Desensitising Brakes???

Plums · #1 (**permalink**) 10-04-2007, 07:46 PM

Took the car for it's maiden voyage around the block..... and nearly killed myself (and the wife to be) with neck snapping whiplash!

I locked the fronts a few times while just trying to stop normally....

I am running the standard G-force set-up with the booster, all of which was provided by G-force.

Is there a way to reduce the amount of vacuum assistance???
I tried disconnecting the booster all together but I needed both feet to stop the car in the driveway.

HELP!!!

bobcowan · #2 (**permalink**) 10-04-2007, 07:59 PM

I don't think you can limit the amount of vacuum assist. Whatever vacuum your engine makes, that's what you get. Maybe if you put in a bigger camshaft?

I'm not familier with the G-Force pedal set up. Maybe you could change the pedal ratio and get less mechanical leverage. You'll also get more pedal travel, which could make them easier to modulate.

New brake pads sometimes are a little "grabby". Before changing anything, I would take them apart, clean pads and rotors, and re-assemble. Bed the pads in per instructions, and try again. Drive the car a bit before changing anything. Maybe you just need to get used to them.

boxhead · #3 (**permalink**) 10-04-2007, 08:08 PM

I am not sure what booster is used but you may want to look at something smaller (Gemini)?

Or you could fit an adjustable pressure regulator in the front brake lines?

pneall · #4 (**permalink**) 10-04-2007, 08:40 PM

Plums,

I had similar experience with a series 2 Jag many years ago.Touched the brakes and instant face plant into windscreen

. Turned out to be missing pad ( size of 50 cent coin)between the master cylinder push rod and the booster.

Phil

stephen low · #5 (**permalink**) 10-04-2007, 08:57 PM

Bobby

My kit came with a sigma booster and ford master but as you would recall the origin of my kit and its bits, that these items were probably sourced by the previous owners than GF. I'd need to check the old GF receipts to confirm this and I'll let you know either way.

Anyway, beyond having a layer of muck on pads and rotors, from sitting around for tens year, my drives and the braking needed have not generated the sort of reaction you are describing.

I've corrected a long brake pedal feel by adjusting the pedal linkage but haven't had another opportunity to test drive. Don't expect any difference in the braking force though.

You need to figure out what the master and booster are from and work the calcs for bore size and effort much as Aussie Mike has talked about in previous threads. But I'd tend to concur with the initial response that maybe something is sticky or not quite working right yet!

Cheers

LoBelly · #6 (**permalink**) 10-04-2007, 10:44 PM

Hello Plums

In the May 2006 issue of Snaketales there's an article on brake upgrades by Hoppers Stoppers (brake specialists near Melbourne)

The article mentions the disc Phil is talking about - and says...

'A reaction disc is a small rubber disc which lives under the head of the booster output pushrod. If this is lost, the brakes will be nearly uncontrollable with stopping proportional to pedal position, not pedal effort, sending you through the windscreen with the slightest touch'

I initially thought that I had this problem but it turned out that i just had very powerful brakes - jag 4 spot assisted in a light car, after a few drives i stopped skidding to a stop and then began enjoying the drive.

LoBelly

Rebel1 · #7 (**permalink**) 10-04-2007, 11:37 PM

Yep, check for the reaction disk first.

Having said that I know both RMC and G-force ( and possibly other full Jag based suspension) suffer unbalanced braking. It is almost as if the rear brakes aren't working well and/or too much front brake or both. and....

It gets worse if you track your car in any way. The front brakes will keep locking up but the rear brakes will just go away.

There was an editorial in Snake tales a few years ago which details the efforts some went to correct the unbalanced nature of RMC brakes.

My take on this is that the jag rear brakes live in a column of hot air from the tunnel so don't cool well enough and the front calipers are just too big.

The rear can be helped by fitting vented rotors. ( available as a kit from Spieri jag in Victoria) and the front by swopping the huge cast iron, very heavy jaguar front calipers for something lighter. (Nissan 4 spot for instance). The nissan pads are a little smaller in area than the Jags.

There are other options but these seem to be the popular ( read affordable) options.

One other thing I'm eager to check this time around is the weight transfer during braking. In it's earlier guise my car would just lock the front wheels even from a slow speed with little pedal effort. The car just sat level with no noticeable weight transfer. It made me wonder whether I had over sprung the thing.

My thoughts this time is to use lighter springs, see what happens and work from there.

Cheers

Plums · #8 (**permalink**) 10-05-2007, 12:56 AM

It appears you blokes are right, I rang a brake joint this morning and that's what they said....

Damn, now I have to empty the fluid and remove the master, and the worst thing is I still have over 24hrs remaining on the permit to drive...

Harrymac · #9 (**permalink**) 10-05-2007, 06:23 PM

Hi Bob,
When you sort it out, could you write a short story about it with a few photo's for the 'Snakeskin' because I have heard about this problem before but never bothered to find out about it. Did you give it a squirt on the maiden run?

Harry.

Plums · #10 (**permalink**) 10-05-2007, 11:39 PM

No worries Harry, will write it up.

I have given it a little squirt a couple of times, seems to go ok with my very weak program (basically a standard VYII Calais program, just turned MAFless)..

I've done 48 k's so far and loving it.... can't wait to get the brakes reacting a little more "progressively"!!!