Front brake M/C question

[ivorbiggin]

The ET Thunderwagon came with no front M/C, i do have a Blandit 600 jobbie, but its a 14mm bore, and  im lead to believe the ET would have been a 5/8. So my conclusion is, smaller bore = longer lever travel to push the right amount of fluid to get the front brakes to do something. Hmm, just how much extra travel do you get? I tend to panick as didly squat happens untill the last second.

Ive managed to pick up a 5/8 M/C from an 83 GSX550 E. It came attached to a pair of twin pots and the ET Thunderwagon has single pot floaters. I cant work out if i am going to get a brake that slams on at a touch, or one thats going to just about work if i pull hard enough on the lever, or is the stoke the same and not make a difference?  Unfortunately i have no original ET M/C to compare with.

Any help would be much appreciated.

[Gixer1460]

You need to work out total piston area of single piston against the double pistons. if comparable then the change from 14mm to 5/8" (15mm roughly) will have a slightly longer pull but pressure will be slightly higher. If the piston area is smaller (single to double) then the pull distance should be similar and opposite if greater. Only you'll know if the feel will be acceptable to you!

[ivorbiggin]

1 hour ago, Gixer1460 said:

You need to work out total piston area of single piston against the double pistons. if comparable then the change from 14mm to 5/8" (15mm roughly) will have a slightly longer pull but pressure will be slightly higher. If the piston area is smaller (single to double) then the pull distance should be similar and opposite if greater. Only you'll know if the feel will be acceptable to you!

Thanks, that gives me something to work with, ill measure the pistons tomorrow.

Just by eyeball the 550 pistons appear smaller than the ET single piston, but the total area of the 2 smaller pistons will probably  be greater than the total area of the single piston on the ET.

If the 550 twin pot and the 1100 single pot have the same M/C bore size (5/8) but the twin pots have a bigger area, am i right in assuming the M/C piston stroke must be different between the two?

If so what determines the stroke? Is it length of piston, the lever, or something ive not thought of?

[gs7_11]

Don't over think it too much. Put the M cyl on and try it.

You might be surprised how little difference it makes in some cases.

I've fitted an extra disc and caliper on a couple of occasions, without changing the M cyl, and it's been fine. In one case, BMW R65 (sorry...), the feel was much better with the "wrong" M cyl.

Edited November 3, 2019 by gs7_11

[Gixer1460]

10 hours ago, ivorbiggin said:

If so what determines the stroke? Is it length of piston, the lever, or something ive not thought of?

You've two ratio's to deal with. 1st is master cyl bore area vs. Caliper piston bore area - this influences the pressure exerted on the pistons and lever travel will be a function to achieve that. 2nd is lever pivot ratio ie. how far the Master cyl. piston is moved relative to lever movement. Both will have an influence on lever movement and feel. With a multi adjustable lever the brake lever can develope the same line pressure but be light pull or hard pull, just be varying the pivot point / mechanical advantage in the perch!

[ivorbiggin]

11 hours ago, Gixer1460 said:

You've two ratio's to deal with. 1st is master cyl bore area vs. Caliper piston bore area - this influences the pressure exerted on the pistons and lever travel will be a function to achieve that. 2nd is lever pivot ratio ie. how far the Master cyl. piston is moved relative to lever movement. Both will have an influence on lever movement and feel. With a multi adjustable lever the brake lever can develope the same line pressure but be light pull or hard pull, just be varying the pivot point / mechanical advantage in the perch!

Thanks for the detailed info,  and I hadnt given adjustable levers a thought. (adjust cable and/or shoes back in my day)

Ive decided to go with the 550 M/C and have ordered a kit for it, ill see if the Blandit adjustable lever can be utilised on the 550 master, if not ill see whats about on the interweb. Worst case is, if it feels crap bin it, and find the right M/C.

[ivorbiggin]

Time to update.

Ive fitted new piston and seals to the 550 M/C, fitted Wezmoto braided lines, and bled it through using a mittyvac.

Brake lever feels fine, about 5 to 10mm of free travel and with the lever fully pulled in i have a 50mm gap between the lever and grip. The real proof of the pudding will be when i road test it, but that will be some time away yet.

Alas the Blandit adjustable lever wont go anywhere near the 550 M/C, but tbh, i dont think its going to need it.