Brake Mods for Servoless TCS

[surfguitar58]

Today's feeble minded question:

There has been a lot written on removing brake servos and switching from the stock 0.875 inch dia tandem master cylinder to a Spitfire 0.70 inch dia tandem master cylinder for the Fed TCS. The piston area reduction is 0.64:1. Could one get the same effect by drilling a new hole in the brake pedal and moving the brake clevis location closer to the fulcrum by the same ratio? (See sketch)

Granted the M/C connecting rod would be off axis with the MC, but that could be corrected with a custom off-axis clevis. (The load would still be off-axis, but the rod alignment would be axial.)

What am I missing here?

Tom

[Clifton]

I converted to a hydraulic clutch and did this to get a better pedal. I did on the brakes too  but I am running dual masters. There was a brake calculator online that I used to figure out everything when I did my brakes. It is no longer there but it took into account pedal ratio, bore, weight, everything. It gave pedal travel and force required for what ever G stop you wanted. I would search for one online.

[gideon]

Surfguitar, it looks to me as though the bracket you drew to bring the connecting rod back into alignment will exactly cancel out the extra leverage from using a lower clevis pin hole on the brake pedal.  That is assuming that the bracket isn't free to rotate when you press the pedal.  If it is free to rotate then I think you will lose leverage overall.

If you can get away with a little connecting rod misalignment, then simply using a lower clevis pin hole might buy you some extra leverage.  Is the connecting rod attached with a horizontal pin at both ends? If so then a small change in the angle of the rod away from the "correct" axis will still work.  In this case a "small" angle is anything where sin(x) ~= x, over the whole range of movement of the pedal.

[Arizona]

When I removed the brake boosters many years ago, I drilled a new hole as in your first sketch. It was actually Richard Banks who recommended it. That's the only modification I made. The pedal effort is higher than with the boosters but not objectionably so. One thing that has worried me is that I've heard reports of brake levers breaking under excessive pressure but it hasn't happened yet.

[jbcollier]

The offset clevis yoke is a cosmetic solution.  The load is still applied off axis the piston is loaded off-axis as well.  So skip the custom yoke.

Increasing the pedal's mechanical advantage will work.  That said, I would fit the smaller M/C myself.

[surfguitar58]

Thanks for the advice everybody.

Arizona: I worry about breaking the pedal in my current configuration with big bore MC and no boosters. I've mostly gotten used to it but I really have to stand on the pedal in a panic stop. The second hole weakens the pedal, but less force would be required.

JB: Are you concerned about the off-axis loads scrubbing the MC piston against the cylinder wall, potentially causing scoring? Frankly, that is my biggest concern.

Tom

[4129R]

Two things would worry me about your offset clevis pin.

1) I would triangulate the clevis pin, so as not to put large pressure on a right angle.

2) If you increase the mechanical advantage, you have to increase the pedal travel for the same piston movement. Can the pedal travel far enough to give you sufficient braking?

Personally, I would not mess about with what is essentially equally the most important control for the car along with the steering.

Brake failure = Huge problem. Next thing to stop the car is the umbrella brake (no chance) or whatever is in front.

[jbcollier]

Yes, exactly that Mr. S. G. 58.  Offsetting the rod on a clutch is one thing, the forces involved are much lower, but a brake m/c sees a far greater applied force.  Best to keep it all in line.

[LotusJoe]

Today's feeble minded question:

There has been a lot written on removing brake servos and switching from the stock 0.875 inch dia tandem master cylinder to a Spitfire 0.70 inch dia tandem master cylinder for the Fed TCS. The piston area reduction is 0.64:1. Could one get the same effect by drilling a new hole in the brake pedal and moving the brake clevis location closer to the fulcrum by the same ratio? (See sketch)

Granted the M/C connecting rod would be off axis with the MC, but that could be corrected with a custom off-axis clevis. (The load would still be off-axis, but the rod alignment would be axial.)

What am I missing here?

Tom

When I removed my boosters, I was advised by Richard Winter at Banks to cut the pedal off and move it up slightly. He gave the measurement at the time but I can't remember. So I cut the pedal off the arm and moved it up. This would be an alternative to drilling a new fulcrum hole.     

[EuropaTC]

Hi,
As the others have already posted, drilling a new hole in the direction shown will increase the pedal ratio and you won't need to move it very much.  Assuming your pedals are the same dimensions as my UK TC, the standard leverage ratio is 4.28, moving it only 1/4" downwards increases that to 5 and 1/2" pushes it up to 6. The force will be applied at a slight angle which will mess things up a touch but your principle is right.

So your approach will work and given the length between the pedal & m/cylinder bore I think you'd get away with it. Personally if you're removing the booster/servo then I would fit higher friction pads and see how you feel about it before making any mods.  I'd reckon Joe's idea of increasing the pedal length is probably the safest but you could try lowering the pivot point and seeing if there's enough clearance, but if you do go that route I'd weld up the original hole.

Brian

[Lotuswins]

I have to agree on the change of pads.  EBC green pads are good, for street use, and will have a significant impact on how much pedal pressure is needed.

Modifying the alignment, or strength of the pedal is really a bad idea.  Welding up the hole even worsens an already compromised arm.  In this day of litigation potential, perhaps just changing the pads will give piece of mind too. 

I'm running a 7/8 bore Tilton, with 1144 Mintex pads and the pedal is firm, but effective.  I used to run the EBC until a track day fried them.....but until the they were great. 

Jerry Rude
4005R

[surfguitar58]

Okay, in my never ending quest to find the laziest solution to any problem, I am revisiting this old thread about increasing boosterless brake pressure by changing pedal geometry. In the thread above I was considering just drilling a new clevis hole in the brake pedal to increase pedal leverage. The consensus was that creating off-axis forces on the MC piston would scrub the piston against the MC cylinder wall and cause premature wear.

Is there any reason I couldn't drill my second hole to increase leverage, then put a block under the whole pedal assembly to raise it up 3/8" or 1/2" to put the forces back on-axis with the MC? I think the clutch cable could accommodate this, and I have big (US size 11) feet, so reaching the higher pedals would not be a problem. What am I missing?

Tom

[jbcollier]

That would work just fine.

[LotusJoe]

Agreed, that will work.

[4129R]

I got a new servo for about £70. I adapted the Girling fixing bracket to suit, and with a bit of pipe plumbing, all is fine. Clearing the luggage box was forgotten for a short while.

A lot easier than changing all the master cylinder etc.