Brake pedal ratio modifications

[IainXE]

Can someone check these measurements and give me any confidence it'll do what I require please?

Removed my brake pedal and took some measurements to get the pedal ratio, as I plan to ditch my servo assist in favour of a twin master-cylinder non-assisted bias-adjustable set-up.

Pics of pedal and measurements:





I make that 19cm between the pivot and the pedal, and 4.5cm between the pivot and the connecting rod hole. That gives a pedal ratio of 4.3:1.

If I redrill where the white dot is in the second picture, that's 3cm away from the pivot, giving me a better non-assisted ratio of 6:1.

Sound about right?

[b2hbm]

I would have thought your ratio was higher than 4.3:1 already. When I've done this I've always assumed the measurements should be taken along the same axis/straight line between pivot & pedal center, something like this;



A+B is one measurement, A is the other to get your ratio. You will get a higher pedal ratio at the point you've shown, but going on the photos I wouldn't expect it to be massively different in feel.

edit to add - actually what I've written could be complete twaddle - what angle does the master cylinder push rod leave the pedal ? I was assuming the pedal is vertical and the rod at 90deg, but it could easily be a different angle and that's what you need to sort out the correct ratio.

Edited by b2hbm on Wednesday 16th December 17:40

[IainXE]

Thanks for your input.

It's a vertical pedal and the link rod moves vertically into the scuttle area.

By your measurements I won't gain much by re drilling. Will have to suck it and see

[b2hbm]

In which case my original assumption was wrong, sorry.

I'd assumed the actuation rod left in the direction of the green arrow below, because mine's set up like that. If your set up is more like the red arrow then your original numbers measured on the radius are right. You'll get a bit more pedal travel but I assume you're ok with that. If it's a mainstream car though, be prepared to tell the MoT man why it's longer than normal

[IainXE]

Not mainstream, and tester knows the dealio with modified cars so should be OK on test. Thanks for your confidence, hopefully should do what I need it to then. Would I be better off welding extra to the top of the pedal or just drilling where marked?

Bias box arrives today so I'll mock it all up and see how I get on.

Cheers

[Dave Brand]

b2hbm said:

I would have thought your ratio was higher than 4.3:1 already. When I've done this I've always assumed the measurements should be taken along the same axis/straight line between pivot & pedal center, something like this;

No. Dimensions should be taken from the centres of the respective holes.

[Mr2Mike]

Dave Brand said:

No. Dimensions should be taken from the centres of the respective holes.

Agreed, but that's probably not the end of the story in this case. The master cylinder pushrod extends vertically upwards from the pedal, so from the pictures it seems likely that the pushrod forms an acute angle to the pivot, as below:



Effective pedal ratio = B/(C * sin(a))

If a=90 degrees, then the ratio = B/C. If a=0 then the ratio = infinity.

This assumes the OPs foot meets the pedal at ~90 degrees

[IainXE]

So by that calculation I'm actually increasing the pedal ratio far too much, if its 45 degrees from the pivot point.

Impressive application of maths (to me anyway!) Thanks

[Mr2Mike]

If the angle 45 degrees, the sin(a) term is 0.707 so your calculated ratio of 4.3:1 becomes about 6:1

Obviously the angle increases as the brake pedal is moved, so you want the angle at the point that the brakes are actually being applied rather than the resting position.

[IainXE]

Great advice much appreciated sir.

[Gsr88]

Dragging this one up from the not to distant past, would the angle that the master cylinder attaches to the brake pedal have an impact on the effectiveness?

Thanks in anticipation.

[Mignon]

Mr2Mike said:

Effective pedal ratio = B/(C * sin(a))

If a=90 degrees, then the ratio = B/C. If a=0 then the ratio = infinity.

This assumes the OPs foot meets the pedal at ~90 degrees

I think it's more complex than that because as the pedal moves so does the pushrod and the angle a changes. That in turn depends on the length of the pushrod.