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Go Back   Madabout Kitcars Forum > Mad Build Area > Marlin Sportster, Cabrio, Berlinetta and Roadster builds

Marlin Sportster, Cabrio, Berlinetta and Roadster builds Enthused or Confused about your vintage Marlin build? Ask away here or show off your build.

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  #21  
Old 1st November 2011, 19:43
Mike Mike is offline
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Quote:
Originally Posted by mashtun View Post
A Sierra one is 8" (assuming the measurement isn't across the full width of the unit including the seam joining the two halves, but 8.5" if it is). The unit in an M50-engined E36 appears to be 10" (a bit awkward to measure it in situ though).
Thanks for that. I reckon the Metro is only 6.25" at most.


Assuming that servo boost is relative to surface area of the diaphram then using the 10" BMW as a base the relative boosts are:

Metro - @6.25" = 39%
Sierra - @8" = 64%
Dual - @2x7" = 98%
BMW - @10" = 100%

If the above is an accurate reflection of the boost available it helps explain why the Metro servoed brakes require significant foot pressure to make them work.
Does anyone have a different take on it?




I am now very interested in following up the Dual 7" as I think it could be made to fit, with a little care and ingenuity.
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  #22  
Old 1st November 2011, 23:37
mashtun mashtun is offline
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I hadn't noticed before that you're tight for space for a servo, but it's clear enough to see there. My Cabrio's like, for example, Simon's in that my servo's mounted directly on the bulkhead i.e. not on a protrusion as on yours. That means the available width on mine isn't reduced by the bonnet and the Sierra servo fits without any problem. My bonnet detail's also different - the lower section is narrower than yours as it's just a tight fold over of the aluminium sheet (my upper and lower bonnet sections are hinged together). Must get some photos online somewhere...

Quote:
Originally Posted by Mike View Post
Does anyone have a different take on it?
I've never considered how a servo works but it does seem reasonable to conclude that the assistive force isn't multiplying the force applied through the pedal but is instead simply adding to it (and usually greatly exceeding it?). Whether or not a dual diaphragm 7" unit gives 98% of the force of a single diaphragm 10" unit presumably depends on the same pressure difference existing across both diaphragms? I suppose that's quite possible but it must make the innards quite complex.

Might you end up with too much assistance from a dual 7" unit though?
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  #23  
Old 2nd November 2011, 09:39
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Modern cars have very light brake pedal so I don't see 'too light' being a massive issue as you kinda get used to it.

I suppose you just need to be very careful of locking up the wheel without ABS and needing to let of a bit again in an emergency stop situation.
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  #24  
Old 2nd November 2011, 21:43
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Quote:
Originally Posted by mashtun View Post
I hadn't noticed before that you're tight for space for a servo, but it's clear enough to see there. My Cabrio's like, for example, Simon's in that my servo's mounted directly on the bulkhead i.e. not on a protrusion as on yours. That means the available width on mine isn't reduced by the bonnet and the Sierra servo fits without any problem.
Hi Mashtun
How do you find your brakes with the standard Sierra servo?
Do you have drum or disc rear brakes?

I am aware that fitting a 200BHP engine means that if I want to use its performance I need brakes to match, and have been concerned from the outset about pedal pressure: particularly as I am now used to modern "over servoed" German performance.

I have fitted the larger Cosworth/Granada discs at the front, and have the Sierra discs at the rear, so I'm happy that the caliper capacity will be sufficient, but guess with the little Metro servo the pedal pressure is likely to be high? Patrick and Robin's comments about their brakes seem to support this.
There is of course more to it than just the servo boost - the bore of the master cylinder, and the mechanical advantage of the pedal arrangement are factors too.
Could I end up with too much boost? - I certainly felt my first Audi was over boosted - it took me ages to brake lightly they were so sensitive, but after 7-8 years I have become very accustomed to them.

I am still very tempted to explore this option, but others' views would be welcome.
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  #25  
Old 2nd November 2011, 22:03
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Quote:
Originally Posted by Patrick View Post
Modern cars have very light brake pedal so I don't see 'too light' being a massive issue as you kinda get used to it.

I suppose you just need to be very careful of locking up the wheel without ABS and needing to let of a bit again in an emergency stop situation.
Both good comments.
Us older folk (anyone learning pre ABS) were taught to cadence break in an emergency, so I may have to revert back to it, instead of relying on the ABS !
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  #26  
Old 3rd November 2011, 07:44
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Having learnt to drive in pre-ABS days, and rallied in a Mk1 Escort I still find ABS disconcerting when it deploys

I rather have full control and decide if I want to let a wheel skid or cadence brake.

I'm not too keen on over-sensitive brakes either, I rather they require a bit of a push rather then being lightly pressed, it seems easier to judge the amount of braking required when there is a wide range of pressures required.
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  #27  
Old 3rd November 2011, 07:55
mashtun mashtun is offline
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Hi Mike,

Quote:
Originally Posted by Mike View Post
Hi Mashtun
How do you find your brakes with the standard Sierra servo?
Do you have drum or disc rear brakes?
Sadly it's so long ago that my Cabrio was last on the road (almost 14 years... ) that I really don't recall its braking characteristics. I do remember other problems though (some of which, like the lack of width with the standard pedal arrangement, you've addressed and I plan to address) but I'll save those for other threads.

My donor was a 1983 1.6 Sierra so 240mm solid discs on the front and drums (8", I think) on the back. However, I'm replacing those with front calipers that'll take 260mm vented discs and a rear disc setup that'll take solid discs (253mm, I think).

Quote:
Originally Posted by Mike View Post
I am aware that fitting a 200BHP engine means that if I want to use its performance I need brakes to match
I think there are lots of factors that affect the braking performance you need - and IMO some have more influence than engine power (how late you leave it before braking, for instance - not advocating a leave-it-to-the-last-second braking style on the public highway of course, just saying!).

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Originally Posted by Mike View Post
There is of course more to it than just the servo boost - the bore of the master cylinder, and the mechanical advantage of the pedal arrangement are factors too.
No question that's true. And while we can be guided by what others have done (which certainly contributed to my decision to replace the drums at the back) and be more than reasonably confident of ending up with something safe and performant, ultimately calculation and comparison against some personal benchmark seems the only way of being sure? But I haven't made any calculations myself so am certainly not preaching!

Mark.
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  #28  
Old 3rd November 2011, 10:04
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My cabrio has 240.0mm vented discs at front, and 9 inch drums at the rear with std Ford servo
The braking is OK. Its possible to lock up the fronts on a dry road at a fairly high speed, and even after repeated use on a windy road there is no sign of fade. Not too sure how it would behave on the track.

I have managed to warp the discs as I do seem to be one of the leave-it-to-the-last-second brigade. Still new ones are relatively cheap and dead easy to fit.
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  #29  
Old 3rd November 2011, 10:13
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Quote:
Originally Posted by Chris Cussen View Post

I'm not too keen on over-sensitive brakes either, I rather they require a bit of a push rather then being lightly pressed, it seems easier to judge the amount of braking required when there is a wide range of pressures required.
Chris

That was exactly my reaction to the Audi's brakes. There was so little variation between soft and hard that it was hard to judge.

Having said that, the kids had an old Peugot 205 to learn to drive in the field and it had rock hard brakes that frightened you because they required huge pressure.........................I guess the trick is to find something in between.
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  #30  
Old 3rd November 2011, 10:44
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Quote:
Originally Posted by Mike View Post
Thanks for that. I reckon the Metro is only 6.25" at most.
Assuming that servo boost is relative to surface area of the diaphram then using the 10" BMW as a base the relative boosts are:

Metro - @6.25" = 39%
Sierra - @8" = 64%
Dual - @2x7" = 98% - revised estimate to - 1 x6.25" + 1 x5.25" = 66%
BMW - @10" = 100%

If the above is an accurate reflection of the boost available it helps explain why the Metro servoed brakes require significant foot pressure to make them work.

I am now very interested in following up the Dual 7" as I think it could be made to fit, with a little care and ingenuity.
I have found that the "(Dual 7")" is actually the same maximum size as the Metro, ie. 6.25" externally. I would also guess there are two different sized diaphrams, as the front chamber is 1" smaller. Although I have not found out what the internals look like, this assumption will reduce my calculations above to: pye x (6.25"/2 squared) + pye x (5.25"/2 squared)
This is then only 66% of the BMW at 10" and is very similar to the standard Sierra.


As the maximum diameter is the same as the Metro unit this will fit more easily in the limited space that I have in the cabrio than I initially thought, and with a similar theoretical boost to the original Sierra, this appeals even more. Watch this space.......................
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