I have a 66 with the proportioning valve and was wondering how to set the bias????

front +

front/rear =

rear +




Steve

 

Steve:

You don't want to lock the rear brakes before the front brakes in a max. effort stop. The car will tend to spin.

With sticky race tires, you'll get more weight transfer to the front, and the rears will tend to lock first. That's why the competition package had the adjustable prop. valve.

Bill

 

Correct. I used to get on a gravel road and do a hard stop with somebody watching the wheels. Then I would adjust the valve so the front wheel braking action kicks in just before the rear wheels.

 

You want to do this on pavement. The car will transfer more weight to the front on pavement. If you set it up on gravel, the rears will lock first on pavement.

 

Whatever. I've always done it on gravel (with three different cars) and never experienced the situation you describe. I'm almost sure my old Wildwood proportioning valve instructions said to test on gravel but they are long gone...

In any event here is a good article and 1/2 way down it describes the adjustment procedure:
http://www.chevyhiperformance.com/te...lve/index.html

 

Mine is a Tilton that I mounted under the dash. It is a lever with 7 repeatable settings. I can adjust it while I drive.

 

I looked at the article but it won't work for my application. According to the Vette Manual, I cannot go more than 2 full rounds in either direction. So I will have to play with it in that configuration. I have already gone 1 round to the rear. I may have to take that out as when I drove it today it felt like the rear was hitting before the front. Will make adjustments when I drive it again. Thanks.

 

Those are nice but I am playing with the old original configuration for a proportioning valve. Not as much adjustment with it as you have with yours.

 

Testing on gravel makes sense to me because you won't flat spot tire$! But I would make a final check on pavement.

 

Actually, since you brought it up I dinked around on the Internet and have found articles that support both types of surfaces for adjusting a proportioning valve.

 

Glad to see this subject come up because my '61 has an adjustable proportioning valve installed but with no logo or other designation on it as to manufacturer. It's definitely not the Wilwood product, or the same version sold by Summit, so I scanned the vendors in that Chevy Hi Perf article listed by Frank and found it's made by SSBC. My '61 has the Force 10 front disc setup so no doubt they got all the parts from SSBC. So far, the only real 'drive' I've had with this car is the 50 mile run in Texas which was about a half year ago. I remember it stopped well enough but I remember thinking that the pedal pressure needed to stop was kinda high. At the time, I wasn't sure if it was the brake setup or the fact that I hadn't really driven a solid axle Vette in about 20 years.

Anyway, the instructions that came with the Summit(Wilwood) valve I have in my stash of parts says that when the adjusting **** is turned fully counterclockwise the pressure to the rear is reduced to about 57%. Turn that **** all the way in (~10 turns) and it's then full line pressure.

Just went out to check the SSBC valve on my '61 and found that (even though the **** was long gone when I got the car), the adjusting shaft is turned all the way counterclockwise which I suspect should be maximum reduction of pressure to the back brakes. Unfortunately, I wasn't able to find the pressure range for this valve on the SSBC site but still expect that I'm currently running around 60% pressure to the rears and maybe that's partly to blame for the extra effort I noticed the last (and only) time I drove it.

For those of you running adjustable proportioning valves (with Summit/Wilwood having about 10 turns of adjustment and SSBC having about 9), where did your final adjustment wind up?

Thanks,
Mike T.

 

On my '64 with front disks and drum rears, I reduce the pressure to the rears by ~53% (notch 2). Notch 1 is the max reduction at ~60% and that one works OK too.

 

When you say you reduce the pressure 'by' 53%, you're saying the rears only see 47% of the pressure that the front discs are receiving or do you mean it's 53% of the amount the fronts are getting?
That's less than the Wilwood valve would allow at it's lowest setting which I think is set for 57%.
I know your post said you had 7 notches, do you see rear wheel lockup at the higher settings?
I wonder what the percentage difference is on the cars with pre-set proportioning valves?
On my '61 it's set for the least amount of pressure allowed to the back brakes and until I get it on the road I'll leave it that way but am hoping to be able to get a bit more braking for the pedal effort by increasing the pressure to the rears.
Mike T.

 

My '61 has NO proportioning valve and the front disks (stock drum rears) were put on by prev owner. The car stops adequately but requires serious pedal pressure for hard stops.

It was a little exciting when I first got the car until I got used to it!

My '60s Mustangs all had proportioning valves but I think the vette is good enough I'm not going to change anything at this point.

 

It is based on a linear scale across the pressure band. When the valve receives 1,500 PSI, it reduces it to ~700 PSI to the rear brakes, So (1 - 700/1,500) = 53% reduction and rears receive 47% of what the fronts get. See the link for more info. It's a little different than the chart included with my valve, but you will get the idea.

http://www.tiltonracing.com/pdfs/98-...rop_valves.pdf

Yes, the rears lock up WAY before the fronts without the valve and to varying degrees at lesser settings (i.e., with more pressure to rear).

If the car is properly engineered, it should vary by brake system and weight distribution of the car.

 

Frank, how long have you had the front discs on your '61? Are you running a dual master cylinder? I'm surprised that you don't find the rear brakes locking up way ahead of the front brakes since you're not running a proportioning valve.
Again, my only 'driving reference' with my '61 was on the day I bought it 6 months ago (before it got perched on jackstands in my garage). I found out today that the proportioning valve is set at the 'maximum reduction' of pressure to the back brakes and while I found no squirrely braking action that day in Austin, I do clearly remember the extra effort needed to come to a stop. The Summit instructions say to make two turns of the adjustment **** (increase of pressure from the minimum pressure setting) and then test drive the car to see how the brakes react and when you reach that point where the back brakes are locking up ahead of the front brakes, back it off. Sounds simple enough.
Now I need to get the car back on the road and start the test procedure.
Mike T.

 

The front disks have been on the car since I bought it two year ago. No dual master cylinder....I insisted on them in my '60s Mustangs but fitting them in a C1 is ugly...you can make them fit but they just look wrong. They ARE safer and more efficient but I drove single master cylinder cars for years and had some failures too...you have to stay on top of things with these old cars anyways and keep stopping distances long, etc.

I don't know why the brake balance seems OK without a proportioning valve...maybe the installer used some other technique or it just plain worked out ok...

 

If you have rear drums, you can reduce the wheel cylinder bore size to reduce rear lock-up. That's what the drum brake racing Corvette packages did (684, 687, Z06).

 

The Wilwood valve will allow up to a max 57% reduction in output pressure. That amount however is not what you are setting.

What's being set is called the knee point; the input pressure point at which the reduction begins. So you cannot (example) count turns and say 'ok, this is half way so I have about 30% reduction' in output pressure.

The way the valve functions is based upon INPUT pressure. If you put in 150psi you may well get 150psi out. If you put in 250psi you might get out only 150psi. The knee point adjusts where the reduction begins and from there the actual reduction is based upon how much input you supply it. By starting at a higher setting however you'll see less final output, sooner. Up to a total of 57% or only 43% of the input.

The purpose of this is to allow more rear pressure under light duty to provide rear brake pressure as the vehicle can use. As the demands of pressure up front are increased the value restricts rear pressure to prevent lock up. As you adjust it you begin the restriction sooner. The question is less about how much restriction do you need but how much flow can you handle. I'd urge you to begin with max restriction and work back- all the while trying to maintain the same hard pedal applications on the same surface. Dial it back from there until you begin to feel the rear twitch under that level of pressure. Then turn it up a quarter of a turn. Under more extreme pressures the cut will be greater which is what you desire.


*It irks me that Wilwood does not provide this but Stoptech has a good article on this and in graph C you can see what the knee point is. This point of pressure reduction is what is adjusted by the spring in the valve body as you "dial it".
http://www.centricparts.com/files/Ce...g%20Valves.pdf

 

I don't know about you, but my Corvette will never get close to a gravel road as long as I own it.

Especially to lock up the brakes and spray gravel everywhere.

Doug