MG Sports Cars

In the past we have included Welwood brake proportioning valves to balance the system In car we built for customers when using disk front and drum rear; admittedly, at least to some degree, because we sold them for more than we paid for them. That’s business, but I’m not so sure they're all that necessary on street cars that will never see a track day. What do you guys think?



Paul

Paul
That's an interesting question! I'm not sure if today's new cars have a proportioning system or not. I do beleive the brake bias should be set but whether adjustable or not (short of setting up to maximize corner braking) I would think with anti-lok brakes it would be a none issue.

The proportioning valves you used made up for the wide discrepancy in line pressure requirements between the disc front brakes and the self energizing rear drum brakes. Most drum brakes have a primary and secondary shoe and the engagement of the primary shoe causes extra force to be applied to the secondary shoe through the adjuster link joining the two shoes at the bottom. So why doesn't the MG have a proportioning valve? The MG drum brake does not transfer force between the shoes, they are solidly pivoted at the bottom so they act more like a disc from a line pressure perspective.

With 4 wheel discs the proportioning is done at the factory by varying the area of the piston(s) in the caliper to achieve the proper balance front to rear. For instance, the front calipers in my C4 Corvette brakes have 2 pistons of 1.5" diameter for 3.53 sq". The rears have i piston of 1.57" diameter for 1.95 sq". To be on the safe side I also installed a balance bar pedal assembly that lets me vary the percentage of pedal pressure that goes to the front and rear master cylinders.

Doug, I think you’re right. Stock MG’s, though most of ours are not actually stock, had a valve that would shuttle if a brake failure occurred and turn the light on, but they didn’t have a proportioning valve as such. For less than $50 I wonder if the addition would help any or would the money be better spent somewhere else. I had one on my Bucket because that’s what everyone else did, drove it for six years and never turned the knob. It always stopped straight but was not fond of corners. The reason I ask this is because I was setting here bored to tears and thought to myself, I need to go buy something for the B's birthday, if you ever forget their birthday they will never let you live it down, and then I thought, I haven't asked a dumb question in two days, this might be worth a discussion.

Thanks Jim, That's great information, I never thought of it. Actually I didn't think of it because I didn't know it, I guess on the B the money would be better spent somewhere else.


Paul



Edited 1 time(s). Last edit at 01/11/2013 10:44AM by pspeaks.

You know guys after all these years I start to think I know all there is to modifying my toys, but every day I learn from you guys. THANKS!

Paul

Proportioning valves are typically installed on the rear circuit where they can only retard the rear brakes, which might be counterproductive. After installing a Mustang / 8.8 axle, I needed to increase the rear's contribution to overall braking so I played around with different wheel cylinders and brake shoes. Now I think I'm pretty well set for dry roads, but a proportioning valve might be nice for damp road driving.

If you're still looking for a birthday present... you might want to consider adding Wilwood residual pressure valves. They typically provide a modest reduction in pedal travel, which you might perceive as a firmer pedal. They're relatively cheap and simple to install.

I know my car is a Triumph and not MG, but when I upgraded my pitifully small spitfire rear drums to MGF discs and calipers I had to fit a wilwood proportioning valve to the supply to the rears as initially they were as good as if not better than the GT6 discs and calipers on the front.
When I first tried the car out after the upgrade the rears were locking before the fronts.
Fitting the proportioning valve and dialing out some of the pressure to the rears solved the problem and brought the front/rear balance back.

Graham B

I have a set of residual valves left over from my T-Bucket days. We used them because the rear brake wheel cylinders were higher than the master cylinder, worked OK for most 9” Ford’s (not mine) but we had a bit of trouble with them causing the rears to lock up on a couple of cars with GM rear ends. I stopped using them but I’ll revisit the idea. You’re right Curtis; we only used proportioning valves on the rear. While my brake system is totally new from the master cylinder to the wheels, it is basically MG and worked OK in the past but with the added weight of the V8 I may have to rethink that.

For anyone interested in Residual Valves, ironically Muscle Cars on power block had a pretty good segment on them this morning and will re-air it tomorrow (Sunday). The 10 psi and 2 psi valves are available from Speedway.



Paul



Edited 3 time(s). Last edit at 01/12/2013 01:57PM by pspeaks.

I do a lot of work with brakes. Proportioning valves are definitely used on new cars, including those with four-wheel brakes. The ideal brake proportioning is not a fixed ratio, but one that changes depending on the level of deceleration. The typical two-ratio valve approximates this ideal proportioning fairly well. You definitely cannot use a fixed front/rear ratio on a street car. Get close with the rotor size/pad size/piston size, then let the valve determine the rest. Obviously, you'll set it up with extra rear bias so you can "turn down" the rears to get the right bias.

Cars with ABS may use electronic brake proportioning instead. This has become more common as ABS systems improve in speed.

Race cars might use a fixed proportion with dual masters and a bias valve, but they also tend to always be at 100% braking effort so they're not really worried about the other cases. Plus that bias valve will be adjusted to suit the traction on the day, and might even be cockpit-adjustable so it can be tweaked as the tires go off.

Street cars can definitely benefit from correct proportioning. Maybe not in normal driving, but when you need to do an emergency stop you need all the help you can get. That 1% stop can make the difference between saying "whew" and writing off your car. A properly adjusted valve can be the best bang-for-the-buck braking upgrade you can make.

Having long had a cockpit control for my balance bar, one thing I can tell you is that the balance changes from dry to wet road conditions, and pretty noticeably at that.

Jim

When I put the Ford 7.5 rear end in my car with the Ford drum brakes, I was surprised that the brakes did not need any adjustments. They worked beautifully. Perhaps that was due to the Ford drums/cylinders being so similar in size to the stock MGB?

Now, I have just about finished installing the Ford Mustang disk brakes on the Ford 7.5 rear (I thought I would finish this job a month ago - It's funny, how, when you become the designated mechanic of the extended family and end up repairing all the cars in the extended family!) and I have a proportioning valve to install as well, just have to button everything up to get it done. I think you guys saw my posts about the rear calipers making contact with the leaf springs, and then welding up the brackets a few degrees up to avoid that leaf spring contact. When I do get it all together, I'll post some more pics. I have not fooled with a brake proportioning valve before, so I might have some questions....

Brad



Edited 1 time(s). Last edit at 01/16/2013 12:19AM by denvermgb.

All of race cars have had a lever type proportioning valve, specially in the endurance events. This will allowed the adjustment as the weight of the car changes as the fuel tank gets closer to empty, so you adjust the proportional valve from the cockpit same as the sway bar setting which can also be adjusted form inside the cockpit.
Braking is a race car is just as important as hp and torque, late braking canbe a big plus to make a pass etc.

On a street car a proportional valve should be installed no guess if you need one or not, it a must.
This can be done in different ways, larger brake line feeding the rear, different compounds of pads and piston size of the caliper or wheel cylinder. some times all of the above in a race car. But on a street car the use of a proportional valve from a GM vehicle can be used and that is all is needed if the piston size is matched and that is not difficult to do. I just installed one in the Red project B

Installed in the car

I don't have the brakes connected yet and have been following this discussion with interest.

Brad's comments got me thinking - I will be using an 7.5" narrowed S10 rear axle I got from D&D. It turns out that the S10 wheel cylinder bore is 0.75" while the MG is 0.8".

So, on a late model B with vacuum assist, and with the cylinders so similar in size is a proportioning valve overkill in street applications?

NO !

To answer your question better, find what you have first, what I mean is do some braking test without a proportional valve and see what the car does; if the rear brakes lockup first and the car spins then you know that will need a valve, if the car brakes straight and in reasonable distance (need something to compare) then you will not need one.
You could also try larger brake lines in the rear, change the 3/16 line with 1/4" very common.

Can not make changes unless you know what have first.

What about VW ? They used to brag about diagonally linked brakes,(right front to left rear) and mirror image.How say you brake experts ? roverman.

Similar to volvo a fail safe system

Not very tunable.

Jim