Jump to content

Brake Bias


zerorace
 Share

Recommended Posts

hi all just a quick one i have the bias bare all the way to one side to reduce the rear brake pressure but it still locks the rears to easy would the next size up master cylinder do the job do you think at the moment i have .625" on the front was.75" and .75" on the rear changing from the .75 to .625 made it better as when they were both the same size the rear bakes locked first.

Link to comment
Share on other sites

This could be incorrect set-up of the components or miss-match of components.

Can you list the components. ie. I currently run bias twin parallel master cylinders.

Front is 0.625 into vented standard 2.0L sierra discs/callipers with mintex m1144 pads.

Rear is 0.75 into standard 2.0L sierra drums and slaves with no-name shoes.

 

Adjustments of the bias bar are critical as there must be sufficient room for the bias bar to swing at least 450 and accommodate the different travel of the two master cylinders. The rear master cylinder stroke to full application on mine is about half an inch, the front about one and a half inches. (When you remember that the original sierra did its braking with same size master-cylinders in tandem (plus a dynamic rear limiter) it makes me wonder why I followed the standard advice to use different sizes in my pedal box but that's another story) That's quite a swing of the bias bar. It would probably be different if I had rear discs.

 

Nigel

Edited by Longboarder
Link to comment
Share on other sites

Can we make the assumption that the pedal and bias bar are properly adjusted? See pic below.

 

If so your options are

1. Better pads on the fronts (M1144) and or 'worse' (try Greenstuff) pads on the back.

2. Larger bore rear master cylinder. Wilwood do a 0.813 or 7/8 inch one. http://www.rallydesign.co.uk/product_info.php?products_id=16617

3. Smaller bore rear callipers.

4. Pressure limiting valve in the rear brake line. CBS do two sorts. Page 22 of their catalogue. Not IVA friendly but then a bias bar isn't if it's adjustable.

 

Are you sure you have sierra front callipers on the back? What is your handbrake arrangement?

 

Nigel

Edited by Longboarder
Link to comment
Share on other sites

Thanks nigel no handbrake fitted so I think you have confurmed what I had thought larger rear master cylinder to start with and try again it is only just over braked on the rear will let you know when I get it out again about March or April Bedford Autodrome thanks again

Link to comment
Share on other sites

zerorace has too much braking pressure in the rear system. The bias bar has already been fully adjusted to reduce brake pressure to the rears and maximise pressure to the fronts. The front and rear systems are hydraulically separate although the bar is a mechanical link. Moving to a bigger master cylinder will reduce pressure but it may not be enough. As a limiter/reducer, commonly with one line in and one or two out, will lie in the hydraulic line between the m/c and rear callipers it can only affect the hydraulic circuit it is in. It can have no effect on the front circuit. It can only affect the rear brakes by further reducing pressure.

If this car is basically a trackday tool I would probably opt for a limiter as my first choice. It's cheaper than a new 7/8 master cylinder and gives a greater range of adjustment. It also gives easier adjustment on the fly for when the day starts wet and dries out.

 

Nigel

Link to comment
Share on other sites

I take your point Nigel, with the information we have, but putting in restictor will defeat the object of a bias system and should not be necessary.

 

The cylinder combination that Zerorace is using will give approx 60/40 split at the cylinders, assuming bias is at mid point and the smaller cylinder is feeding the front.

 

A matched caliper combination would change that to nearer 76/24 at the pads, there are other variables such as pad area, disc size and pads/disc friction coefficient, but the split is already low to the rear, before adjustment, but even fully adjusted Zerorace still has rear lock problems.

 

Similar, not the same, my sons garage could not get the MOT standard on a mark one Golf, fully stripped out competition car with bias brakes.

 

The problem was that the owner had plumbed the smaller cylinder to the rear, swapped over it went through with the bias locked mid way, he was running 0.685/0.75 which gives a 55/45 split at the cylinders with standard with matched calipers, pads and discs it achieved MOT so must have been between 60/40 and 70/30, which I believe is the acceptable standard.

 

My point being that there may be another issue here.

Link to comment
Share on other sites

I have made that point in each post. Setting up a bias bar is not intuitive. Spacing to allow full swing, differential piston range of movement and the static position is often skewed, not at right angles to the pushrods. But I can't check the car, plumbing and positions myself. Only rely/hope it is set-up right.

Just noticed I didn't add a pic to one of my posts.

 

Nigel

 

ps. To clarify. Maybe your first step would be to increase the clearances of the clevis joints just to make sure the bias bar can swing. I remember that when I first fitted mine the spacing was too little, the bar was almost locked and did lock as soon as the rear drums were full on. This resulted in no further increase in pressure to the front calipers so I had virtualy rear wheel braking only.

post-21-0-85717700-1448722264_thumb.jpg

Edited by Longboarder
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...