Brake bias
 

From studying the build diaries, I see a wide range of brake rotors, caliper types, engine weights, and I would think, front/rear brake loadings.
Has any one found it necessary to get into brake bias adjustment from the seat.
In the Cobras there can be a 90lb difference between a cast iron 427 side-oiler and a new Shelby aluminium(I spelt that correctly, eh?) block, and with the weight so far forward it makes a difference in the mountain corners.
I have seen set-ups with dual master cylinders connected with a balance bar, and dash mounted needle valve adjusters in the hyd lines.

Also, any experiences with gearbox to driveshaft to diff universal angles, torque.v. diff wind-up, not a problem with BMW diff mounts?

Hi Maurice,

No - I don't think anyone has had need to do this. And the IVA (UK roadworthiness test) wouldn't allow it anyway, as any adjustable brake bias mechanisms must be locked off in an unajustable state after it's initial setting - i.e dual masters with a bar, but locked so it's not adjustable once the balance has been set.

I can't answer your other questions, as everything driveline related on Vikki is from the BMW donor. Well, apart from the prop shaft, which is specific to the Sportster due to the reduced length.

Cheers

Jason

Thanks Jason.
My Marlin learning curve is quite steep,but I always find the tech comparisons interesting.

These are interesting questions.

Brakes
Until now it has not been possible to generate enough brake pressure to lock the wheels, so bias has not been an issue.
if you have been following the Dual 7" booster thread it now looks like we have reached a situation where the brake pressure now exceeds the mechanical grip of the tyres and can be forced to skid. This may mean the balance between front and rear braking needs more attention.
The truth is we are just embarking on a learning curve.
So the answer is " We do not know".

Drive Line
Regarding the drive line I was told by a propshaft manufacturer that I must NOT use the BMW E36 rubber donut, as it will only tolerate around 1 degree of angle, otherwise it will shred it self in no time. My Cabrio has the engine at a significant angle to the centre line of the chassis, so would definitely have caused problems. Not sure what the Sportster is like?
You will need a special length propshaft, as the original is too long complete, and too short in part, for the Sportster: you will not need the centre UJ.
Therefore get one made, with a spline to accommodate diff. wind up, and ease of removal.


http://farm3.staticflickr.com/2207/2...27bbff2071.jpg

Although I am building a Cabrio, with a Ford diff, the Sportster is designed to support the diff in the same way as the donor, so there should not be any problems with the diff - unless you are going to fit a muscle car engine up front!!

Had mine made to measure by Dave Mac propshafts. He reused as much of the BMW propshaft as he could minimising costs. Get no vibration at all. Highly recommended.

Robin

Mike

Regards the brakes.

At one stage during the build of my Cabrio I was agonising on the merits of a disc rear end over drum rear end setup. I had an interesting conversation with Pete Morris (moc bmw cabrio tech rep) on the subject.

When he put his car through SVA he found the brake bias was right on the limit, ie borderline that the rear brakes would lock up before the front. He was running standard Sierra disc all round with the standard Sierra servo which gives substatially more boost than a Metro servo.

It may be interesting to compare the specs of the dual servo to the specs of the standard Sierra servo, if the dual is giving more boost, maybe a bias would be needed when running disc all round. I gues it would be less of a problem with a drum backend. I guess the only true way to test it would be to run a car with the modified brake setup on a rolling road brake tester as used for IVA.

John

Mike,

We are on the same page.
These two items, brake bias and wind-up, are significant factors in other builds I have been involved with.
The dual booster and tire grip got me thinking a little deeper.

John
This ties in with a question from MBM - they were asking if we needed a brake bias valve to go with the servos?
Sounds like we may do?

Better ask Jason to go and do some more brake tests!
Stand on the brakes
Hang head out the side
Look forward - is wheel skidding?
Look rearwards - is wheel skidding?

Report back on whether break bias valve is required!

John

If the MBM spec sheet is anything to go by then a 7" Dual gives almost the same boost as an 8" single.
In US speak, I think the UK Sierra servo would be classed as 8.5" as they take their measurement across the seam.

If this is the case then you may be right that we are getting close to needing a bias adjuster.

Mike

Who keeps their car on the road over winter?
 

How many owners have registered cars that they will be running through the winter?

We will need you to give us all more feedback on the new servo!

I know I'm a bit different having drums at the back. My cars off the road for its engine refit so I can't do any tests at present but I'm pretty sure I could lock up all four wheels when doing an emergency stop at slow speed. It's one of my pre-MOT tests that I do in nearby street. (Maybe my tyres were a bit rubbish?)

I fitted the BMW rear brake restrictor (bias valve?) which limits the amount of pressure going to the rear wheels to stop them locking up before the fronts. (These are not fitted to ABS donors as the ABS prevents it in a different way).
I am fairly sure that Patrick fitted the same to his car and he has disks all round.

It's BMW part number 34331152494 (you can look it up on Realoem.com)

Unfortunately, I can't find a picture....

My Cabrio has all the standard brakes straight off the Sierra donor (disc front/drum rears, std servo). I have managed in a panic moment to lock up the fronts at about 60m.p.h. on a dry road.

I found a picture of the BMW restrictor on Patrick's website....
http://www.msportster.co.uk/gallery/sva-day/sva015.jpg

Chris

can you remember if there was a brake bias control valve in the original Sierra set up? Or is this unique to the BMW donors like Peter and Patrick?

Peter

A good picture of the pressure reducing valve as fitted to the E30 BMW. In fact the same PRV was fitted to my 325i Sport donor and is still in place on my spares donor, both fitted with ABS and rear discs. It it located on the bulkhead just before the brake pipe is routed to the rear of the car.

I imagine the front/rear weight distribution will be similar on the Sportster so I intend to fit the PRV as on the BMW.

Hi Mike,
Yes there was, but I didn't incorporate it into the build. I wasn't too sure if I would get it mounted at the right angle and left it off.
Chris

I don't recall the Siera having a brake bias in its standard form.

I tend to agree with others, if the brakes are standard from the donor the bias should be OK unless the weight distribution was vastly different. The only difference should be the reduced effort in terms of pedal pressure.

I still have standard disc front and drum rear and don't intend to change that with the new servo.

I think the only time a problem could develop is if the rear brakes were upgraded without considering the front. For instance if you had a standard Sierra disc/drum setup and you upgraded to discs at the rear without upgrading the standard 240mm discs on the front.

A straight lift of the donor brakes should be fine.

That of course is just guess work and could be complete tosh!!

John

Mine had some kind of limiting valve fitted in the rear brake-line, which I didn't use. I didn't know if it was inertial and had to be mounted at the correct angle or just sensitive to the pressure.
Apart from that I just couldn't be arsed to fit it.

Chris, such honesty is refreshing!

I'm resurrecting this thread after some issues on the way back from Stoneleigh. I believe I have a brake imbalance, which is only coming to light on longer runs since I fitted the dual booster.

Vikkis rear brakes where binding, and where if not red hot, bloody hot and smoking when I got home. There was a distinct lack of oomph from her, as she was continually fighting the friction of the brakes.

This could be down to sticky calipers - and I won't know for sure until I take a look at them... But with the mileage I've done, and the lack of problems prior to the booster change, I think it could be residual pressure building up in the rear lines, keeping them engaged.

I'm wondering if I need to fit the brake pressure regulator that was on the donor, to reduce the pressure to the rear brakes. Thing is, I didn't keep mine - so I need to source one. Does anyone have one to hand?

Everything I find online is in the states - so assume that the only source will be at the parts desk.