Brake setup?? 1

[willeng]

We have a car in that has 4 wheel disc brakes, 4 spot AP race calipers on the front & AP single spot race calipers on the rear. On further inspection it has dual master cylinders & a brake balance bar fitted, it has only the front brakes being boosted by a remote booster & is impossible to brake correctly with the fronts locking all the time.
Could anyone advise me to how this should be setup correctly, my feelings are that the rear brakes should also be boosted & a brake bias valve or proportioning valve should be fitted into the system somehow?
Being an engine builder i need some advice with this to help the customer out, so any & all help will be appreciated.

Thanks

 

[Tmoose]

Could you disconnect the booster and see how it works?

I guess I've read that the bias bar is supposed to have within its adjustment range the best front / rear proportion.

 

[willeng]

Tmoose, the bloke who owns it disconnected the booster because it was to dangerous with the fronts locking all the time. As a result of this you need Mr universe legs to get enough pressure on the brakes to get them working!!

Thanks

 

[patprimmer]

Can you apply a booster to the pedal before the balance bar, or two boosters after the balance bar.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[willeng]

pat:

I think your right with the suggestion of a second booster, i will look into it & see how i go!

Do they make a remote booster with twin hydraulic cylinders on it, the remote booster at the moment has a single cylinder on it.

Thanks

 

[moparmark]

willeng, i have a similar setup on my car but dont use a booster. the master cylinder size on these setups usualy vary from 5/8inch upto 1inch. you may want to check what size is fitted as if they're the 1 inch ones pedal effort is a lot higher than if you us say a 3/4 or 5/8. the best compromise is to size it so that pedal travel is exceptable but firm pressure is required thus allowing the greatest control. simple test, remover the booster and centralise the balance bar and test car by braking firmly from 30mph and observe which brakes lock first. if its the rear get a smaller master cylinder for the front circuit(depending on current size) and try again. the pedal effort will go down the balance would be better and the car will be safer.
it would help to know what size masters are already on there and the current amount of pedal travel, if you know these please post them.

 

[willeng]

moparmark:

Thanks for the reply, the masters are both 5/8", not sure of the current pedal travel as we have the rear end out of the car at the moment, also the engine is out being re-built!
From what the owner has told me the fronts keep locking all the time with or without the booster connected especially when the balance bar is adjusted equally!

Thanks again

 

[patprimmer]

Can you make the pedal leverage on the master cylinders better?

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[moparmark]

willeng, thanks for that. given that your pedal assembly is of the 6:1 ratio type you probably have already gone as small in the master as practical for any given front disc setup. without the booster is the effort required to lock the front brakes considered excessive? is the car used for track work or street. if its street you best option will be to go back to a normal booster setup. pedal boxes are usualy setup so that firm pedal effort will not lock the brakes easily maximising brake control. if the rears are drum brakes and the pedal effort required to lock the front brakes is exceptable you could try going up a size in the rear wheel cylinders which will make the balance better and then just fine tune it with the balance bar, but i would not recommend over adjusting the balance bar to try and compensate at the moment. idealy if with a neutral balance bar if the pedal effort to lock the brakes is ok with your customer you will be able to make this work well fairly simply . let me know how you go and also maybe what sort of front and rear brakes are being used if you are going to try and make it work so we can better judge the easiest fix.
thanks
mark

 

[willeng]

Pat:

Yes, this can be done easily!

Thanks

 

[Warpspeed]

I would think it would be best to have a conventional mechanical booster before the balance bar setup, and then adjust relative master cylinder size or balance bar leverage to get the desired front/rear brake bias right.

Power boosters often do not have a linear force multiplication, because the vacuum from which they operate is finite. So at low force levels you can get more boosting, and when the driver really leans on the pedal there is less boosting.

This is not so bad if the booster operates the brakes at both ends of the car with a conventional tandem master cylinder. But if only the fronts are boosted, with a hydraulic/hydraulic booster I would expect premature front lockup where traction is not so good such as wet or uneven surfaces, and premature rear lockup where traction is excellent on a smooth dry road.

Having a hydraulic/hydraulic booster at each end of the car might only make it MORE unpredictable in my experience, because the force levels and hydraulic pressures will be different at each end of the car.

Best to keep the hydraulics 1:1 and use mechanical boosting between the pedal and the balance bar.

 

[willeng]

Warpspeed

Could you tell me more about the conventional mechanical boosting setup before the balance bar.


Thanks

 

[Warpspeed]

The mechanical booster is just the common standard factory arrangement these days, where the pedal pushrod acts directly on the rear of the vacuum servo booster on the firewall, and there is a tandem dual circuit master cylinder bolted onto the front.

The vacuum booster amplifies the mechanical effort direct from the pedal pushrod to the master cylinder. What you would do is replace the single tandem master cylinder with a balance bar and dual master cylinders acting in parallel. That way the booster acts before the balance bar.

I assumed that if only the fronts are boosted on the existing setup, there was a hydraulic/hydraulic vacuum servo booster on the front circuit only. PPR used to make a VH44 booster which did this, but I think it may now be obsolete, as I have not seen one for a very long time. They used to be fairly common to boost the old single circuit brakes with drum/drum, or disc/drum combination.

Some thought will also need to be given to master cylinder bore size versus, pedal stroke, and fluid displacement. With a tandem cylinder if the total available master cylinder stroke is one inch, each piston moves (displaces) only half that. With a balance bar that has one inch total stroke, each master cylinder could move the full inch. So to displace a similar amount of fluid over the full stroke, the bores of both master cylinders will need to be smaller.

For safety, you will need to still have some pedal height left if you totally lose one hydraulic circuit, so master cylinder bore size is important.

 

[funnelguy]

This link is to a spreadsheet that thread participants might find useful...

http://sports.racer.net/brake_bias.htm