brake master debacle

[Hak]

Hey guys! Had a bit of an adventure workin on the ol XS this weekend. As I said in a thread before my front calipers have been sticking since I got the bike. Did the master and that didnt work so I decided to do all the rest, lines, pads, and rebuild the calipers. Well the calipers were fun lemme tell ya. Also I ordered EBC pads according to their catalog and even the listings stated one set was LF one set was RF but I ended up with 2 LF sets of pads... who knows. then finally everything is back together and I knew from before my master I had gotten was a bit undersized, as a full lever pull would net maybe 60 or 70% of the braking effort I had with the OEM master. But apparently it's so small that I couldnt even get the pistons to pump out of the calipers.. just push out a little then retract. So I threw my old master on just to ride the thing home and now I'm wondering if anyone can help me figure out what size master cylinder I need? I think the one I got is a 14mm or 14.5mm piston.. I previously did hours of research trying to figure it out and apparently failed miserably, so here I am asking for advice from the benevolent masters of XS lol

[jetmechmarty]

Maybe this will help.



From Vintage Brake:

Front Master Cylinder Ratio Chart

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.

[bikerphil]

If you have a Special with SS braided lines, you need a master with a 16MM piston. A 17MM piston is the stock size and would be correct if you have rubber brake lines.

[Tenorman]

SS Braided Lines on 1980 XS1100LG?

Hello,

I'm a bit confused....

I want to replace the 35 year old rubber brake lines on my 1980 XS1100 Midnight Special.

Can I use the stock master cylinders (front & rear) for the SS lines or MUST I change to smaller master cylinders?

On my 2005 BMW R1150R, the brakes really grab and I like the feel of having to squeeze pretty good to lock the wheels...like the XS1100...

I'd rather stick with rubber brake lines than having to change master cylinders...

Any thoughts and thanks!

Tenorman

[jetmechmarty]

Install the stainless lines and keep the master cylinders. They'll just feel slightly more wooden than before. The effort it takes to stop will be the same, if not less. Your brakes will be slightly more effective. Those 35 yo lines have to go!

[Hak]

Awesome! Thanks guys! I had found that chart before but I guess I goofed on what size the calipers are or somethin. i think I'll go with the 17 tho even tho I have the stainless lines. I'd rather have a little too much brakes than just not enough lol

[bikerphil]

Just to clarify, 17MM = 11/16"

[jetmechmarty]

Awesome! Thanks guys! I had found that chart before but I guess I goofed on what size the calipers are or somethin. i think I'll go with the 17 tho even tho I have the stainless lines. I'd rather have a little too much brakes than just not enough lol

I'm thinking maybe you don't understand how this works. The smaller master cylinder means less effort to move the lever and a more linear feel. Too small and the lever reaches the bar before you stop. If you're replacing the master cylinder with an aftermarket unit, the stock size is not optimum. I think BikerPhil covered it.

[Hak]

I mean it's basic hydraulic principle.. bigger piston more fluid is transfered for a given stroke of the piston.. replacing rubber hoses with stainless means less swelling and even more fluid travels to the calipers upon a given piston stroke. If 17mm is factory I'd rather stay with that and gain increased fluid transfer to the calipers rather than go down 1mm to gain an easier or less sensative lever. But all I'm really finding so far are 16mm for aftermarket masters so I'll probably go that route anyway.

[jetmechmarty]

If 17mm is factory I'd rather stay with that and gain increased fluid transfer to the calipers rather than go down 1mm to gain an easier or less sensative lever.

I think you'll find it more sensitive as far as feel.

[Hak]

16mm more sensative than a 17mm? That defies basic hydraulic theory, although in reality you have to take into account the mechanics of the lever, the length and the fulcrum would have more to do with "feel" than anything else. How much stroke of the piston vs how far the lever itself travels but that's a whole new can of worms.

[bikerphil]

The 16 will give you nice smooth linear travel, the 17 will give all your braking power in a very narrow range. The 16 (5/8") is the ticket for the Special calipers. I run one as you can see in my signature, and am very happy with it.

What size is the one you have now?

[phatts27]

I think sensitivity is the wrong word. A smaller piston mc would be more accurate with ss lines.

A larger piston would move more fluid to the calipers, reducing the required travel of the piston in order to fully engage the brakes. With ss lines, the stock master cylinder moves so much fluid that the lever hardly has to move to achieve maximum braking.

A smaller piston bore would restore much of the lever movement lost when converting to ss lines. The smaller piston pushes less fluid requiring more movement of the brake lever to fully engage the brake.

[Hak]

I understand all that. Seems to be a mixup of terminology lol I don't mind a more sensative brakes, I was just saying the 17mm will provide more clamping force than a 16mm and I'd rather have too much than not enough, I don't mind the quick response I have now with the 17mm stock master back on the bike, but a full pull on the 16 or the 17 is probably going to provide more clamping force than the tire has traction anyway so I see why you wouldn't mind trading some clamping force for a smoother lever pull.

[phatts27]

Rereading my post, I've decided accurate is still the wrong word. Precise is a better word.

I have ss lines with the stock 17 and my lever feels hard like a rock. So much, in fact, that my lever only moves about an inch on the far end before I can't physically pull it any farther. Such little movement makes it hard to accurately apply the brakes during normal riding conditions.

Use whatever feels right.