GRRR
|
| posted on 3/2/12 at 06:06 PM |
|
|
Specifying Brake Disc Size?
Hi all,
I'm running through brake disc options for a Mid Engined kit car project with the following target specs:
Weight (wet) ~ 750kg
16" Wheels
150 - 250bhp depending on spec
Useage is expected to be 65% road use 35% track day.
and I'm picking components at present. Lots of options for brakes, with custom bells and rotors etc, but to start with I want to use OEM stuff,
my first thoughts were 280mm x 10mm solid discs all round with HiSpec Ultralites ont the front, VW alloy 38mm piston handbrake caliper on the rear.
The trouble is most front engined cars have tiny solid discs leaving me with few options that will require the centre bore to be opened up, and new
stud holes on a new pcd drilled.
So before I plough ahead with sorting out this discs size, does anyone have any experience with sizing of discs? Does 280 sound ok? Or is it a case of
try it and see?
I note the elise has 288ish vented all round, my old MR2 had 308ish venteds, but then the lightweight MEVs reuse standard focus discs which are pretty
small. I wouldn't have thought I'd need vented at this vehicle weight and would be better building and keeping sensible heat in the solid
discs.
Thanks in advance all, I know there's some bulging brain cells on this forum. As long as you've not all hit the friday night beers yet..
|
|
|
|
|
loggyboy
|
| posted on 3/2/12 at 06:53 PM |
|
|
My 750 kilo 180bhp Nova stopped perfectly on 256mm standard vented GM callipers/discs.
Not sure how it would have stood up to long term track abuse, but i know 288s cavalier turbo/v6 brakes were a popular upgrade. Is it a road or track
targetted car?
[Edited on 3/2/12 by loggyboy]
|
|
|
GRRR
|
| posted on 3/2/12 at 07:00 PM |
|
|
In black and white terms it looks like a track car, but certainly will get plenty of road use. But I think people would expect it to be fully capable
of several hard laps on a track day without fade.
|
|
|
MakeEverything
|
| posted on 3/2/12 at 07:05 PM |
|
|
I would stick with Sierra stuff. Its cheap and can be upgraded later. Ive got solid discs all round, and plan to cross drill them when i get a new
set, to relieve some heat.
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
|
|
|
loggyboy
|
| posted on 3/2/12 at 07:05 PM |
|
|
vented discs, circa 280mm would be my choice. It not like you need to retain much heat in metal discs. 750kg is 'quite' heavy in kitcar
terms.
|
|
|
loggyboy
|
| posted on 3/2/12 at 07:06 PM |
|
|
quote:
Originally posted by MakeEverything
I would stick with Sierra stuff. Its cheap and can be upgraded later. Ive got solid discs all round, and plan to cross drill them when i get a new
set, to relieve some heat.
Isnt drilling discs bad, I thought this can lead to cracking under extreme heat.
|
|
|
mark chandler
|
| posted on 3/2/12 at 07:07 PM |
|
|
I have vented discs sat within 13" wheels on my little car, I cannot even get them hot on the track.
Do not under estimate the power of little brakes, they were designed for much heavier cars.
|
|
|
rdodger
|
| posted on 3/2/12 at 07:21 PM |
|
|
On my GTM Libra, 750ish KG with 170bhp I ran 305mm discs with AP calipers on the front and 260mm MGF solid rears. It was immense on track and never
even hinted they might fade. They very rarely got hot enough on the road though.
The best all round set up on the Libra I found to be 280mm with Hispec 4 pots and MGF on the rear. My prefered pad was Mintex 1144.
|
|
|
GRRR
|
| posted on 3/2/12 at 07:24 PM |
|
|
There is an element of looks inevitably, 252 rear sierra discs might look a bit feeble under 16" wheels! I can use 274 vented on the front, 280
solid on the rear with no machining required, and that means i can use VW rear calipers, then use a dual master cylinder pedal box to trim the bias,
that might be a reasonable setup? Definitely useful to hear you guys' track experiences though as I have none yet!
Thanks
|
|
|
GRRR
|
| posted on 3/2/12 at 07:29 PM |
|
|
quote:
Originally posted by rdodger
On my GTM Libra, 750ish KG with 170bhp I ran 305mm discs with AP calipers on the front and 260mm MGF solid rears. It was immense on track and never
even hinted they might fade. They very rarely got hot enough on the road though.
The best all round set up on the Libra I found to be 280mm with Hispec 4 pots and MGF on the rear. My prefered pad was Mintex 1144.
Was going to PM you but I guess forums are for sharing the knowledge! This is useful info, so would you say your track setup was too big for the road
i.e. cold slippy brakes half the time?
The 'all round setup' sounds close to what i was planning, was that a 4x100pcd, if so what were the fronts (280 discs) from?
On both, how did you balance them, master cylinder with bias-bar?
|
|
|
tadltd
|
| posted on 7/2/12 at 01:00 AM |
|
|
Are you designing this car?
If so, why aren't you working out what size brakes you need? Do the calc's...
Best Regards,
Steve.
www.turnerautosport.com
|
|
|
coyoteboy
|
| posted on 7/2/12 at 02:15 AM |
|
|
quote:
Originally posted by tadltd
Are you designing this car?
If so, why aren't you working out what size brakes you need? Do the calc's...
Slightly harsh, from the post I read that he was planning on doing the calcs but identifying what's the generally accepted sizings out there on
similar vehicles.
Certain well respected designers advocate the "see what everyone else is doing that works and work from there" principal, why start from
scratch on every problem?
|
|
|
tul214
|
| posted on 7/2/12 at 06:30 AM |
|
|
If you are looking for a 4x100 fitment, what about the R56 Mini Cooper S brakes. That uses 294mm x 22mm vented on the front and 259mmx10mm solids on
the rear.
1.6 Raw Super6 sold
|
|
|
britishtrident
|
| posted on 7/2/12 at 08:23 AM |
|
|
Fitting rear discs that are a larger diameter than the front discs even on a mid-engined car is not a good idea, even a rear engined car like a
911 or Davrian works best with equal sized discs all round.
A main objective should be to get similar pedal response characteristics from the front and rear brakes as they warm up.
Race & rally davrians used 10.25" (260mm) dia discs all round and like the rdodgers's Libra brakes were pretty immense.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
|
|
|
rdodger
|
| posted on 7/2/12 at 09:14 AM |
|
|
quote:
Originally posted by GRRR
quote:
Originally posted by rdodger
On my GTM Libra, 750ish KG with 170bhp I ran 305mm discs with AP calipers on the front and 260mm MGF solid rears. It was immense on track and never
even hinted they might fade. They very rarely got hot enough on the road though.
The best all round set up on the Libra I found to be 280mm with Hispec 4 pots and MGF on the rear. My prefered pad was Mintex 1144.
Was going to PM you but I guess forums are for sharing the knowledge! This is useful info, so would you say your track setup was too big for the road
i.e. cold slippy brakes half the time?
The 'all round setup' sounds close to what i was planning, was that a 4x100pcd, if so what were the fronts (280 discs) from?
On both, how did you balance them, master cylinder with bias-bar?
The MGF PCD is something odd at 98. whatever.
If you fancy 280mm the VW Corrado G60 disc is cheap and 280mm 100 pcd.
Balance was "designed" in. It used a Metro master cylinder and no balance bar etc. I spent a some time getting the balance right using
pads and different rear discs. At 1 point I had 280mm on the rear but that led to some interesting rear lock ups.
To answer the question yes the fronts just never got hot on the road. Well not UK roads. They got plenty hot on the Col de Turini. They still worked
well just not as sharp as you would expect.
To be honest the Libra set up is handicapped by the pedal ratio at something like 4:1. That was on my list if I had kept the car.
|
|
|
coyoteboy
|
| posted on 7/2/12 at 10:30 AM |
|
|
quote:
A main objective should be to get similar pedal response characteristics from the front and rear brakes as they warm up.
Naturally. But that would surely rely on having the same braking power at each axle, which generally you don't, even on a mid or rear
engine'd setup? Even mid/rear you still have a fairly large forward brake bias, similar sized discs would heat up faster at the front, reducing
this?
And I have to say, on the 911, the rears don't look even close to the same size as the fronts:
http://911car.net/wp-content/uploads/2011/04/911-Side-View.jpg
[Edited on 7/2/12 by coyoteboy]
|
|
|
britishtrident
|
| posted on 7/2/12 at 11:55 AM |
|
|
The trick is to balance the way the brakes warm up so you don't end up with the brake balance being different hot and cold, the key is to
match the rate of increase in brake temperature ie if the rear brakes do 40% of the work the rear brakes should have 2/3rd the heat dispersal
capacity of the fronts.
FIAT had major problems with this on the Lamcia (Beta) Montecarlo so bad they halted production for a couple of years FIATs quick and dirty
solution to the premature front locking problem was novel they simple deleted the brake servo ---- Yes I know !!!!!!! ho-hum ;-) Needless
to say the revised Mk2 Beta Montecalo till had lousy brakes.
Incidentally the reason why the MG TF and later Metro wheel PCD size was odd is it was changed so the they could use production tooling from
the Morris Marina/Triumph Herald era. In contrast the Rover 25/45 PCD was 100mm ---- ??????
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
|
|
|
coyoteboy
|
| posted on 7/2/12 at 12:03 PM |
|
|
quote:
The trick is to balance the way the brakes warm up so you don't end up with the brake balance being different hot and cold, the key is to match
the rate of increase in brake temperature ie if the rear brakes do 40% of the work the rear brakes should have 2/3rd the heat dispersal capacity of
the fronts.
But doesn't that lead to smaller rears, assuming as I do from literature and some level of practice, that usually front brakes do 70-80% of the
work? Meaning same-size rotors would be a bit odd? Not trying to hound you but you seem to be contradicting your original statement, unless you are
saying most mid/rear cars have 50:50 braking distribution, which seems unlikely.
|
|
|
britishtrident
|
| posted on 7/2/12 at 12:54 PM |
|
|
In a rear engine racer/lightweight sports car the static weight weight distribution is likely to be around 30% front and 70% rear A
mid-engined will have a bit more weight on the front but still no more than about 40%. front 60% rear
Under braking due to dynamic transfer the rear engined car will have 50%-50% The mid engined car car more like 60%-40%. In contrast a front
engined car will be anywhere from 70% front 30% rear on a Seven style car to 95% front 5% rear on a smallest fwd
On Imp engined Davrian race and rally cars the brake were identical front and rear with same bore master cylinders in each circuit, I remember the
verbal instructions from Tim Duffee at Davrian were spot on "Set the the balance bar dead centre there should need no more than one turn
adjustment in either direction to suit conditions"
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
|
|
|
hughpinder
|
| posted on 7/2/12 at 02:35 PM |
|
|
Ok, Ive done some calcs for my midi:
The weight including 80kg passenger will be ca 690 kg, with a 61R/39FR weight split when static.
At 1.3G braking the weight split will be 42R/58 front.
At 1.9g braking the weight split will be 33R/67F (maximum with warmed up track tyres and perfect surface before fronts will lock)
Don't forget the braking force is proportional to the disc diameter (at the centre of the pad roughly, not the outer diameter) and the area of
the pistons pushing the pads onto the disk. I'm assuming you use the same type of brake pads front and rear. I'm using a dual circuit
master cylinder with the same bores to front and rear (so I could do a traditional split circuit with one front and the opposite diagonal rear on each
circuit if I wanted)
I plan to use mx5 calipers at the front, since I'm using mx5 uprights, which have a piston diameter of 51mm, and mk1 disks are 184mm OD.
To balance this, I have chosen VW golf rear calipers (31.8mm diameter pistons) and 200mm disks)
This will give a situation in wet or dry with road or track tyres, where the fronts lock before the rears. So you can have a larger rear disk at the
rear - it all depends on your caliper cross sectional area. The VW calipers only have something like 64% of the front caliper cross section in my
case.
Hope that helps
Hugh
|
|
|
coyoteboy
|
| posted on 7/2/12 at 03:11 PM |
|
|
FYI the 2011 Atom V8 runs 290mm Vented 4 pots front and rear. Seems over-braked intuitively but I've run no calcs and I'm sure they
have...
|
|
|
britishtrident
|
| posted on 7/2/12 at 04:06 PM |
|
|
An interesting thing that comes in the maths is assuming the the braking force acts at the radius of centre of the caliper piston/pad then in ball
park figures the majority of the braking energy is absorbed by the part of the disk swept by outer 1/3rd of the pad.
The ability of the brake disc to dissipate heat energy of course goes up in proportion to the outer diameter of the disc.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
|
|
|
rdodger
|
| posted on 7/2/12 at 04:29 PM |
|
|
I think that if the car being built is around the 700kg mark then I would be having a close look at what the S1 Elise and exige use. Both in terms of
disc size but caliper and master cylinder. From there a small increase both front and back should see it about right for track work IMO.
|
|
|
coyoteboy
|
| posted on 7/2/12 at 04:39 PM |
|
|
FYI Elise S2 (190hp):
282mm diameter ventilated discs with Lotus/AP Racing 2 piston aluminium front callipers and racing pads.
Generally accepted as plenty for track use too, from what I read.
|
|
|
tadltd
|
| posted on 7/2/12 at 07:32 PM |
|
|
Sorry, I fail to see what's harsh about this. I thought it was a fair statement, and so did my colleagues in the design office. In fact they
found it baffling that someone even mentioned that my questions were harsh.
With all due respect, if you're designing a car from scratch, you cannot randomly select braking performance based on opinions, conjecture and
what other solutions look like. At best it's poor engineering, at worst it's fatally dangerous. Not just to the driver and passenger(s),
but to other road users, pedestrians, etc...
Doing the calc's will define the parameters of the system that are important, then allow the proper sizing of the components to deliver the
solution. From this information you can select an appropriate set of components from what's available off the shelf, with contingency/factor of
safety, and therefore with minimal risk. You can then look at what other solutions are applied on other vehicles to get a feel for whether
you're in the right ball park with your own design.
In the original post, I didn't see any mention or indication that calculations were going to be carried out, which is why I mentioned it. It
also concerned me that "looks" seemed to be important in the selection of parts.
(BTW - It's also worth remembering that most OEM brake systems rely on vacuum assistance to aid performance...)
If the 'harsh' questions make someone stop and think about what they're doing, then lead to a safe design solution that keeps them
out of harm's way when that solution is called upon to perform, then I'll keep asking them.
These cars can be enormous fun, but don't ever lose sight of the fact that they can be dangerous. A good braking system is the first line of
defence when things start to go wrong.
quote:
Originally posted by coyoteboy
quote:
Originally posted by tadltd
Are you designing this car?
If so, why aren't you working out what size brakes you need? Do the calc's...
Slightly harsh, from the post I read that he was planning on doing the calcs but identifying what's the generally accepted sizings out there on
similar vehicles.
Certain well respected designers advocate the "see what everyone else is doing that works and work from there" principal, why start from
scratch on every problem?
then
Best Regards,
Steve.
www.turnerautosport.com
|
|
|
