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850 Le Mans Brakes

2

24heureslemans

Just got it firing!
Joined
Jul 5, 2011
Messages
3
Got a 76 Le Mans with dual control rear/front brake system and single front brake on handle bar, would like to convert to dual front brakes on handle bar. Seems to me you had to change the handle bar master cylinder before the dual fronts would work right, does anyone have info on that? What happens if you cap off the rear line going to the front and just split the front line to both calipers?

BTW: why is it so difficult to post a topic anywhere or to all on this site, does it always have to be by region? What if the person who knows my answer is on the east coast, or europe, or Napal?
 
Early big blocks had 12mm M/C for single disc ie T3, Le Mans, Convert etc and the 750S twin disc had a 5/8" M/C(15,9mm) bore
This is an excerpt from some where that I've forgotten and lost the link to! but nonetheless I quote........

While attending Vintage Days West, and thoroughly enjoying it, I was reminded that many of the people I had occasion to talk to, lacked an understanding of the importance of master cylinder to wheel cylinder ratios. This critical ratio is of paramount importance in determining "feel". It has been my experience that there is a "sweet spot" in the range. I like ratios in the 27:1 range-2 finger power brakes, feeling some line and/or caliper flex. 23:1 is at the other end of the spectrum-firm. Ratios lower than 20:1 can result a feel so "wooden" as to have a toggle switch effect: nothing happens until the wheel locks. Disc and wheel diameters must be taken into consideration. A 10 inch disc working against an 19" wheel just doesn't have the leverage ratio that a 13 inch disc working a 17" wheel does. The hand lever ratio counts too: witness the adjustable master cylinders from Lockheed and Brembo.

A case in point: I had a complaint from a racer about Ferodo CP901- a compound renown for its great feel. His comment was that they worked poorly until the wheel locked. He had been thrown on the ground twice. Intrigued, I inquired as to the application. "Yamaha RD350" he replied. A red flag went up. CP901 was not available for the 48mm Yamaha caliper. I asked "How that could that be?" He had up-graded his braking system with the 41mm Lockheed unit, but was unaware that a master cylinder change was in order. A stock RD 350 has an already poor ratio of 18.3 :1, and with Lockheed, became an unhealthy 13.3 :1. The "sweet spot" formula said a change to a 11 or 12mm master cylinder was in order: my personal preference and recommendation would have been an 11mm. He was able to switch to a 1/2" , and although not ideal, he was keeping the rubber side down.

For 2 piston opposed calipers, I like ratios in the 27:1 range, feeling some line and caliper flex. For a firmer lever, use 23:1. I think ratios lower than 23:1 produce a lever feel so "wooden" as to have little, if any feel. Combine "low" leverage ratios with sticky pads, and unpredictable lockup is the result. The high effort required at the lever also results in undesired input to the bars. Single piston calipers are much happier in the 14:1 to 12:1 range. Disc and wheel diameters, as well as hand lever ratios, must be considered.

I CANT IMPORT THE TABLE SO HERE ARE SOME BITS OF IT

38mmx2 pistons (1 disc) gives a ratio of:
M/C
11mm = 24:1 better option ? suitable for rear
12mm = 20:1 Poss why standard is a bit wooden
14mm = 14:1
16mm = 11:1

38mmx4 pistons (2 Disc)..
12mm = 40:1 lots of lever travel
14mm = 30:1 poss OK
15mm = 26:1 looks like the one ;)
16mm = 20:1 poss a bit wooden

All this would be different if you change to later 4 piston calipers etc :ugeek:

note never had any probs posting to any of the catagories :?:
 
Thanks for the reply Kym,

I'm still trying to figure out the navigation of this web site, so its taken me awhile to find this spot again. I'm wondering by your formula that I have 12mm front M/C to cover the single side of the front disc, and therefore something like a 15mm rear M/C to cover the duplex control of rear disc + other front disc? If that is the case it seems I would have to enlarge the front M/C to 15mm and reduce the rear M/C to 12mm to have the correct feel on both brakes, right?
 
the 15mm for the front seems about right for a twin disc using the original type calipers and either a 12 or 11mm for the rear. the 11 would give better feel and I believe they are still availablewith a bit of hunting down. Try some duc sites if your having trouble
 
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