Victors Mutterings part 3

[Victor Meldrew]

I needed disks foe the rear so the Ali bells that were on the front are now on the rear. I didn't go for the upgrade calipers, just the bigger disks.

My idea is that when sprinting the brakes never really get hot so I will use a pad that works well from cold with the extra leverage you will get with bigger disks.

All part of the master plan of getting the car ready for the Audi engine conversion in 2010.

[mac]

Has anyone done the maths to prove or otherwise if the large discs have any real effect on braking?


Mac

[steve_weegie]

Has anyone done the maths to prove or otherwise if the large discs have any real effect on braking?


Mac

As torque equals force x distance, as you increase the size of the disc the torque increases as the outer most edge of the pad is further away from the center of rotation. The force emelent is derived from the friction between the disc and the pad. If this remains constant, the applied braking torque is increased with the size of the disc.

As you also have a larger surface area, the heat lost from the disc will be increased also.

All this REALLY does however is move the brake balance towards the front of the car, until you take into account the extra ability to dissipate heat.

On the down side, if you take 2 discs of the same material density, the larger the disc will have a greater gyroscopic effect than the smaller one. Probably less important on cars than on motorbikes however, but you may still notice these effects on turn in.

Before I became terminally skint, i looked at machining out the Brembo rear calipers to take bigger pistons while fitting large AP rotors at the front, to bring the brake balance back to normal.

Cheers,

Steve

[mac]

I understand the theory of it, but does it work in practice?

How much difference is there by moving the caliper out by x mm - could it be that to be a worth while gain it would need to move by centimeters rather than millimeters.


I don't belive that the braking system on the elise was produced by chance and that all of these things would have been investigated at some point prior to the car going into production.


Mac

[robin]

Until you're sure you are pressing the pedal as hard as you can for the required duration, it's academic to worry about pad material A vs B or disk radius X vs Y.

If you can lock your wheels at speed then you have more brakes than you actually need, and that makes it harder (because now you have to deal with modulating the brakes at speed, which means you travel a long way whilst fannying about cadence braking or whatever you call it). I've only ever had one car that did that ... the first S2 I had on RS15/RS19 combination, standard disks, standard tyres ... at 80 mph you could stand on the pedal and lock the front wheels

What I am suggesting to Victor is that he cuts his losses on the APs and just buys a set of standard rear disks and slaps them on ... reason being that they will work just as well at the rear and he can then move his APs back to the front where they belong ... although the brake balance on the Elise is not ideal, it is moderately safe. Increasing rear braking effort without doing something similar at the front is inadvisable IMHO ... the effects of locking just the rears and not the fronts are unpleasant to say the least (unless you're Ape-like).

Don't forget you can add rear braking by well orchestrated down shifting and once you compress your braking into the smallest possible area, you'll have to be doing that anyway.

Anyway, was really just trying to help Victor get his car moving again, not start a huge argument about whether or not you need big disks, which you obviously don't

Cheers,
Robin

[steve_weegie]

Here's some rudimentary calculations for torque increases against disc size....

std 288mm
torque applied at 1 newton force = 1 x (288/2) = 0.144 N/m

295mm
torque applied at 1 newton force = 1 x (295/2) = 0.147 N/m = 2% increase in torque over standard

305mm
torque applied at 1 newton force = 1 x (295/2) = 0.152 N/m = 6% increase in torque over standard

Going down the AP route isnt just about increasing disc size tho - although the rotors are larger than standard, they are lighter than the EP ali belled rotors at 288mm. Also, the probs i've had with standard discs and the EP ones are related to warping / run-out at high temps... The AP bell to rotor mount deals with the different expansion rates of the alu and steel better IMHO.

Cheers,

Steve

[tut]

I think that I have locked up twice on the track, going into a corner too fast, that is with EBC discs on the front and MMC's on the rear.

tut

[alicrozier]

I think that I have locked up twice on the track, going into a corner too fast, that is with EBC discs on the front and MMC's on the rear.

tut

Found one of them...

[tut]

Nice one Ali, flat spotted my tyre on that one.

tut

[robin]

Is that charade?

Cheers,
Robin

[mckeann]

Is that charade?

Cheers,
Robin

Yep

And Tut, your just not trying hard enough

[Victor Meldrew]

Until you're sure you are pressing the pedal as hard as you can for the required duration, it's academic to worry about pad material A vs B or disk radius X vs Y.

If you can lock your wheels at speed then you have more brakes than you actually need, and that makes it harder (because now you have to deal with modulating the brakes at speed, which means you travel a long way whilst fannying about cadence braking or whatever you call it). I've only ever had one car that did that ... the first S2 I had on RS15/RS19 combination, standard disks, standard tyres ... at 80 mph you could stand on the pedal and lock the front wheels

What I am suggesting to Victor is that he cuts his losses on the APs and just buys a set of standard rear disks and slaps them on ... reason being that they will work just as well at the rear and he can then move his APs back to the front where they belong ... although the brake balance on the Elise is not ideal, it is moderately safe. Increasing rear braking effort without doing something similar at the front is inadvisable IMHO ... the effects of locking just the rears and not the fronts are unpleasant to say the least (unless you're Ape-like).

Don't forget you can add rear braking by well orchestrated down shifting and once you compress your braking into the smallest possible area, you'll have to be doing that anyway.

Anyway, was really just trying to help Victor get his car moving again, not start a huge argument about whether or not you need big disks, which you obviously don't

Cheers,
Robin

Slight misunderstanding I think..

Current set up (pre-rebuild) Elise Parts Ali bells on front, EBC groove on rear..
Planned set up (post-rebuild) AP Ali bells(larger) on front, Elise Parts Ali bells on rear..

If I am right, the std Elise parts ali bells are the same diameter compared to normal discs, so moving them to the rear should not increase the rear braking effect.. The fronts will get the AP larger diameter discs, more brake effect on the front..

I have had on several occasions had the rear end begin to lock up under braking.. several things could have caused this, probably poor gearchange technique, or possibly a missmatch of pad materials.. i.e. unknown original rear pads with EBC Greens on the front.

So the car will now have relativly new 14's all round with good disks..

Will probably change them to EBC Greens as they have better "cold" bite than the 14's

[fd]

it's academic to worry about pad material A vs B

Hmmm, don't agree at all, it's not about ultimate friction or being able to lock wheels (which any monkey can do by standing on the pedal) it's about only just not locking wheels and being able to do that predictably time after time, corner 1 or lap 100 . . . and pad material is, in my experience, critical to that . . .

On this whole subject I don't understand the need for bigger brakes on an Elise . . . I have never, once, had brake fade in my elise with significantly over standard power and a harsh braking style . . . meanwhile I can kill the (tiny) caterham brakes in 10 laps of KH (with some pad compounds) . . . still . . . each to their own . . . whatever floats your boat . . . second I guess changing the brake balance on, say, an elise can far more easily be achieved by a rear circuit pressure reg than by changing the calipers or disks . . . common practice on other cars and works very effectively for, ooooh, 50 quid ? . . . having said that I've found the elise brake bias to be very good when you don't dick with the engine and gearbox while braking . . . like you're supposed to . . .

$0.02

Fd

[pete]

it's academic to worry about pad material A vs B

Hmmm, don't agree at all, it's not about ultimate friction or being able to lock wheels (which any monkey can do by standing on the pedal) it's about only just not locking wheels and being able to do that predictably time after time, corner 1 or lap 100 . . . and pad material is, in my experience, critical to that . . .

On this whole subject I don't understand the need for bigger brakes on an Elise . . . I have never, once, had brake fade in my elise with significantly over standard power and a harsh braking style . . . meanwhile I can kill the (tiny) caterham brakes in 10 laps of KH (with some pad compounds) . . . still . . . each to their own . . . whatever floats your boat . . . second I guess changing the brake balance on, say, an elise can far more easily be achieved by a rear circuit pressure reg than by changing the calipers or disks . . . common practice on other cars and works very effectively for, ooooh, 50 quid ? . . . having said that I've found the elise brake bias to be very good when you don't dick with the engine and gearbox while braking . . . like you're supposed to . . .

$0.02

Fd

Hmm. I am very hard on brakes. On the road or track I have never cooked the elise brakes. On every other car I have owned I have cooked the brakes on the road.

My Elise has braided hoses and Mintex 1144s. They have made no difference from standard a far as I can tell, but I can't drive for toffee.

IMHO the Elise brakes are fine, until I read FD's post I though I was alone.

[mckeann]

On this whole subject I don't understand the need for bigger brakes on an Elise . . . I have never, once, had brake fade in my elise with significantly over standard power and a harsh braking style



second I guess changing the brake balance on, say, an elise can far more easily be achieved by a rear circuit pressure reg than by changing the calipers or disks . . . common practice on other cars and works very effectively for, ooooh, 50 quid ?

$0.02

Fd

First point, i agree with. I only JUST found the limits of the brake system in my car, with 223bhp, on very heavy braking tracks, on sticky tyres, and with my also harsh braking style. Thats why i said not to bother with the AP discs to victor. EP bells will be good enough for 99% of people.

Second point, i think i maybe misunderstood. Generally, the people in the know (i.e randy and walshy) advocate MORE rear braking. You cant achieve that with a pressure regulator, as they reduce pressure. Sure you could probably fit it to the front system, but thats probably not the best way to go about it. In my experience, i found my yellow car was on the limit for rear braking in the dry. In the wet, it could have done with a bit more rearwards bias.