Wondering about wheel cylinders

[Steve49841]

It's that time of year to think about making the race car better for next season, I was wondering if installing a larger rear wheel brake cylinder would be worth the effort. My rear brakes don't seem very powerful and I don't get much heat out of them so I think there is room for improvement. My car is a 68 with a standard master cylinder and 3/4 wheel cylinders. I see some other size cylinders are available so I was wondering what you guys thought was the optimum size.

[Gregs672000]

I'm no expert and have never done anything to the rear brakes, but I know several racers here run different compounds that may help. Have you already gone that route?

[Steve49841]

I've been using Green Stuff brake shoes on the rear. I was thinking maybe a bigger wheel cylinder would exert more force on the shoe. The car stops okay but racing is about looking for every little bit.

[Gregs672000]

For sure. I would imagine this issue has been considered by many a team (unless your brakes just are not working as well as they should), but who knows... Maybe you'll be pioneering a new and improved Datsun Roadster racing rear brake system mod! Good luck and let us know what you find!

[SLOroadster]

The thing about making the rear brakes stronger is that it will also cause them to lock earlier. Not a good thing if you happen to get forced into going for the brakes mid corner. I'm running the same set of NISMO green stuff shoes that I put on 8 years ago, and they are still in spec. My rears work fine, but I don't know that I want more rear brake simply to avoid the risk of locking the back end up while trail braking.

Just a thought,
Will

[spl310]

I don't know how active it still is, but the forum on Risensonracing.com was where the racers shared views. Find guys that are doing the kind of racing that you are doing, and pick their brains...

[RCMike]

For my use, I am about to go up one step in rear pad compound to try to bring more brakes to the rear.. When I run time attack (basically autocross) my fronts lock WAY before the rears, so I don't get nearly the benefit trail braking I would get if it was more balanced.. Of course it is considerably easier to drive that way, but it could be faster with a more controllable balance..

I will worry about the car being too tail happy the first time I spin it out in a grip run..

[Steve49841]

The idea would be to have enough rear brake force available to lock the rears then dial it out with a rear brake bias adjuster for track conditions. I've given some thought to putting a bias adjuster on the front brakes to control front lock and exert more brake force to the rear with heavier pedal pressure but that might just open another can of worms. As evidenced by brake temp readings my current balance is probably 95% front and 5% rear so there's room for improvement, just got to figure out how to get it done.

[Gregs672000]

95% front and 5% rear? I'm talking way outside of my "knowledge" (more opinion) base, but that seems way low for good operating rear brakes. I mean, what would be the point other than to have a parking brake if that is all they provide!

[Steve49841]

I'm guessing at the percentages, but after a 30 minute race the fronts are around 900 degrees and the rears are in the 150 range. I would like to see the rear brakes around 300 degrees so I need more brake force in the rear which goes back to my question about larger rear brake cylinders.

[Gregs672000]

Ya, 150 degrees sounds cold! Lilke they certainly are not working... wonder at what temperature the green pads start operating well? Are you using an infra-red thermo? Just curious, but seems to me that other racers should be able to give you a typical temp reading vs yours using that method. I just can't get past the idea that maybe yours are not operating at even stock levels... but I truly have NO idea...

[dbrick]

It would seem to me the way to go is to make a step up with the rear cylinders, 13/16 or 7/8 along with a balance/bias valve to dial it back. IIRC, 13/16 was available from Nissan, not sure on 7/8, but you could bore what you have.
Now this is assuming your master is working 100%, so if any doubts you may want to get that done first.
The only other worry is with the change that it is consistent as the brakes warm up. I like the lever type valves, as you can have presets and easily change as you drive, within a range, without guessing. Mounted on the tunnel it would be easy to reach and away from heat.


I think you can also get some balance changes by adjusting the rear brakes a little bit looser to delay engagement so the front brakes bite first.

[Steve49841]

There is a guy advertising 15/16's wheel cylinders on e-bay, will they fit my 68? I have also used the style bias adjuster you noted and it worked fine.

[jake7140]

Does sound way off. I'd make sure the master is working correctly though with a brake pressure gauge, and that the shoes are adjusted correctly and working freely too. Borrow or they're about $50. Don't want to fix the symptom of another problem!

I think front lock up is controlled by the right leg of the steering wheel driving nut!

[fj20spl311]

Did you ever get this sorted out?

The temperature of the rears is too low for most brake shoe material......you are correct that 300°F is a good minimum.

Brake shoe adjustment is very important and maybe adding a 10 psi residual pressure value would help.

Check the shoes are arced to the drums....making contact completely.

I have seen people trim some of the material from aftermarket "full" contact shoes to more closely resemble stock shoes with one small and one large shoe.

I have posted in the past a brake system (balance) calculator and how to modify it for rear drum brakes.
IIRC, the roadster should have about 20% rear braking effort.

I would add a brake bias valve to the rear as it also flattens the line pressure vs pedal pressure curve, so the response from the rear drum brakes is closer to the linear response of the disc brakes.


Here is the link:

http://www.311s.org/phpBB3/viewtopic.ph ... or#p142963