as possible. In the case of Iron 600 Deg is as high as possible - any more and you'll crack as you say.
My whole statement was theoretical - I know the limits of materials, but the statement was constructed to try and ignore those variables.
The reason higher friction pads are moot in our case is because you are limited by tyre grip.
If you have enough power to force the ABS to kick in and you ride the ABS, you are stopping at the limit of the tyres, thus you will be on the brakes the same amount of time
Lets ignore tyre grip for a moment and lets say it is infinite - pad friction is limiting factor.
Remember what I said about kinetic energy.
The same amount of energy would be transferred to the disc REGARDLESS of how long you were actually on the brakes for.
You brake for 10m you brake for 100m, the energy transfer, assuming initial and final velocity is the same in both instances, which means that the heat in the disc would be the same. Pure theory.
Real world there are other factors.
Air/tyre/bearing/drvetrain etc etc resistance. This resistance means the longer you take to brake, the more air resistance time you have so less energy is being required by the brakes.
Despite the brakes being on, they are still subject to being cooled by the surrounding airflow so the statement about offering the brakes longer to cool the less time you are on them is incorrect as they are constantly being cooled regardless of whether you are using them or not
As for the Auzzie guy - pinch of salt.
20m is a MASSIVE distance to shed.
I put it to you - do you really think a racing team wouldn't be able to work out the maximum friction of the tyre and whether or not their braking distance was closely approximating the friction limit of the tyre?
We also don't know what the weather was like - Temperature/wind/humidity - I know you know they play a massive part.
As for as high a temperature as possible - of course you want them to run as hot as possible. In the case of Iron 600 Deg is as high as possible - any more and you'll crack as you say.
My whole statement was theoretical - I know the limits of materials, but the statement was constructed to try and ignore those variables.
10-26-2009, 07:15 AM
#3
Thread Starter
Ok, so ignoring the limits of materials.....
There are more heat generating mechanisms at work than just the kinetic energy conversion. I agree that the kinetic energy should remain the same. But imagine for a second a spinning bike wheel. If you jam a stick between the spokes the wheel stops instantly. That is the ultimate in high friction..... but neither the stick or the wheel gets hot so where does the energy go?
To get the same braking effort with OEM style pads compared to high friction pads you have to really stamp on the brake pedal. You are therefore adding a load of energy into the system which is again going to get converted to heat. If you glaze your pads through overheating you end up with a low friction surface on your brake pad and it doesn't matter how hard you press the brake pedal you get very poor braking performance but you still get the heat problems.
Secondly, you do get better braking performance with high friction compared to OEM. I understand your point about the ABS and tyres being the limiting factor, but in reality with high friction pads its is possible to brake harder without the ABS kicking in and certainly for a lot less pedal effort.
I bet i could stop my car in a shorter distance with the Dixcel pads than i could with OEM pads. I suspect that the increase in friction and consequent reduction in pedal pressure allows you to get a lot closer to the tyres frictional limit before the wheels lock.
I don't understand the shearing mechanism that the technical manager at DBA refers to, but everything else he says does make sense.
There are more heat generating mechanisms at work than just the kinetic energy conversion. I agree that the kinetic energy should remain the same. But imagine for a second a spinning bike wheel. If you jam a stick between the spokes the wheel stops instantly. That is the ultimate in high friction..... but neither the stick or the wheel gets hot so where does the energy go?
To get the same braking effort with OEM style pads compared to high friction pads you have to really stamp on the brake pedal. You are therefore adding a load of energy into the system which is again going to get converted to heat. If you glaze your pads through overheating you end up with a low friction surface on your brake pad and it doesn't matter how hard you press the brake pedal you get very poor braking performance but you still get the heat problems.
Secondly, you do get better braking performance with high friction compared to OEM. I understand your point about the ABS and tyres being the limiting factor, but in reality with high friction pads its is possible to brake harder without the ABS kicking in and certainly for a lot less pedal effort.
I bet i could stop my car in a shorter distance with the Dixcel pads than i could with OEM pads. I suspect that the increase in friction and consequent reduction in pedal pressure allows you to get a lot closer to the tyres frictional limit before the wheels lock.
I don't understand the shearing mechanism that the technical manager at DBA refers to, but everything else he says does make sense.
10-26-2009, 04:14 PM
#4
Remember also that with braking more gently and over a longer time, there is more heat dissipation time too.
A result of air convection (air cooling) and also conduction (heat into the wheels).
I see Ron touched on this above.
Its a very interesting theory.
What about carbon F1 brakes then.
How to they work?
A result of air convection (air cooling) and also conduction (heat into the wheels).
I see Ron touched on this above.
Its a very interesting theory.
What about carbon F1 brakes then.
How to they work?
10-26-2009, 08:48 PM
#5
Registered User
Originally Posted by lower,Oct 26 2009, 04:15 PM
But imagine for a second a spinning bike wheel. If you jam a stick between the spokes the wheel stops instantly. That is the ultimate in high friction..... but neither the stick or the wheel gets hot so where does the energy go?
The momentum of the wheel is tiny compared to the momentum of the bike being ridden.
Throw a stick into a bike wheel when it's being ridden and you instantly:
a) Break the stick
or
b) lock the wheel and overload the tyre causing it to skid, thus transferring the heat to the road.
Stop your car in a shorter distance with your Dixcels than OEM? No Way - the brake pads are completely independant of the tyres and the tyres are the limiting factor.
10-26-2009, 10:48 PM
#6
I have read one or two opinions on brakes lately 
A couple of our experienced cousins have found BBK can extend stopping distance.
The 'exact' reasons are still unclear but may be down to effects on brake bias.
It may be down to overbreaking making the brake zone on track harder to judge.
Also others have found that using Dixcel fronts, the backs appear to run far hotter, where the fronts cooler.
I would offer that this 'may' be because you are hitting the pedal harder or they are not so efficient.
Thus, like when the fronts fade - the rears pick up the load (you are pressing harder on the pedal to stop).
Just my 2p
A couple of our experienced cousins have found BBK can extend stopping distance.
The 'exact' reasons are still unclear but may be down to effects on brake bias.
It may be down to overbreaking making the brake zone on track harder to judge.
Also others have found that using Dixcel fronts, the backs appear to run far hotter, where the fronts cooler.
I would offer that this 'may' be because you are hitting the pedal harder or they are not so efficient.
Thus, like when the fronts fade - the rears pick up the load (you are pressing harder on the pedal to stop).
Just my 2p
10-27-2009, 12:10 AM
#7
Originally Posted by lower,Oct 26 2009, 03:15 PM
There are more heat generating mechanisms at work than just the kinetic energy conversion.
Tyres certainly aren't the limiting factor otherwise nobody would ever upgrade their brakes. Obviously they can be in slippery situations, but in the dry, typically you can brake pretty hard without ABS cutting in. And having BI pads on mine now I can tell you it certainly doesn't stop as fast as it did with Dixcels.
I think what Lower says is probably correct, and Hyper is correct but over-simplifying. I'm just not sure why.
Trending Topics
10-27-2009, 12:46 AM
#8
Registered User
Join Date: Mar 2005
Location: Kineton, Warwickshire
Posts: 1,050
Likes: 0
Received 0 Likes
on
0 Posts
There are other reasons you would upgrade your brakes aside from reducing the stopping distance, for example, improving the heat dissipation properties would mean less fade on a track...
10-27-2009, 12:53 AM
#9
Thread Starter
Originally Posted by Dembo,Oct 27 2009, 09:10 AM
Like what? The energy comes from the momentum of the car. The only other place any energy comes from is you pressing the brake pedal, but typically you'd only be moving the pedal for half a second and then you'd hold it in position - a very small amount of energy.
Tyres certainly aren't the limiting factor otherwise nobody would ever upgrade their brakes. Obviously they can be in slippery situations, but in the dry, typically you can brake pretty hard without ABS cutting in. And having BI pads on mine now I can tell you it certainly doesn't stop as fast as it did with Dixcels.
I think what Lower says is probably correct, and Hyper is correct but over-simplifying. I'm just not sure why.
Tyres certainly aren't the limiting factor otherwise nobody would ever upgrade their brakes. Obviously they can be in slippery situations, but in the dry, typically you can brake pretty hard without ABS cutting in. And having BI pads on mine now I can tell you it certainly doesn't stop as fast as it did with Dixcels.
I think what Lower says is probably correct, and Hyper is correct but over-simplifying. I'm just not sure why.
As for the tyres being the limiting factor, Ron you need to try some uprated pads.
I've used OEM Honda, Brakes International, Porterfield RS4, Ferodo DS2500 and current Dixcel Type Z.
You can certainly stop the car much faster with the Ferodo and Dixcel pads. With the uprated pads you can brake much harder without the wheels locking, i'm guessing because the reduced pedal pressure and therefore pad on disc pressure allows you get much closer to the lock up point without the tyres loosing traction than you can with OEM type pads.
This becomes obvious on the track when you can see the S2000s with uprated brakes braking much later and harder into the corners. With DS2500 pads the braking performance of my car wasn't that far away from PJL's car with his tarox setup. With OEM pads is wasn't even close.
10-27-2009, 01:36 AM
#10
UK Moderator
Some thoughts:
1) The amount of grip available in the tyre doesn't change whatever changes are made to the braking system.
2) Braking system changes only really affect the length of time before overheating/fade/failure
3) However, they also affect the way the brakes feel to the driver, who is probably the weakest link. I know that I have to push the pedal much harder to get the same amount of deceleration with OEM than I do with the Dixcels. This is more difficult to do under the stresses of track driving (at least for an average Jo like me) and hence I get much better braking more easily with the Dixcels, which certainly reduces braking distances. Add in the longevity mentioned in 2) and it's all good
1) The amount of grip available in the tyre doesn't change whatever changes are made to the braking system.
2) Braking system changes only really affect the length of time before overheating/fade/failure
3) However, they also affect the way the brakes feel to the driver, who is probably the weakest link. I know that I have to push the pedal much harder to get the same amount of deceleration with OEM than I do with the Dixcels. This is more difficult to do under the stresses of track driving (at least for an average Jo like me) and hence I get much better braking more easily with the Dixcels, which certainly reduces braking distances. Add in the longevity mentioned in 2) and it's all good