coyoteboy
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| posted on 3/1/13 at 12:33 AM |
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Caliper heat percentage
I've read a few locations suggesting the calipers get approximately 20% of the heat rejected by the brakes, while the rotors receive the
remaining 80%. While this is clearly a vast generalisation (Caliper at 400C won't accept any heat from a pad at 400C, but will when the caliper
is at 20C etc), does anyone have reason to doubt this rule of thumb? I don't have figures for heat transfer between pad and caliper.
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snapper
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| posted on 3/1/13 at 06:29 AM |
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Really not sure what your trying to understand here
But given time all the materials will try and achieve the same temperature, that's the laws of physics
What your saying is that the calipers are exposed to 20% energy created by pad to disc friction which is converted to heat and disapated by the disc
to atmosphere
You mention 400*c are you concerned about the boiling point of your brake fluid?
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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MikeRJ
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| posted on 3/1/13 at 07:56 AM |
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quote:
Originally posted by snapper
Really not sure what your trying to understand here
I think it's just a question of thermal impedance, i.e. given the heat energy which is generated at the disc/pad interface, how much of that
energy goes into heating the disc vs. the caliper. I suspect 20% may be a little on the high side to be honest, there is relatively little contact
between the pad and the caliper.
On a vaguely related note, I was recently looking for some paint to sort out a set of slightly manky calipers on my Civic Type R. I didn't want
any bling colours, idealy something to bring them back to the silver/grey OEM look. I discovered Halfords sold some
caliper
paint of about the right colour, but it was only rated to 80C! The Halfords blurb says the caliper never gets hotter than this, which is utter,
utter BS of course.
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Ben_Copeland
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| posted on 3/1/13 at 08:06 AM |
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I used to use Hammerite on my calipers. Lasted the best out of most paints.
Ben
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britishtrident
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| posted on 3/1/13 at 08:33 AM |
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Brake pad material is a very good insulator, it has high resistance to the flow of heat, so very little heat energy flows through the pad to the
caliper.
Probably only about 5% of the heat energy from braking is dissipated through the caliper, the surface of the disc is cooled by airflow and the
temperature of the rubbing surface of the pad is kept under control by the temperature 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]
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Dingz
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| posted on 3/1/13 at 08:41 AM |
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Not quite sure why you are asking but standard brake fluid (without water) boils at around 200°, seals used in the calipers are rated at 150°. ‘We’
designed some special seals rated at 220° for big forklifts that carry containers, they were cooking the standard seals.
Phoned the local ramblers club today, but the bloke who answered just
went on and on.
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MikeRJ
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| posted on 3/1/13 at 10:54 AM |
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quote:
Originally posted by Dingz
Not quite sure why you are asking but standard brake fluid (without water) boils at around 200°
Depends on how much moisture it has absorbed. Standard DOT 5.1 is supposed to have a minimum dry boiling point of around 270C, and minimum wet
boiling point of 190C.
Seems odd you should never see the fluid boil without melting the seals however (unless it saturated with water).
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Bare
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| posted on 3/1/13 at 04:47 PM |
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pertinent question is ; under what fantasy would one's homebuilt car ever get the calipers hot enough to be of concern.
Unless one fitted Austin Mini discs :-)
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coyoteboy
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| posted on 3/1/13 at 11:37 PM |
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BT - you're erring towards the direction I was going (rotors and pads are the primary concern, ignore the calipers). Thanks for confirming my
thinking.
Bare - I'm selecting/designing my brakes to be as light as possible without being too light. The endless discussion of "you don't
need vented discs on these cars" interested me (I agree to some extent) to go back to basics and do the full calculations (admittedly with some
assumptions - I could do a full 3D model of each and simulate it but I'd like to get the car on the road some time soon). I've constructed
an excel "package" that takes everything from CofG position and height, brake piston diams, bias bar settings, pedal ratio, rotor
dimensions and and spits out:
Single stop from 100mph to view heating of disc at whatever required G.
10 stops from 100mph at required G, ignoring convection and assuming no cool-down time to give worst case scenario (possibly overkill).
Calculates brake force (torque) bias based on piston sizes, rotor sizes, balance bar settings and approx pad friction coefficient, to check if the
combination is stable (in two cases, single occupant and two).
Calculates if that'll overcome the approx force the tyre can generate (based on an average of a number of tyre data found from numerous
sources), including longitudinal weight transfer.
A few other bits an pieces.
I have absolutely no doubt that by choosing calipers and rotors without thought you could easily end up with a lethal situation and possibly exceed
the caliper and pad heat dissipation, especially if your car is a bit portly  I'll have a 300+hp mid-rwd car, I don't want to be unsure
about the brakes! I have an open book, blank page - I can use whatever I like, so I want to pick something that makes sense, not something that
everyone else uses.
I'm going to revise my estimate and assume 10% into the caliper as a happy medium.
Cheers all!
[Edited on 3/1/13 by coyoteboy]
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hughpinder
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| posted on 4/1/13 at 10:05 AM |
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Just for amusement I just did a calc for caliper heating assuming 10% of the cars kinetic energy goes into 1kg (each) aluminium calipers (This may not
be the best assumption, but its the maximum since all the energy you want to dissipate is kinetic energy) :
Max speed 140 mph = 62m/s
weight 650kg
=1.25Mj of kinetic energy
heat capacity of aly is 879J/kg/K
If your brakes are set up ok, you can probably get a midi to almost 50/50 front rear braking force under maximal braking (because your midi will be
weight biased to the rear static and will get closer to 50/50 as you brake), so I'll assume the energy is equally split from front to rear
calipers
for 4 kg aly (4 calipers), and 10% of the energy = 36 Kelvin temp change!
From 100mph (44 m/s) it's only 18 DegC.
Regards
Hugh
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coyoteboy
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| posted on 4/1/13 at 10:18 AM |
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Aye, a single stop isn't much of an issue, even rotors don't get much over 60 with a single stop even if I plumb in 750kg from 100mph.
After 10 stops the rotors are up near 600, which is borderline fine too. But the calipers are going to be limited to ~150 for safety reasons, though
you can effectively include the upright in the caliper mass too.
I too have worked on conversion of kinetic and splitting it via the brake force balance, but I've included radiation losses (negligible at lower
temp but quite a few watts when hot!) but I've absolutely no way of getting reliable data for convection without doing a full model - the
possible variations are huge so I've ignored it and called it worst case
It's quite enlightening to stick in different rotor dimensions and see just how quick you can take a solid rotor up to 1200
When I've tidied it up a bit I'll post the file so others can review/have a play.
[Edited on 4/1/13 by coyoteboy]
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britishtrident
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| posted on 4/1/13 at 12:55 PM |
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The brake disc cool will pretty rapidly when moving, even a short straight will bring the disc temperature down quite quickly, the cooling effect
will be even more pronounced with a vented disc as being a centrifugal fan the faster the rotational speed the more air gets pumped through the
cool slots.
Also don't forget the cooling effect of a spoked alloy wheel, when Rootes took the Sunbeam Tiger to the Le Mans 24hr race back in the early
1960s durring the Le mans test weekend they were running out of brakes at the end of the Mulsanne Straight . This was hardly surprising the Tiger
had brakes that were more or less the same as the MK3-5 Cortina. They fitted DS11 pads and Minilite Magnesium wheels and found they had had just
enough brakes to do the job.
Even with DOT4 brake fluid provided the brake fluid isn't saturated with water the problem is always going to be with the pad friction
material temperature rather than fluid boiling.
Provided the brakes have (1) The correct friction material for the job (2) a good flow of cooling air (3) and he discs have enough cooling area
then no problem.
Repeated 100 to 0 stops would be a very harsh test even for a circuit racer, for a normal road car three 70 to almost 0 mph stops in a
row without an excessive rise pedal pressure is a realistic test, for a very high performance road car after three 120mph to 30mph in quick
succession I would expect the pedal pressure to rise but not to rise excessively.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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coyoteboy
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| posted on 4/1/13 at 01:06 PM |
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OK, well I can happily adjust the test stops to suit a more realistic value - I've built that in as an easy variable, but there's no way I
can quantify conduction or convection. My plan is to keep disc temps under control (under 450C worst case) to hopefully keep pad temps under control.
Reducing the duty down to 3 100-0 stops you can bearly get the rotors up to 250 even with the assumption of 280mm solid rotors. If you add conduction
and convection to that I suspect the brakes will never be up to temp! hell, I could probably put motorbike rotors on there!
In fact, the more I look at it, the more I reckon I could get away with something like 4x R1 rotors and calipers!
[Edited on 4/1/13 by coyoteboy]
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hughpinder
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| posted on 4/1/13 at 01:39 PM |
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I think your entheusiasm is fantastic, but I also think this will prove very difficult!
I think it would be easier to just try it, or find some who races a car similar in performance/weight to yours, find out what they use and decide if
what you are thinking of is better or worse!
10 stops from 100 with no losses would get your caliper to 180 (Potentially, if you could achieve a steady state and assuming no heat losses anywhere
-thats easy to calculate), but of course thats based on a guess of 10% split to the caliper, and people have already proposed that this could be 5% or
20%, which gives a factor of 2 difference either way.
You have included radiation loss from the caliper, but what about the radiation GAIN from the huge glowing red lump of iron right next to it?
You will get into some pretty hairy multi variable/partial differential equations if you want to calculate this properly and will need to know the
thermal conductivity profile of your pads/calipers/disks and many other variables - I can think of:
temperature of the disk - varies with time, if you are braking or accelerating, start temperature etc, thus varying the convection, conduction and
radiation gain to the caliper, which in turn depend on the area facing the disk or the air, and distance between the disk and the pad .....
The heating of the disk/pad surface depends on how much energy you can dissipate in a unit of time - this will vary with the tyre temperature/level of
grip/road speed .....
temperature of the air - will affect the disk temperature, convection/conduction losses. I can remember trying to work out the cooling profile of a
hot spinning disk mounted horizontally in air placed between a vertical hot face in one direction and a cold one in the other, and that wasn't
easy. That had the advantage that the distances between disk and the faces was fixed, and both were considered to be at a steady temperature too, and
no heat input to the disk from other sources!
The effect of the time to accelerate and brake - obvioulsy the disk in in pure cooling during acceleration (assuming its hotter than the surroundings)
and a mixture of heating and cooling when braking, and the times for each will affect the temperature profile through the pad/caliper/disk - it
isn't a steady state.
Obviously you are in a condition of energy flow, and even a good conductor like aluminium will have some significant thermal gradient in a calculation
like this, so the centre will accumulate heat more than the edge furthest from the disk.
If you manage to actually do the calculations, I'll be hugely impressed (I know it would be beyond my maths now), and even more so if it is
possible to demonstrate they were accurate to +-20% when you finally get built!
Best of luck
Hugh
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coyoteboy
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| posted on 4/1/13 at 04:05 PM |
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I think it'll be tough too But I have to start from somewhere logical, while it's easy to suggest going from similar vehicles - that
depends on what compromises they chose, are willing to live with etc. The problem with designing a car from scratch is there's very few fixed
items - I could fit anything from 240mm solids to 350mm venteds, using single piston or 4 piston. And I'd probably find they'd all work
reasonably too. Of course I'm looking for examples with good heritage but I'm also wary not to use other people's info blindly I
could go 350s all round and just deal with the 14kg rotors, but that sort of defeats the purpose, so I just want to be able to do some calcs and stick
a stake in the ground where I think my best bet is.
I've included rad loss from the rotors and calipers but not included rad gain on the assumption that if it's at around the same temp
it'll see net zero, so it will only lose to ambient, the area of the caliper means it'll only see a few tens of watts at max temp. Good
point though, I could include it with some ease (and I like doing the maths!).
quote:
You will get into some pretty hairy multi variable/partial differential equations if you want to calculate this properly and will need to know
the thermal conductivity profile of your pads/calipers/disks and many other variables - I can think of:
Yeah, I'm quite comfy with that side of it (thermal calcs and simulations was my job for 4 years!) and I don't have that info (though I
could guestimate it from experience), but doing a real transient response simulation is possibly too much. You could write a thesis on the task, which
is fine for an academic but I don't have time to do stuff like that anymore! To be fair there's plenty of software options that will solve
a lot of the problems but ultimately the assumptions and estimates are the thing that kills the accuracy - which is why I am thinking it's
reasonable to do a stab at a steady state result and get it +- 33%, rather than do a full transient simulation with at least 1D calcs and get +-10%.
Don't get me wrong, I'd love to do the sims but I have the entire rest of the car to build, I'm just trying to strike a balance
between theory and just buying what's off the shelf. This should at least give me a "280's look too small, probably worth going to
300s, which confirms X, Y or Z's use of 300s".
You've got me wanting to do it "properly" now!
[Edited on 4/1/13 by coyoteboy]
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froggy
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| posted on 4/1/13 at 06:13 PM |
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I'm now on 285mm vents and Mazda rx7 4pots with a medium ebc pad and 260 vents on the back with sierra calipers and yellow ebc pads under
15" wheels . I ran 300mm solid rear discs but they couldn't cope with more than a few Trackday laps before getting too hot . The car is
850kg and running 450hp
[IMG]http://i144.photobucket.com/albums/r187/froggy_0[IMG]
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onenastyviper
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| posted on 4/1/13 at 07:16 PM |
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What were the symptoms?
quote:
Originally posted by froggy
I'm now on 285mm vents and Mazda rx7 4pots with a medium ebc pad and 260 vents on the back with sierra calipers and yellow ebc pads under
15" wheels . I ran 300mm solid rear discs but they couldn't cope with more than a few Trackday laps before getting too hot . The car is
850kg and running 450hp
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froggy
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| posted on 4/1/13 at 07:30 PM |
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Rear discs and calipers were a lot hotter than the fronts but I had set the balance as best I could on the road before its first trackday ,I was
running a wilwood powerlifting caliper with a ds2500 pad which are usually pretty good but winding more front on started locking the fronts . My car
needs a lot of work regarding handling etc so the brakes got a lot more punishment than they would if I could carry more speed through corners .
[IMG]http://i144.photobucket.com/albums/r187/froggy_0[IMG]
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coyoteboy
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| posted on 5/1/13 at 05:36 PM |
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Cheers for the info Froggy, looks like the sort of sizes I had planned for too (285/260, with 4 pots front and rear) - I'll throw in the new
stopping profile suggested by BT and adjust for caliper heat removal using 10% of the energy (rather than going down the correct route of setting heat
transfer coefficients) and see how the numbers fall out.
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onenastyviper
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| posted on 5/1/13 at 07:53 PM |
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Road setting of brake balance is not ideal - you would not (ordinarily) find out the limits on the road. Besides, if the rears are heating more than
the fronts - it indicates that the balance is wrong. If it is not a bias issue then could be a fundamental brake size issue.
Someone else will have a better idea but brake balance is not as simple as a 50/50 split - a lot depends on the weight distribution of the car - for
example, I have a fiesta with discs & drums: FWD with 80% of the weight over the front axle - I would expect to have near the same brake balance
front/rear.
Some thing to check (you probably have done this already):
1. check master cylinder sizes if running a dual setup with front/rear split - this is for sanity reasons.
2. check caliper data is what you would expect (i.e. piston size etc.) against other similar car types
One thing that others have demonstrated to me was that a car with a good brake balance will "tend" to have good handling characteristics -
I drove a Formula Ford and in my rookie tests, I started playing with the brake balance (a la schumacher) when I thought the car was not
"handling". Anyway, the tutor reset the brake balance "by feel" and told me not to touch it. After a few corners getting use
the the changes, I was immediately quicker and the car felt more planted and stable.
By feel, he jacked up the car so the wheels were free asked me to sit inside and press the brake pedal whilst he turned the front and rear wheels and
was adjusting the bias and was looking for the front to just lock (turning by hand) whilst the rears were still free(ish). OK, a FF has something like
a 50/50 weight split and a very low CG location.
I have just typed all this and not yet asked what type of car this is (duh!)...so what type of car is it?
quote:
Originally posted by froggy
Rear discs and calipers were a lot hotter than the fronts but I had set the balance as best I could on the road before its first trackday ,I was
running a wilwood powerlifting caliper with a ds2500 pad which are usually pretty good but winding more front on started locking the fronts . My car
needs a lot of work regarding handling etc so the brakes got a lot more punishment than they would if I could carry more speed through corners .
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froggy
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| posted on 5/1/13 at 09:33 PM |
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It's a reliant kitten with a transverse mid mount Saab turbo engine running 450+ hp . I did do the spin all wheels and alter the bias bar til
the fronts came on a touch before the rear and did a bit of fiddling on the road but its geared for 150 mph and gets there pretty sharpish so I
couldn't do any real adjustment until I got on a track . I was seeing 140+ mph on the long straights at Bedford and didn't have any issues
with the fronts locking so there was room for more front but on the road with colder tyres I did lock the fronts a few times . . Size wise I
can't go much bigger under 15" wheels . My track and wheelbase are the same as an Elise but I'm a bit on the heavy side at 940kg
fully fueled with me in the car .
[IMG]http://i144.photobucket.com/albums/r187/froggy_0[IMG]
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coyoteboy
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| posted on 5/1/13 at 10:49 PM |
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My calculations will tell you where to set your bias based on calipers and rotors. with certain common sizes it is actually fairly easy to get a
setup that will never be tuneable to ideal, it seems.
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onenastyviper
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| posted on 5/1/13 at 11:05 PM |
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Just found some pictures on another thread - very cool car (http://www.locostbuilders.co.uk/viewthread.php?tid=152056)
I would seriously recommend finding your static corner weights and estimating your front/rear split - it may surprise you (and me  )
Your cold road locking problem could be an indication that either your pads are too "trackday" orientated and have poor cold
"feel". Alternatively, you may not have enough dynamic weight transfer under road-conditions and so your car effectively becomes
over-braked at the front.
You have several options open to you if you don't mind testing:
1. reduce front Master Cylinder Bore or increase rear master cylinder bore, possibly both
2. reduce rear caliper acting radius (reduce disc size)
3. increase front caliper acting radius (increase disc size)
4. change calipers to different piston sizes
A lot of suck-it and see testing. I would seriously consider the corner-weighting solution to find out what actual braking capabilities are
required.
You have to estimate what your hardest deceleration you would expect under braking, i.e. 1g or 2g. From this and the other figures, you can find out
your static and dynamic brake balance required. Using your existing brake components, you can then figure out where in the envelope you are and what
you can do about it.
The problem with custom cars, especially with lots of horsepower is that you don't tend to have a team crunching the numbers to find out the
best compromise for the braking system. You may end up with figures that show your system is OK for track but poor for the road. At this point you
will have to make a choice and I would recommend that the road performance outweights the track performance for a safety perspective but it is
entirely possible that changing a single component could change your performance envelope.
Also, to show available solutions: NASCAR Sprint Cup cars use 15inch rims, steel discs and manage to stop a 3000lb stock car from 180mph+ (well, just
about stop )
As a comparison (and to show off really :cool F1 use 13inch rims  .
Edit:
A thought occured to me (a rare event ): what is the handling/balance like in the corners? Is it neutral, under or oversteer?
quote:
Originally posted by froggy
It's a reliant kitten with a transverse mid mount Saab turbo engine running 450+ hp . I did do the spin all wheels and alter the bias bar til
the fronts came on a touch before the rear and did a bit of fiddling on the road but its geared for 150 mph and gets there pretty sharpish so I
couldn't do any real adjustment until I got on a track . I was seeing 140+ mph on the long straights at Bedford and didn't have any issues
with the fronts locking so there was room for more front but on the road with colder tyres I did lock the fronts a few times . . Size wise I
can't go much bigger under 15" wheels . My track and wheelbase are the same as an Elise but I'm a bit on the heavy side at 940kg
fully fueled with me in the car .
[Edited on 5/1/13 by onenastyviper]
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froggy
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| posted on 5/1/13 at 11:15 PM |
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I haven't corner weighted it but my mot brake rollers give me axle weight which is near enough 60/40 but my main issue is the comedy handling
which is giving the brakes such a hard time , it feels like I have a fridge strapped to the roof as the rear roll angle just keeps building through
corners so I have to scrub so much speed off to hold a line through a bend but the new layout at the rear with a fairly big anti roll bar should help
. It's my first mid engined car so a pretty steep learning curve after 25 yrs of front engine rwd .
[IMG]http://i144.photobucket.com/albums/r187/froggy_0[IMG]
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onenastyviper
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| posted on 5/1/13 at 11:23 PM |
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Front and rear anti-roll bars fitted?
quote:
Originally posted by froggy
I haven't corner weighted it but my mot brake rollers give me axle weight which is near enough 60/40 but my main issue is the comedy handling
which is giving the brakes such a hard time , it feels like I have a fridge strapped to the roof as the rear roll angle just keeps building through
corners so I have to scrub so much speed off to hold a line through a bend but the new layout at the rear with a fairly big anti roll bar should help
. It's my first mid engined car so a pretty steep learning curve after 25 yrs of front engine rwd .
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