Improving brakes on rally Imp

[Gemini]

Hi All,
I am currently running the following brake set-up on my 998 Rally Imp

Viva discs front, standard drums rear
Master Cyls - Front .7 Rear .625
Material - Front, Mintex 55's (second softest comp pad) - Rear, Mintex comp linings
Bias pedal box
No Servo
13" wheels
All cylinders/hoses are good and system's well bled

The brakes have never been that effective with a hard pedal requiring lots of pressure to slow the car although I have been competing on Forestry stages only for the last 4 years where you can throw the car sideways if you need to!! This is not ideal as you can imagine and my stage times would be quicker if I could brake later. I have tried different adjustment on the bias.

What is the best set-up for loose surface stages where rear bias is preferred?

(I have to run Viva discs, so says scrutineer, and I only have .625 or .7 Master cyls to choose from (or do I?) . Also a Servo on twin webers is problematic (so I read on another thread here?). Maybe .625 front and rear for more pressure and softest pads?).

Finally what would you change if running on tarmac stages as I'm off to Ulster (August), Belgium (Sept) and Manx (Oct)?

Any help and discussion would be good so thanks in advance. (I have also posted this on the Brake info section)

Geoff

[OldImp]

Hi Geoff
Just a few thoughts.
When the pedal was modified for bias brakes was the pivot point moved?
I have always found the .7 master cylinder hard work without a servo with comp linings.
In the forest if you can get away with standard linings on the rear you might find they will give more braking and feel especially when cold. You could try standard pads on the front and do a test run to see if this helps. If it does then the problem could be the pads.
For tarmac rallies you need most of the power to the front brakes as locking rears can cause you to spin. This means you will need the comp linings as this generates a great deal of heat.
Best of luck with the rallies.
Bob

[glenzierfoot]

Had all the some problems with the brakes om andrew cowans car and also did not want to run a servo, in the end i used o.7 master cylinders ,triumph spitfire front calipers. viva discs with green stuff pads, but the best mod was rear cylinder, there is lots of different piston sizes 3/4 is standard imp , i found a girling cylinder the same with a larger bore at a local auto jumble, not sure what they came from but should not be to hard to find out .it might be hillman hunter?
Well he is very pleased with his brakes now.
clark.

[onomatopoeia]

(I have to run Viva discs, so says scrutineer, and I only have .625 or .7 Master cyls to choose from (or do I?) .

There are also 0.75" master cylinders in the same fitting, but that would make the pedal even firmer. From memory, standard Imps were fitted with 0.625" for brakes and clutch, sport models had 0.7" for the brakes. The 0.75" size was never fitted to a production Imp that I recall.

I'd have thought you can fit any discs you want - generally scrutineers are more concerned that the car can stop in my experience (sprints & hills though), unless you're doing historic and mods need to be period. My Davrian started its life as a tarmac car and has 10.2" vented discs at the front, with a separate bell and rotor, 10" one piece solid rears with motorbike type handbrake acting on the inside of the disk bell (I think that's the original Davrian disc conversion).

I've always found dual circuit brakes to be very firm in an Imp (my current Chamois has them with 2x0.625 master cyls, Fiesta disks front and standard rear drums, the old Chamois did as well with disk / drum), but on the Davrian the pedal pressure required is much less - the pedal box arrangement on the Davrian is somewhat different though.

[pimpdriver]

The normal problem with some bias boxes is you cannot replace one master cylinder with two master cylinders with changing the pedal ratio. The standard imp 0.625" master cylinder has an area of approx 0.31 square inches, so if you fit two of them you should really fit two off 0.44" master cylinders (0.155 x 2 = 0.31)to move the same amount of fluid. Unfortunately 0.625 is the smallest Girling cylinder you can get. AP racing do a 14mm 0.55" (0.24 area) cylinder but they are £70+ each.
The way to do it properly is change the pedal ratio, to do this you must move the bias tube welded into the pedal nearer to the pivot to change the leverage. The standard pedal is about 44mm, the ones i've made are about 32mm.

Hopes this helps

Eric

[bks974c]

I took the bias pedal box of my rally car for the same reason - hard pedal, no feel and I was running drum brakes all round even tried standard shoes to no avail,
on events it was never a problem just on the road due to the fact that in competition you are much more aggresive in your braking.

As Eric said the problem is you require twice the amount of effort running two .625 master cylinders and even more effort required with the .7 and .625.

Eric
If you moved the mounting point did you have to raise the master cylinders?

I had thought about lengthening the pedals to increase the leverage but have not done anything about it yet.

Scott

Link to previous topic but no more info than here
viewtopic.php?f=14&t=14810

[brakedisc]

What are the dimension of the Viva discs?

[onomatopoeia]

The way to do it properly is change the pedal ratio, to do this you must move the bias tube welded into the pedal nearer to the pivot to change the leverage. The standard pedal is about 44mm, the ones i've made are about 32mm.

I worked out that this was the solution , but there is no way I'd trust my welding on any part of the brake system, never mind one so critical as the pedal!

As it stands I live with it, it's not too bad and for a vehicle primarily intended for classic reliablity trials, brake feel is not really an issue.

[Rod]

Hi Geoff, following lifted from the HRCT site, guess you've studied it anyway but would the twin piston Girling replica calliper mentioned help? "Easier" back in the old days, couldn't afford discs, I seem to remember the Rootes Disc Brake Kit cost around £250.00 in the '70's. The only thing you could rely on was that you couldn't rely on the drums!

Brakes.

Front. Brake drums or Solid brake disc. Triumph Herald or Vauxhall Viva Lockheed twin piston brake calipers.

The front brake calipers are are available under the follwing Pt numbers.

Viva Lockheed CA132 Left Hand CA132R Right Hand. Triumph Girling CA2 Left Hand CA2R Right Hand.

There is also a Period replica Girling twin piston brake caliper available from Performance Braking based in Monmouth Tel 01600713117. or e mail info@performancebraking.com The part no is 14LF and utilises 48mm pistons.

Rear: Brake drums.

Tandem master cylinder. Adjustable brake pedal box. As long as it cannot be adjusted by driver or navigator when normally seated.

[Meltdown]

Girling 14LF also available from Demon Tweeks. Tim Duffee does a bracket for both front and rear (welded onto the trailing arm) that uses modded Saab solid discs. It's designed for a twin master cylinder bias system. I didn't get around to using it before selling the car it was intended for.

[RGWM]

Interesting topic - I've just resurrected my modded 998 Chamois after a considerable period out of action - and the brakes are sticking - so much that I thought I'd break the Rotoflexs moving off - I'm guessing the master cylinder is objecting to being disturbed after its long rest

Ever since I've had the car (about 14 years....) I've intended to change the master cylinder as the car has always had a long pedal travel - something to do with it having Escort vented discs on the front, I think - so I'm now minded to fit either a Sport master cylinder - 0.7 compared to standard 0.625, or maybe a Girling 0.75 one for Demon Tweeks...

Any thoughts?

[SimonBenoy]

It's really a matter of personal preference regards how short a pedal you want and how much your pads knock off. On my racer I have a .75 for the fronts and a .7 for the rears (discs all round). This gives a very hard pedal initially but it is great on the track and the bigger front cylinder compensates for more knock off on the fronts. Works for me though it might not be everyone's choice.

[bks974c]

Sticking brakes are more likely to be rusted drums/discs sticking to the pads/drum unless they were OK until you pressed on the pedal and then they locked on in which case its likely to be the master cylinder, but always worth changing after sitting about as you are rather dependant on them working to stop

Increasing the master cylinder will reduce travel but will increase the pressure required to excert the same force suggest you go up a size at a time rather than going straight to .75

Scott

[Gemini]

Thanks Guys,

Really appreciate all your thoughts and advice - very informative and thought provoking.
In short, we are doing the BHRC Championship and the rules dictate we have to run only period spec pre -68 but we get a little dispensation if all Impers (no others this year, I'm afraid) are running the same kit (within reason).
Based on your advice, we plan to do things step by step with step one being change to .625's back and front, run the softest Mintex comp pads (we have a new set) and standard rears and see how that goes on a shake-down.
I will let you know the results.
Cheers Geoff

[Mike Hanna]

Hi Geoff. Just picked up this thread. On my Rally Sport Imp I had .625 m/cyls on original pivot point distance. On the front:- Viva discs and hubs, IIRC Sunbeam/Avenger callipers with standard pads. Rear :- Swap back plates one side to the other and mount upside down (so adjuster at bottom and cylinder at top. This saves the pipe from stone damage and makes for easy adjustment) Vg95 linings. This setup seemed to make the rear brakes more efficient although the shoe wear was rather uneven. I think I put spacer tubes on the cables at the h/brake end. This setup was spot on for loose or tarmac, but we did have asbestos in the brake material in those days!! My race car from a few years ago used viva discs and very light weight Wilwood alloy 2 pot calipers with a 1.75" square pad and similar pedal box. Rear used Micra disc and same calliper and pad as front with bias valve in the brake line. Fantastic brakes. If I get to build a new racer I will do the same setup on it . Cheers. Mike. p.s. Good luck in Antrim.