1990 rear brake SWITCH adjustment

[Windmill]

Ah but, old brake fluid can be very compressible  .......it absorbs moisture and becomes aerated. Hence only storage in sealed containers and one of the reasons to change the brake fluid completely at regular intervals..... The difference does relate to volume though, as demonstrated in the difference in response of systems using narrower or wider bore pipes.

Sorry Alan, getting hooked up on the theory and your thread drifting off a little. Sure someone else more experienced here will tell you but are you sure the new brake pedal is for the correct model.........there seem to be many minor variations in the same item between various models, as I know to my cost having bought wrong stuff I was sure was correct. It would only take a small difference for the pull length to be different. Sure previous advice to thoroughly free off the switch is correct too though. Like the PO's theory by the way, install new pads but remove brake pedal and the pads will last longer unlike the rider I suspect........

I can empathise with your bike condition, mine was the same but riding it even with faults was enough to ensure I persevered with it. Keep at it, it will all be worth it in the end      

[ribbert]

Ah but, old brake fluid can be very compressible  .......it absorbs moisture and becomes aerated. Hence only storage in sealed containers and one of the reasons to change the brake fluid      

Yes it absorbs water, but water is not compressable in its liquid form and its presence alone won't aerate the fluid unless it boils, which it will do at a lower temperature than the fluid.
Its presence will also cause corrosion and rust, it is less viscous than brake fluid so more prone to leaking and it lowers the boiling point.
You can often stop leaking / weeping seals for a considerable time simply by refreshing the fluid.

[andyb]

If you have a container full to the brim with an incompressible fluid (and we'll ignore surface tension), adding a cup of liquid to that container will cause a cup of liquid to spill out.  The size of the container is utterly ignored.

If the bore of the caliper's piston is slightly conical, getting larger as the piston adjusts further out, then there will be a mild difference in the travel you get at the lever with new vs old pads.

I wonder if the incredible heat that a racebike's brakes undergo has any influence on a rider needing to change the adjustment of their controls, possibly.

[ribbert]

I've always noticed brakes are firmer with new pads. I understood that it is to do with the lower volume of fluid in the piston chamber, hence less fluid to move before the pressure is applied.  

Thin pads, thick pads, should feel the same.
You are talking about an extra thimble full of fluid in the chamber. The extra force required to move it only has to overcome weight and friction. Given the enormous advantage the lever and the hydraulics give you, I don't' think even NASA could measure it.

Flossing before you ride would have a bigger impact on you power to weight ratio than this would have on the brakes.

Noel

[Pat Conlon]

Noel, I agree, the "feel" remains unchanged, but have you noticed a difference in lever travel when going to new pads?

[SlowOldGuy]

Pat,
I was merely stating the simple fact of fluid incompressibility and hydraulics.

To tell the truth, I haven't really changed brake pads that often to notice any difference in lever "travel."  I might notice a difference in lever "feel" but that should be attributed to fresh pads providing better stopping power than worn out pads.

But even then, I recall new pads feeling a little "squishy" until their bedded.  The pad material does compress slightly during the break-in and bedding period.  But the lever travel should be identical from a new pad all the way to grinding metal on metal.

DavidR.

[Pat Conlon]

..... But the lever travel should be identical from a new pad all the way to grinding metal on metal.

Should be...but it isn't. That's why the racers use these:

http://www.constructorsrg.com/levers/remote.html
"Derived within the top levels of motorcycle road racing, remote (left hand) adjust brake systems allow riders to compensate for front brake system variations due to system fade, and brake pad wear during race situations...."

Of course that is a extreme example, but the basic principal applies. Lever/pedal (free play) travel is 'slightly' different between thin pads and new thick pads.

[rktmanfj]

Ah but, old brake fluid can be very compressible  .......it absorbs moisture and becomes aerated.

Well yeah... once it's seriously contaminated with water (and at what point can you consider it still 'brake fluid'), and under braking the water turns to steam (and steam is VERY compressible).

But regular maintenance keeps the brake fluid pure, and as David stated, it's just not compressible.

[ribbert]

..... But the lever travel should be identical from a new pad all the way to grinding metal on metal.

Should be...but it isn't. That's why the racers use these:

http://www.constructorsrg.com/levers/remote.html
"Derived within the top levels of motorcycle road racing, remote (left hand) adjust brake systems allow riders to compensate for front brake system variations due to system fade, and brake pad wear during race situations...."

Of course that is a extreme example, but the basic principal applies. Lever/pedal (free play) travel is 'slightly' different between thin pads and new thick pads.

My understanding of these is they do exactly what our adjustable levers do but without having to take your hand off the throttle. The left hand for a Motogp rider is the only one that has any free time.
The only reference to compensating for pad wear appears to be in the ad writers mind.  They also mention compensating for brake fade. I would be very surprised if these high tech, cutting edge bikes suffered from this. I have never heard of brakes going off as the race progresses.

I do believe (I have read) that riders like a different feel to the brakes at different stages of a race though.

Noel

[Pat Conlon]

The only reference to compensating for pad wear appears to be in the ad writers mind......

Heh heh, ok Noel, it must be someone's creative imagination...who do they think they are kidding?..

[fj johnnie]

 Another issue racers have is head shake. When the front wheel goes into a mini tank slapper the side to side force causes the pads to push the pistons away from the pads/rotors. I have had a tank slapper and had no brake pressure right after. One or two pumps and it is back to normal. Watch any of the motogp guys and they often check brake pressure  after a steering episode.