fitting big brakes ? read this first!!

[budd]

I've owned various preformance cars over the years from a TVR to a 360bhp Nissan track car and lots in between (infact I still have the TVR, I haven't been able to bring myself to sell it even though I could use the money and with 2 small kids it rarely gets used)anyway I've now bought a Volvo 850R to lug the kids about, but me being me I can't just leave it alone so naturally I check out what everyone is doing, and cwhen I come to brakes all I see is talk of big brake kits (Porsche brakes on a volvo!!!) it's the same on every forum people seem to think by putting the biggest front brakes possible on a car it will improve it's stopping ability, anyone seriously thinking about this course of action should read this first:

http://www.mgf.ultimatemg.com/group2...ake_myth_1.htm

I know the artical talks about MGF racers but the math is the same no matter what car you drive.

Getting good brakes is all about balance plus the condition of the system seems to me the best Volvo 850 option is 302 discs and S70/C70 caliper bracket with a suitable pad (dependent on intended driving situation), with new fluid, braided hoses and everything free, clean, lubed and in tip top condition this set up should prove a big improvement over stock while retaining it's overall balance.

[FaKaWi]

Most of us are more than aware that there is no increase in braking force/friction due to the nature of how brakes work, unless the servo is altered to allow greater pressure on the brake piston.

The major reason why most people opt for the bigger brake kit is to reduce fade issues, which is a known problem with running the T5 cars on track owing to the weight of them. The larger surface area of the components within the BBK means it takes longer for the heat to build up in the first place, and also allows for faster cooling of the disc/pad surface area.

With regards to the best option from Volvo themselves, there was actually a factory option of a 320mm brake setup on the Phase 1 cars.

[tt82]

id have to disagree with that.

1st: a larger diameter disc will have more braking power, it works simply like a lever. next time you are taking your wheels off, try putting force on the wrench right next to the nut, at the end of the wrench and then with a breakers bar. you will notice you need progressively less effort the further away you are.

2nd:

After 50 milliseconds (it's actually much faster than that - 7 to 10 milliseconds is typical - but it makes the math easier) the ABS takes another snapshot of the wheel speed information in an attempt to figure out what's going on. This time the wheel speed sensors are all reporting a speed of 69 mph. Doing a quick calculation, the ABS determines that in order to have slowed 1 MPH in a 50ms period the wheels must be decelerating at a rate of 0.91g's. Because you are driving a sports car, the engineer who calibrated the system 'taught' the ABS that your car is capable of decelerating at this rate, so the ABS continues to hang back and watch the event from the spectator's booth. No problem so far.

The next 50ms, however, are a little more interesting. This time around, the wheels are reporting 66.5 mph. Now, it may not seem like a big jump, but to slow 1.5 MPH in a 50ms window equates to a deceleration of 1.36g's. Not alarming, but the ABS 'knows' that based on this deceleration level, the wheels are probably beginning to slip a little more than they should - after all, your car is probably not decelerating at quite 1.36g's..and any error between the two indicates slip.

ABS is now in "ready mode." It's probably too soon to jump in, as the wheels might spin back up on their own in the next 50ms loop, but things are definitely looking bad!

This is not an ABS (Anti-Lock Brake System) he is describing, it would be EBD (Electronic Brake Distribution)

But going with his theory, does this mean that there is no point in putting high end premium tyres on your car, as no matter what rubber you have, the EBD dictates when your wheels are slipping based on a fixed measurement. Your tyres could be performing well and well within their grip levels but the abs decides your stopping to fast and reduces the braking pressure.

I think this part of his theory is flawed, surely it is a difference in wheel speeds that activites EBD and not a pre-determined braking force/speed.

Personally i think the problem with bigger brakes is not the size of the disc but the amount/size of the pistons in the calipers in relation to the original amount/size of pistons and the brake master cylinder.

If your normal brake set up is a 1 piston set up, then your brake is on a 1:1 ratio, fitting a caliper with two pistons the same size as the original piston, then puts you on a 1:2 ratio, meaning you have to put twice as much fluid into the system to get the same pressure. if you have a 4 piston caliper, all 4 pistons the same size as original piston, then you are on a 1:4 ratio, meaning you have to put four times as much fluid into the system to get the same pressure.

your problem would then be that you would hit the floor on your brake peddle without ever getting any substantial force on the disc and resulting in a longer braking distance.

[Simon J]

Originally Posted by FaKaWi

Most of us are more than aware that there is no increase in braking force/friction due to the nature of how brakes work, unless the servo is altered to allow greater pressure on the brake piston.

I hope most of us are more than aware that increasing the diametre of the disc will increase the braking torque and that increasing the coefficient of friction of the pads will also increase braking torque!

[Simon J]

Originally Posted by tt82

id have to disagree with that.

Personally i think the problem with bigger brakes is not the size of the disc but the amount/size of the pistons in the calipers in relation to the original amount/size of pistons and the brake master cylinder.

If your normal brake set up is a 1 piston set up, then your brake is on a 1:1 ratio, fitting a caliper with two pistons the same size as the original piston, then puts you on a 1:2 ratio, meaning you have to put twice as much fluid into the system to get the same pressure. if you have a 4 piston caliper, all 4 pistons the same size as original piston, then you are on a 1:4 ratio, meaning you have to put four times as much fluid into the system to get the same pressure.

your problem would then be that you would hit the floor on your brake peddle without ever getting any substantial force on the disc and resulting in a longer braking distance.

This is complete nonsense! The volume of fluid has nothing whatsoever to do with pressure. And two opposing pistons will displace exactly the same amount of fluid as one when the brakes are applied.

And who would be crazy enough to think of putting four pistons the same size as the single one in a caliper? That would give you twice the clamping force (not four times) which would totally upset your brake balance. Bigger brakes in the sense of bigger disc diametre, make sense (more or less) but just putting bigger calipers and pistons on is madness. Even the 320mm brakes that Volvo offered use exactly the same piston size as the standard calipers thus maintaining the brake balance for the ABS.

[tt82]

Originally Posted by Simon J

This is complete nonsense! The volume of fluid has nothing whatsoever to do with pressure. And two opposing pistons will displace exactly the same amount of fluid as one when the brakes are applied.

And who would be crazy enough to think of putting four pistons the same size as the single one in a caliper? That would give you twice the clamping force (not four times) which would totally upset your brake balance. Bigger brakes in the sense of bigger disc diametre, make sense (more or less) but just putting bigger calipers and pistons on is madness. Even the 320mm brakes that Volvo offered use exactly the same piston size as the standard calipers thus maintaining the brake balance for the ABS.

nonsense?????? how do you come by that conclusion????? you need to be able to put the braking fluid under pressure to create pressure on the disc with the pads, if your master cylinder piston reaches the end off its travel before generating the required pressure then you will not get the same braking performance

as for the sizes, im talking about off the shelf aftermarket alternatives, it was just for example to make it easier to explain, in reality a small difference in caliper sizes can have adverse effects on your braking system.

[thebigv]

The braking system will only hold so much fluid and only create a maximum amount of pressure. The reason bigger calipers with more pistons in works is because they even the pressure over the entirity of the pad, allowing for maximum pressure to be excerted right across the braking surface. Bigger discs and pads creat a greater amounts of friction which is what braking is, momentum turned into heat. Meaning bigger discs (to a certain extent) are better. If i'm wrong i suggest you contact every racing team in the world and tell them to stop buying expensive big brakes as cheap small ones will do.

However big discs on the back should be a no no unless the fronts have more bite and are bigger as it will cause the car to swap ends under heavy breaking

[Simon J]

Originally Posted by thebigv

The braking system will only hold so much fluid and only create a maximum amount of pressure. The reason bigger calipers with more pistons in works is because they even the pressure over the entirity of the pad, allowing for maximum pressure to be excerted right across the braking surface. Bigger discs and pads creat a greater amounts of friction which is what braking is, momentum turned into heat. Meaning bigger discs (to a certain extent) are better. If i'm wrong i suggest you contact every racing team in the world and tell them to stop buying expensive big brakes as cheap small ones will do.

However big discs on the back should be a no no unless the fronts have more bite and are bigger as it will cause the car to swap ends under heavy breaking

More nonsense! I suggest you (and others) read this article which explains in straightforward terms the physics of braking. You've clearly forgotten what you were taught in O level physics at school!

[thebigv]

Originally Posted by Simon J

More nonsense! I suggest you (and others) read this article which explains in straightforward terms the physics of braking. You've clearly forgotten what you were taught in O level physics at school!

I know from experience, Physics plays a part in everyday life but if we all calculated how fast our breakfast hit the stomach wall we'd starve ourselves.

By the way i'll send the link to Brembo, AP, TMD etc and tell them to shut up shop, and take the day off.

I was taught music because it was on the curriculum but it didn't help in my choice of stereo.

Some people need to calm down a little me thinks. The original concern was about brake balance, which incidently is adjustable from corner to corner on a track and if your being picky, on the road if your driving hard. The reason manufacturers put the brakes on they do is because they are balanced for that car under all possible driving conditions. People who put big brakes on are adjusting the conditions and are adjusting their braking to suit, i'm sure someone who managed to get a few hundred horsepower more from their car and crashed it cos they left small brakes on will appreciate your physics lesson. Until then take life a little less seriously.

[Simon J]

Originally Posted by thebigv

By the way i'll send the link to Brembo, AP, TMD etc and tell them to shut up shop, and take the day off.

Hmmm....interesting idea.

But I think you'd better read the article first. You wouldn't want them to think you didn't know what you were talking about in your covering email, would you?