Jump to content


Photo
- - - - -

Rear Brake Cylinder Size


  • Please log in to reply
8 replies to this topic

#1 alex-95

alex-95

    I am THE CLAMP MAKER

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPip
  • 5,566 posts
  • Location: Cornwall

Posted 03 March 2019 - 10:57 AM

I'm currently running 3/4" bore rear cylinders along with 7.5" front discs, yellow tag master cylinder and standard brake limiter valve (it's a 1982 HLE). There are a few threads on this already but want to make sure before I swap them that fitting 5/8" rear cylinders would be better as there is less brake force on the rear, or does this depend on the rest of my setup.



#2 hazpalmer14

hazpalmer14

    Super Mini Mad

  • TMF+ Member
  • PipPipPipPipPipPip
  • 571 posts
  • Local Club: cumbria mini cruisers

Posted 03 March 2019 - 11:34 AM

I've read something about this. I'm going from drums all round to 7.9 vented 4 pots. Everything is standard 1980 clubman estate. Think it was on KC site I read about switching the rear wheel cylinder sizes.

#3 Ethel

Ethel

    ..is NOT a girl!

  • TMF Team
  • PipPipPipPipPipPipPipPipPipPipPip
  • 22,725 posts
  • Local Club: none

Posted 03 March 2019 - 11:43 AM

What brake regulator valve are you running?

 

It will mean less rear brake force with either pressure regulator as they will operate at the same pressure regardless of rear cylinder size. The maximum force the cylinder applies is therefore simply dependent on its piston area.

 

 

(edited as thinking aloud)



#4 alex-95

alex-95

    I am THE CLAMP MAKER

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPip
  • 5,566 posts
  • Location: Cornwall

Posted 03 March 2019 - 12:10 PM

What brake regulator valve are you running?

 

It will mean less rear brake force with either pressure regulator as they will operate at the same pressure regardless of rear cylinder size. The maximum force the cylinder applies is therefore simply dependent on its piston area.

 

 

(edited as thinking aloud)

I've got this type https://www.minispar...|Back to search



#5 Ethel

Ethel

    ..is NOT a girl!

  • TMF Team
  • PipPipPipPipPipPipPipPipPipPipPip
  • 22,725 posts
  • Local Club: none

Posted 03 March 2019 - 03:13 PM

The beloved FAM7821  :D

 

So you're going from drums to 7.5 discs? 

 

If you already have 3/4" cylinders fitted I'd be inclined to try them first. It won't take much effort to swap them if you find the rear's are a bit grabby.

 

I think I'm decided that a FAM7821 does more limiting with a yellow band master than it would with earlier masters and a yellow band master has more bias to the front than a single circuit setup to make comparisons with the original 7.5 "S" setup a useful indication of what you need.



#6 Moke Spider

Moke Spider

    Crazy About Mini's

  • TMF Team
  • PipPipPipPipPipPipPipPipPipPip
  • 8,729 posts
  • Location: NSW
  • Local Club: South Australian Moke Club

Posted 03 March 2019 - 04:44 PM

Try the 3/4" cylinders as Ethal has suggested but I think you'll end up going to 5/8" cylinders.

 

The smaller rears will also improve the pedal travel and feel.



#7 alex-95

alex-95

    I am THE CLAMP MAKER

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPip
  • 5,566 posts
  • Location: Cornwall

Posted 03 March 2019 - 08:37 PM

The beloved FAM7821  :D

 

So you're going from drums to 7.5 discs? 

 

If you already have 3/4" cylinders fitted I'd be inclined to try them first. It won't take much effort to swap them if you find the rear's are a bit grabby.

 

I think I'm decided that a FAM7821 does more limiting with a yellow band master than it would with earlier masters and a yellow band master has more bias to the front than a single circuit setup to make comparisons with the original 7.5 "S" setup a useful indication of what you need.

 

 

Try the 3/4" cylinders as Ethal has suggested but I think you'll end up going to 5/8" cylinders.

 

The smaller rears will also improve the pedal travel and feel.

 

 

Cheers, I've done about 2500miles with the 3/4's and didn't really notice them being grabby, but then I haven't really pushed them to the limits if you know what I mean as it was a slow 998 engine but am now fitting a 1293 so wouldn't want the rear trying to overtake.

Thats handy to know abut the bias of the yellow master cylinder.

 

Would the pedal travel be shorter then as it's pretty short at the minute. The brakes aren't bled at the minute so might change them over.



#8 Moke Spider

Moke Spider

    Crazy About Mini's

  • TMF Team
  • PipPipPipPipPipPipPipPipPipPip
  • 8,729 posts
  • Location: NSW
  • Local Club: South Australian Moke Club

Posted 04 March 2019 - 12:16 AM

Alex, I'd suggest trying the brakes on a quiet road in the wet to be sure that they in fact OK, rather than find out in a real situation.

 

Yes, the pedal would get shorter with the 5/8 cylinders.



#9 Ethel

Ethel

    ..is NOT a girl!

  • TMF Team
  • PipPipPipPipPipPipPipPipPipPipPip
  • 22,725 posts
  • Local Club: none

Posted 04 March 2019 - 11:18 AM

Agreement here,

 

The engine doesn't matter for stopping power the same effort is required according to your speed. How quickly you accelerate to reach it might bring on brake fade if means the time between braking is reduced (not likely on the road, if you still have a driving licence). Find somewhere safe & quiet to have a play and take in to account going downhill and cornering will reduce the weight on one or both the rear wheels making it more likely to lock.

 

You'll get about a 30% reduction in force going from 3/4 to 5/8 bore. I doubt you'll see quite as much in braking effort as drums generate some of their own force.

 

It really is a complex situation where suck it and see is going to be best.






0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users

Mini Spares