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MG MGB Technical - brake pedal ratio?

I've seen a thread on this in the past, but can't seem to locate it. Has someone here altered their brake pedal to increase the pedal ratio? I assume you would have to have a lower pivot point for the brake pedal - separate from and below the clutch pivot. Has anyone tried this?
I now have a contact patch about double the stock size (205 race tires) and feel the increased braking would be significant. If someone could elaborate either from experience or theoretically, I'd appreciate the input!
Thanks!
Jeff Schlemmer

Jeff, I don't recall the thread and I have not changed the lever ratio on the MGB. I have worked with clutch pedal ratios on a race car. It had about a 1/1 ratio and the driver couldn't push the pedal down. The race car pedal setup was different from the MGB, the cylinder push rod was attached between the pedal pivot and the pedal pad. The solution was to move the push rod closer to the pedal pivot. I have a photo of MGB pedals in my Yahoo photos;
http://pg.photos.yahoo.com/ph/flash1929/album?.dir=/6b7d
The brake pedal is on the bottom. I don't think changing the pedal ratio will give you increased braking, but it will change the pedal pressure required for braking. You would need to move the pedal pivot hole closer to the brake cylinder push rod to reduce the pedal pressure. Moving the pivot would create some problems. The push rod alignment would change, it would be operating at an angle, not good. The pedal travel distance for braking would increase. It would be easier to extend the length of the pedal arm between the pivot and the foot pad, but the pedal travel would increase. I don't recall if your car has a servo, if not you may want to consider installing one.

Here are some brake links that may give you some ideas if you haven't already read them.
http://www.sportcompactcarweb.com/editors/technobabble/0212scc_techno/
http://www.mpbrakes.com/mpfaqmasters.htm
http://www.carbibles.com/brake_bible.html
http://www.trillium-bmwclub.ca/site/page.jsp?pageid=144

Clifton



Clifton Gordon

Thanks Clifton, maybe I should expand on my intentions.

I already have a fully rebuilt system on a '71B (non-servo) with Automec pipes and stainless hoses. I am looking to soften the pedal feel (increase mechanical advantage) as I currently can't lock up the brakes without using both feet and would like to be able to heel-toe brake efficiently.

If I've done the math correctly, the brake pedal ratio is about 4.35:1. With an applied force of 120 pounds to the brake pedal, that gives you 1227 psi of brake line pressure. By lowering the M/C pivot 1/2", the pedal ratio change actually increases line pressure by 300psi!

What I'm trying to overcome is the issue of an improper pushrod angle at the master. I have a spare pedal box assembly that I can cut/weld/brace to achieve this, but it may be easier/stronger to fabricate a new pedal box!

I found the thread I was looking for in the V8 board, but there were no details about how the pedal ratio change was made. Is the servo pedal shorter/longer than the non-servo? Can they be swapped to quickly change pedal ratio?
Jeff Schlemmer

Jeff,
Did you search the archives in the engine conversion and V8 sites. I believe Bill Guzman as well as others have commented on this in the past. Post back your results.
Frank

Jeff, as I understand things, if you can't lock the brakes up (with reasonable effort), you've either got an improperly setup stock system, or (as in your case) your tyres have more grip than the brakes can overcome. So, with your big, sticky tyres, you've earned yourself some leeway to increase the overall braking power.

To this end, it would seem that rather than fiddling with pedal ratios, you might consider larger discs, larger/stickier pads and/or more pots, until you can again lock the system up with a good (one foot) stomp.
Curtis Walker

Thanks Curtis! I have already made the modifications to my spare pedal box. The M/C now sits lower by 1/2". I used 3/16" plate added to the front side of the brake box To support the master. The old mounting holes were cut out and welded back in - in the lower position to match the holes in the 3/16" plate. It appears much stronger than the stock arrangement, with significant welds on both sides.

The pedal box also had to be modified for the brake fittings, which also sit lower. Basically, I heated up the box (which was well braced and mounted firmly to my bench) and "adjusted" the flange on the box until it cleared the fittings on my mock-up master. The strengthening flange on the top of the pedal box is still completely in tact.

I still have to modify the pedal, but I've currently run out of C25 and acetylene, so I'm a bit handicapped.
Its amazing how much I can get done when the wife is out of town!
Jeff Schlemmer

Hi Jeff,
I am not sure what the problem is with your brakes but you should be able to lock-up those tires with the stock ratio. I'm running 9 1/2" slicks with balance bar and two M/Cs which doubles the pedal effort and can lock those up if required; ideally you never lock them up. Higher pedal effort allows easier brake modulation a real plus when heel and toeing. I drove a Miata for two hours of a four race this March and I prefer the high pedal effort of my B to those power brakes but each to their own.
Lee Bradley

Lee, it seems to take a substantial amount of force to lock up my brakes. Far more than when I had stock wires and tires - at least partially because I have the ultra-heavy knock-off minilite replicas. My brakes are in top-notch condition, but I guess maybe I just don't like the feel. Brake modulation while heel-toe driving is difficult with a sensitive throttle and a very hard brake pedal. I'm not fond of power brakes either, which is why I'm trying to find a happy medium. I just finished modifying the pedal ratio from 4.35:1 to 6.15:1. Maybe I'll hat it and change it back?

Jeff Schlemmer

This thread was discussed between 09/04/2006 and 12/04/2006

MG MGB Technical index

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