Hi Guy's

Not Locost, but I know you will have the right answers:

I am looking at upgrading the brakes on my Scimitar.

Current spec: 267mm solid discs, twin pot calipers with 2.25" pistons, giving total piston area per caliper of 7.95"^2. (5130mm^2)

If I use wilwood 4 pots with 1.62" pistons the total area would be 8.25"^2 (5319mm^2) .This represents an increase of about 4%.

If I use 4 pots with 1.75" pistons, the total area would be 9.62"^2 (6207mm^2), about 21% increase.

I will also be increasing the discs from 267mm to around 280-290mm, so braking torque would increase by 5-10% just due to disc size.

I will also use Greenstuff (or equivalent) pads, which I am lead to understand give 20-30% more friction over standard compounds.

If all 3 changes gave maximum improvements, I could be looking at 73% improvement in front brakes.

I do not intend to change MC size, or make any changes to the rear brakes (big drums), although I would consider upgrading the compound of the shoes.

My questions are:
1) Does anyone know how much difference this will make to pedal travel? (see q.2)
2) Broadly speaking, do drum braked rear wheel cylinders need to displace more fluid volume than disc brakes? i.e, I know the piston diameter is far smaller, but typically by how much (mm) do drum brake pistons travel (assuming they are well adjusted)
3) Following on from Q2, is it reasonable to assume that front calipers displace about the same as rear cylinders? If so, as I am only changing the front end, the overall increase in pedal travel would only be 10½%.
4) Would 10-20% extra pedal travel feel disasterous?
5) What would be considered to be the maximum increase in front brake efficiency before I have to increase the rears? I'm guessing that 73% increase would screw up the balance big time, but what could I get away with?

Sorry for all the questions, but I'm sure many of you will have done this before.




[Edited on 11/5/07 by TL]

That almost reads like an exam question.

You will end up with an over braked front end, and an arse end that does nothing (if you go for all your top optins that is). Utterly pointless IMO.

David

[Edited on 11/5/07 by flak monkey]

1) Does anyone know how much difference this will make to pedal travel? (see q.2)

Answer; Increased brake pad friction will reduce travel for each decel' level, as will increasing disc size, whilst the bigger brakes will increase fluid consumption (per mm of piston travel).

With so many changes you may find there is no change in pedal travel.

2) Broadly speaking, do drum braked rear wheel cylinders need to displace more fluid volume than disc brakes? i.e, I know the piston diameter is far smaller, but typically by how much (mm) do drum brake pistons travel (assuming they are well adjusted)

Answer; Drum brakes consume much less fluid. They are smaller diameter pistons and they only travel 3-6mm each (Max' ).

3) Following on from Q2, is it reasonable to assume that front calipers displace about the same as rear cylinders? If so, as I am only changing the front end, the overall increase in pedal travel would only be 10½%.

Answer; No - calipers will consume much more fluid.

4) Would 10-20% extra pedal travel feel disasterous?

Answer; Very subjective, you might like it while I might think it awful.

5) What would be considered to be the maximum increase in front brake efficiency before I have to increase the rears? I'm guessing that 73% increase would screw up the balance big time, but what could I get away with?

Answer; Only way to know this is to do the maths. But a scimitar will probably have 60% (approx' ) of it's weight over the front wheels (big, heavy Essex V6 weren't they??) when stationary so the front brakes should be doing between 60 and 100% of the braking. This all depends upon the position of the CofG, wheelbase, overall vehicle weight etc...

If the front brakes are doing more than their fair share you will end up increasing the stopping distances not reducing them. Balance is everything!

[Edited on 11/5/07 by JAG]

Jag said it all except:

Why are you wanting to upgrade the brakes - are they overheating - then do something about improving cooling (ventilated discs and/or cooling ducts or harder pads).

If you can't lock the front wheels then look at changing master cylinder to caliper ratio or pedal ratio etc etc.

Bad stopping distances or early locking up of wheels- could be shock/spring rate problem - define the problem and then research the solution.

Don't make major changes until you know what the problem is and what the least action to solve it is.

There is a calculator (spreadsheet) available somewhere on this forum that can assist in component size selection.

I'd second the question " why do you want to change them". I owned a Scimitar & seem to remember it had very good anchors, if you can fairly easily lock wheels without massive pedal pressure then changing the brakes may well be a retrograde step - as Flak says, overbraked. If the brakes don't work properly then surely much cheaper to fix the existing setup. The only other reason I could see for going for bigger brakes is if you're going for bigger tyres/wheels.

it always ammuses me with the "if you can lock up your front wheels your brakes are fine" old chestnut.

Anyone can stamp on their brakes and lock their wheels, just cause the wheels lock does not mean you have good brakes. it just means you are gonna slid until you stop.

think of the discs as a pivot. if you push a lever very close to the pivot point, it will take alot of effort, and sure, you will push the lever , but it will be a sudden movement with no control. push the lever further away from the pivot point and you will have more control over the lever. it will not be so much of an "on/off" effect.

The point of bigger discs is that you have a much larger leaver. you can control the wheels better so that you can get them to the point just before they start to lock.

Modern cars have larger servos and bigger bore master cylinders so that you dont need to apply as much pressure at the pedal to achieve head smacking results.

i owned a 2.8 capri with 247mm discs. the brakes were ok. sure i coudl lock them up, and skid off into a ditch. but i upgraded the discs to 283mm ones and instead of immedialy locking the brakes with my foot slamed on the floor, i was able to control the braking ability more. and i avoided skidding off into the ditch.

also, it has to be said, brakes and master cylinder sizes, brake disc sizes etc seems to be the most argued over point in all the forums i have been on ( although i swear its not cause of me )!!!!!

i bet thats how WW2 started....

just fitted 4 pots all round my lambo ( hi spec ) would only do all round so to not over brake the front & yes, as expected i have a longer peddle now, just takes a bit of getting used to, but going to fit larger dia master cylinder this winter to compensate
Was it worth it ???
Hell yes !!!!!
now got 330mm disc's all round & it pulls up brilliant

Unlike drum brakes Disc brakes that are correctly set up ie the caliper is true to the disc in all axis and with correctly adjusted wheel bearing will result in very little pedal movement as the running pad to disc clearance is only a fraction of a mm.

Where master cylinder diameter and resevoir capacity come is when something goes wrong, such as a pad breaking up or jumping out, in which case obviously a big caliper takes a lot more pedal pumping than a small one. In my years of experience I have seen pads break up three times and a pad jump out and go completely awol twice.

I know Scimitar specs change a bit over the years but the ones I had contact with all had Tr6 brakes with solid discs on the front, they stopped fairly well so unless you have made massive changes to the grip of the tyres bigger brakes might not actually bring any improvement.

However fade ressistance is a different is a different matter -- bigger discs will improve things but so would a swap to vented discs.

If it were me I would just fit vented discs on the frontand a rear disc conversion to make the rears work for thier money. The Scimitar used a Ford V6 but it was sat quite far behind the front wheels to give decent weight distribution.

Also drum brakes don't give good pedal feel because they respond differently to discs.

It might also be worth using a dial guage to test for pedal box and master cylinder deflection under heavy pedal pressure as this can spoil pedal feel.

I second the 'bigger brakes give better control' argument. I also second the statement that you should first define what you want to achieve.

Some remarks; sorry if I'm telling grandma etc.

- bigger piston area improves control and spreads the heat over a bigger area; less prone to overheating (also more difficult toe get proper heat into the brakes if you run the type of pads that need that). It doesn't in itself give greater braking power

- What gives 4 pistons calipers more braking power over 2 piston calipers on the same discsize is the increase in distance between the center of the pistons and the center of the axle (bigger arm; more torque when applying the same power).

- In an ideal situation (zero distance between pad and disc at rest and no flex at all in you brakelines) pedal travel will not increase. Increased pedaltravel is for a large part due to the distance between pad and disc when at rest. Another thing to look is are the flexible brakelines. Are they the rubber type or the ss braided ones.

When I changed to outlaw calipers from the original ford ones on my first seven and at the same time changed to ss braided flexlines this actually reduced pedaltravel.

quote:

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Originally posted by miegru
- What gives 4 pistons calipers more braking power over 2 piston calipers on the same discsize is the increase in distance between the center of the pistons and the center of the axle (bigger arm; more torque when applying the same power).

- In an ideal situation (zero distance between pad and disc at rest and no flex at all in you brakelines) pedal travel will not increase. Increased pedaltravel is for a large part due to the distance between pad and disc when at rest. Another thing to look is are the flexible brakelines. Are they the rubber type or the ss braided ones.

When I changed to outlaw calipers from the original ford ones on my first seven and at the same time changed to ss braided flexlines this actually reduced pedaltravel.

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I would agree with David, also the more pedal pressure required to acheive lock up the more controllable the brakes are rather than less as has been implied by others.
If you can get the rears to do a little more work then sure you'll gain a bit braking, however get it even slightly wrong with too much on the rear & you've got the fastest ditchfinder going!
My car is very hard to get to lockup, but I doubt you'll find many things that can stop quicker!

4 or 6 pot calipers have three advantages none of them make an earth shattering difference on indvidually but all thre together are worthwhile especially on larger size calipers.

(1) they are stiffer
(2) they distribute pressure more evenly across the pad.
(3) The radius of the working centre of pressure is nearer the edge of the disc.

Most builders tend to underestimate the importance of stiffness of the hydraulic parts it very important.

[Edited on 11/5/07 by britishtrident]

quote:

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Originally posted by russbost
I would agree with David, also the more pedal pressure required to acheive lock up the more controllable the brakes are rather than less as has been implied by others.
If you can get the rears to do a little more work then sure you'll gain a bit braking, however get it even slightly wrong with too much on the rear & you've got the fastest ditchfinder going!
My car is very hard to get to lockup, but I doubt you'll find many things that can stop quicker!

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Thanks for all the replies, Guys, some very useful stuff there. Forgive me for not replying to all your individual points.


My "problem" is that I need to replace the discs anyway, so why not make them better? I would describe my current brakes as "OK", but not brilliant.

I appreciate that you all want me to avoid ending up in a ditch by spoiling the balance, but I was just wondering what sort of % improvement I should be safely aiming for; I know I would be in trouble if the fronts were suddenly (theoretically) 73% better, so using 1.75" pistons is out of the question.

However, at the other end of the scale, if I keep everything else untouched, just by fitting greenstuff pads "could" increase the friction by 20-30%; Is this considered too much? I very much doubt it, as 90% of Locosters wouldn't be using them.

The simplest option for me is to buy grooved & drilled solid EBC discs (standard diameter) & greenstuff pads from powerstop, which would improve friction and help avoid fade. This would cost me £140 and involve no brainache. I already have s/s braided hoses fitted.

From your responses, this would sound like the sensible option.

Thanks for the help, keep it coming!

[Edited on 11/5/07 by TL]

Increasing pad friction by 20% doesnt automatically make the brakes 20% more efficient.

Stick some better pads in, either Mintex M1144 or greenstuffs, and see what improvement you get.

If it aint broke, dont fix it.

David

PS how the heck does fitting 4 pot calipers change the effective radius? I cannot see how this can be the case at all, the centre of the pad will be at the same distance from the centre of the axle, therefore the effective radius will be the same. Am I missing something?

i found that greenstuff pads took a while to heat up, but once at tempurature they worked REALLY well. but you had to keep using them. 10 mins on a road with no heavy use and they went cold again.

Grooved & drilled discs are more liable to crack and warp, don't use them unless you have a really good reason to.

Also keep in mid if you want to use the car as an everyday car that you will use n wet weather retaining the original splash shields is desireable.