Does Anyone Else Have A Front/rear Brake Bias Adjustment?

My CJ has a brake bias knob inserted in the rear brake line, just off the master cylinder.

I'm familiar with brake bias for race cars, but not for offroad, though I understand the concept. I'm really curious why the prior owner felt it was a good idea to go ahead and put one in (simple install).

Anyone else have one?

It recently occurred to me that this might be great for climbing since my CJ doesn't have a locker in the front diff. I can lock-out the rear brakes and lean on he front brakes to keep one wheel from spinning in various conditions.

And downhill I guess it would be good to keep the rear wheels from locking up and swapping ends in certain conditions.

Is that pretty much the purpose of the bias?

 

You set it and forget it.
Same as anything else, here are the instructions to adjust:
https://shop.wilwood.com/blogs/news/how-does-a-proportioning-valve-work

 

You don't have stock brakes so you need it to keep from spinning around in a panic stop.

I have gm disks on all 4 corners so I have a stepped bore master cylinder then the adjustable valve to further knee the curve.

 

Thanks so much for that link. I do indeed have a Wilwood master cylinder with the inline proportional valve (i.e. brake-bias valve). I wanted to post a pic but the site says it's too large to upload. I'll try reducing it.

After reading the Wilwood info, it seems that my understanding of the valve was exactly the opposite of how it actually functions. I was thinking of it like a standard faucet valve, like a garden hose spigot, and thought screwing it all the way in was shutting the rear brakes off. But it's exactly the opposite. Has to do with the spring preload valve inside. I had consequently screwed it all the way out, and have noticed that my braking seemed weaker, which it actually is since I've nearly cut the rear brakes out of the loop. I'll have to screw it back in now.

I have front discs and rear drums (Toyota).

I still like the idea of cutting off the rear brakes so I can brake the fronts to help gain traction when climbing. I'll have to give it a shot.

All the best!

 

You could end up twisting off the front driveshaft. Ask me how I know.

 

Ouch...

 

I'ld use a line lock if you want to fully shut them off.

 

IMOP, shutting off your rear brakes while climbing would be a bad idea. If you should need to back down suddenly you would need your rear brakes to control your reward speed. With the front wheel only brake you will drag your front wheels locked going rearward making it impossible to steer.

 

brake bias/proportioning valve is pretty common when doing a dual reservoir setup and disks.

 

I did that back in the mid 80's with my 3B and used an early Wilwood proportioning valve. Those calipers were all 78-81 Jeep versions with 2.5" dia pistons. On my latest rendition, it now has the 3.1" dia Scout II/Ford pistons on the front. That is close to a 50% increase in piston area and I may end up reverse plumbing the proportioning valve.

 

That's not a great idea. The proportioning valve is 1:1 to a knee point and then a fixed ratio pressure reduction happens. You can move the knee point up and down but you can't change the reduction ratio past the knee point.

There is a good graph in the Wilwood link that Jw60 posted:


The intent of this is to approximate the ideal braking curve with two lines. The ideal braking curve has the rear pressure proportionally dropping as the front pressure rises. This is due to weight transfer under braking. The harder you brake, the lighter the rear gets and the easier it is to lock it up. The ideal curve is vehicle dependent and is effected by f/r brake piston sizes, brake diameter, brake friction, wheelbase, and CG height. There is a good explainer here: https://www.theautopian.com/an-engi...ke Curve is,rear axles under ideal conditions.

A well adjusted proportioning valve will closely match the ideal brake curve like so:


However, if you put a proportioning valve in the front brake line, the knee point is in the wrong direction and you are trying to use one of the blue curves to match the red ideal line and it isn't going to work:


The only proper solution in your case would be putting larger pistons in the back too. But in practice, our High CG, short wheelbase Jeeps don't need much in the way of rear brakes anyway and you might be fine as is.

 

You make a good point.

As shown in the chart above, even when swung all the way forward in the bias, the rear brake still has braking pressure; just not as much. It would brake, but likely not lock up if rolling backward on a steep climb.

But this also reinforces my need for a hand lever-controlled emergency brake out back. Keep the button pushed and you can control the pressure on the rear drums.

You can also 'sort of' use the clutch to apply pressure, regardless of which gear it's in, by lightly pressuring it in and out. I have a manual trans, but with an automatic that wouldn't be an option.

But suggestion well noted! This is where finding that balance and doing lots of testing makes sense.

 

Sooooo, Roy, are you suggesting I tie the FC 150 counter weight on the back of the B? I certainly was going to try it initially as is with the proportioning valve completely backed off. I guess if that is still to much forward biased, like you suggest, I can fit the same larger piston calipers on the rear and use the valve as intended. That thought did occur to me. The rotors are the same diameter.

 

Also put your transmission in reverse so you can still steer the front when descending.

 

Sometimes the trans won't go into reverse when I'm sitting flat and dry, so, on a hill, well... I might be apprehensive to try to shift at all, preferring SOME gear to 'nothing but neutral'.