TL
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| posted on 11/5/07 at 12:48 PM |
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BRAKES: HOW BIG IS TOO BIG?
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]
Cheers,
Terry
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emsfactory
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| posted on 11/5/07 at 12:53 PM |
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That almost reads like an exam question.
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flak monkey
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| posted on 11/5/07 at 12:58 PM |
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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]
Sera
http://www.motosera.com
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JAG
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| posted on 11/5/07 at 01:13 PM |
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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]
Justin
Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!
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Ivan
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| posted on 11/5/07 at 01:43 PM |
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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.
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russbost
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| posted on 11/5/07 at 01:50 PM |
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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.
I no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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saigonij
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| posted on 11/5/07 at 02:23 PM |
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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.
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saigonij
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| posted on 11/5/07 at 02:29 PM |
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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....
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khm
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| posted on 11/5/07 at 03:07 PM |
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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
I'd rather be scared to death, than bored to death !
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britishtrident
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| posted on 11/5/07 at 03:44 PM |
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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.
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miegru
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| posted on 11/5/07 at 03:56 PM |
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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.
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flak monkey
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| posted on 11/5/07 at 04:02 PM |
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quote:
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.
Not exactly true:
What gives 4 pot calipers a greater power than 2 pots is the greater piston area in the caliper. If you fit 4pots to the same disc you arent
increasing the effective radius just the clamping force that the caliper generates for a given pedal pressure. The only way to increase the effective
radius is to fit bigger discs.
Pedal travel will increase slightly if you fit bigger bore calipers, you need to move more fluid, so pedal travel has to increase. The situation isnt
ideal, otherwise the brakes would rub. I think pads in a caliper usually back off somewhere around 0.3-0.5mm when the brake is released.
Unless you are having problems with the current set up (ie the brakes are over heating) then leave them alone. If you mess the balance up you will end
up with longer braking distances.
David
Sera
http://www.motosera.com
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russbost
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| posted on 11/5/07 at 05:01 PM |
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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!
I no longer run Furore Products or Furore Cars Ltd, but would still highly recommend them for Acewell dashes, projector headlights, dominator
headlights, indicators, mirrors etc, best prices in the UK! Take a look at http://www.furoreproducts.co.uk/ or find more parts on Ebay, user names
furoreltd & furoreproducts, discounts available for LCB users.
Don't forget Stainless Steel Braided brake hoses, made to your exact requirements in any of around 16 colours.
http://shop.ebay.co.uk/furoreproducts/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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britishtrident
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| posted on 11/5/07 at 05:40 PM |
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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]
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britishtrident
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| posted on 11/5/07 at 05:45 PM |
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quote:
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!
Agree 100% with above, pedal presure is what gives you control of the brakes --- for proof just try driving an automatic using left foot braking.
[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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TL
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| posted on 11/5/07 at 09:10 PM |
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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]
Cheers,
Terry
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flak monkey
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| posted on 12/5/07 at 06:45 AM |
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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?
Sera
http://www.motosera.com
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saigonij
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| posted on 12/5/07 at 08:40 AM |
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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.
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britishtrident
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| posted on 12/5/07 at 06:42 PM |
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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.
[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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