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The drum brakes are from the earlier /5 model, your bike would have had a single 39mm slave cylinder front disc with dual front discs as an option.

Thank you "chasbmw" I am not at all up on the chronology of these bikes and since it did not occur to me to check the ETK, so I wanted to ask; much obliged.

dwire wrote:For my own education here, what is stock on this vintage? Still the self-actuating style drum?

Stock on a 74 R90/6 would be a single solid (undrilled) disc.

I'll bet since there is a consensus not to like them, they were floating rather than opposed piston designs - besides, a front disc on a M/C would be hanging way out a bit to have opposed pistons, or my imagination makes me think so...

Funny about the drilling. Two reasons to do it, weight and heat yet that meant the brake needed already be over adequate to make it feasible. Amazing how much surface area "aint" there after you chock a disc full of holes - so I'd bet BMW had come to a pretty good balance of disc diam. and both piston and M/C displacements and such.

God the leading trailing self actuated mechanical drums are so much simpler from a design, test and operation standpoint; if only it were not for needing to get a bit of heat in them so you did not get that "insta-grab" on the first few applies (always in someone's 10" of gravel driveway) and if the rider simply has poor modulation skills, I can see why they'd think the drums were yucky.

I am only ever disturbed by my front drum when it needs that turn around and unexpected sort of stop in a foot of gravel right after it was started and has not moved more than a few feet; otherwise, I drag that baby a bit through first gear and could not ask for more. The disc do "LOOK" sexier, but I'm not so sexy I guess myself so I am told, so I would gather the discs would hardly make up for that... Thanks for all the info guys - I'll drop out here as I have run this off topic and off the rails of the track. Thanks again.

Douglas

Jo think that BMW drilled the discs to reduce brake lag in rain.

One thing BMW got very wrong was the ratio between the brake cylinders, the standard ratios give you very wooden brakes, moving to a smaller mastercylinder really helps to give feel and power with more lever travel.

When BMW designed the system they must have been worried that riders would lock the front wheel

dwire wrote:I'll bet since there is a consensus not to like them, they were floating rather than opposed piston designs - besides, a front disc on a M/C would be hanging way out a bit to have opposed pistons, or my imagination makes me think so...
Douglas

It's not that it's on a motorcycle that makes the clearance issue, it's spoke wheels. They were running pretty small rotors in those days, which puts the caliper quite near the hub. Even today, with much bigger rotors (putting the caliper further from the widest part of the spokes), it is very tight putting opposed piston calipers on a spoke wheel, and todays rims and forks are wider. It often takes offset carriers to put the rotor further outboard. Cast/forged wheels make it MUCH easier to fit that stuff in there.

Hard to say really. And surely many ways to skin a cat; less surface area by drilling holes = less vaporized superheated steam resisting the brake working as it should - only lubricant I can think of you can put on a brake that does not cause instant lockup pretty much. Also, maybe they got a lot of feedback on the drum's sensitivity from those first couple cold applies?

D.A. might know more of that. If you want a good brake, just take the friction material off entirely. Maximum stopping power on EVERY APPLY. They just always lock up... Sadly everyone once this crazy thing called a balance. Gosh how picky of us...

Major Softie wrote:

dwire wrote:I'll bet since there is a consensus not to like them, they were floating rather than opposed piston designs - besides, a front disc on a M/C would be hanging way out a bit to have opposed pistons, or my imagination makes me think so...
Douglas

It's not that it's on a motorcycle that makes the clearance issue, it's spoke wheels. They were running pretty small rotors in those days, which puts the caliper quite near the hub. Even today, with much bigger rotors (putting the caliper further from the widest part of the spokes), it is very tight putting opposed piston calipers on a spoke wheel, and todays rims and forks are wider. It often takes offset carriers to put the rotor further outboard. Cast/forged wheels make it MUCH easier to fit that stuff in there.

Yes, you're saying the same thing. You could not fit a tri-bore or quad-bore under a car if it had a BMW rim of any kind and was only supported by two forks mounted against its hub; it puts your brake IN the wheel - spokes, what have you... I don't have any cars with wagon wheels on them anymore - think they just drug something on the edge of the wheel for braking and I think that would be a bit hard on tires... lol

a R65 front end will bolt up and you get Brembo and 18" front wheel with a bigger axle

chasbmw wrote: ...One thing BMW got very wrong was the ratio between the brake cylinders, the standard ratios give you very wooden brakes, moving to a smaller mastercylinder really helps to give feel and power with more lever travel.
When BMW designed the system they must have been worried that riders would lock the front wheel

My sentiments exactly !
Subsequent MC tech advances prove the point to be valid.

The best advantage of resizing the MC on the older systems
will be a more progressive application of braking
which will allow the rider to precisely modulate the brakes to his/her advantage.

IMO, and given that the rest of the system is adjusted properly and in good condition,
the choice of an MC swap may be the best single improvement to the antiquated disk brakes –
perhaps even more effective than twinning
which has the disadvantage of adding unsprung mass.