Welcome to our resource for MG Car Information.
MG Midget and Sprite Technical - Hydraulic System sizing (switch to sebring style)
|Before anyone says read the archive I've done a huge amount of reading and would appreciate a check of my thoughts
I have a modified frogeye with the 9 spring clutch.
It was converted to later discs at the front but initially retained the 7/8 dual master cylinder and 7/8 single piston rear cylinders.
I am considering switching over to a dual cylinder sebring style set up now i've realised the risk of rear brake lock ups and 3/4 GWC1116 rear cylinders.
In terms of sizing of a couple of the hydraulic systems I am unsure of the best approach
Current: 7/8 dual MC to 7/8 slave: 9 spring clutch: ALOT of driver force required.
Plan:would I best changing to solo MC to 3/4 and a 7/8 or even 1" slave? This should sofen the pedal but increase the travel. but do I risk increasing the slave cylinder push rod travel distance? Could anyone advise on what they would suggest?
Current: 7/8 Dual MC, 8 1/4 inch 2 pot discs, 7/8 rear single cylinder drums
3/4 solo MC, discs and pots as above, 3/4 GWC 1116 single rears. Or would it be better with a 5/8 Solo MC? again softer pedal and longer travel?
Should I put a residual valve in the rear brake line (?8PSI) with this set up?
Summary: I currently have appropriate 3/4 (Clutch) and 5/8 MCs (Brake) for a sebring pedal box but pondering if thats the best choice and whether need to change anything else i.e. clutch slave cylinder, pedal ratios etc.
Lastly: anyone know anywhere to get an appropriate spacer made?
Thanks in advance, this is all a bit new to me.
|If you are using discs then the norm is 3/4 bore MC and 3/4 bore rear slave.
Frogeye clutch, whether 6 or 9 spring would be 7/8 bore MC and 7/8 bore slave.
I have made a couple of Sebring pedal boxes, sorry no pics, for people in the past and made tubular plinths - a tube with washers on the ends, as a substitute for the large alloy spacer on the regular Sebring set -up.
Even with my regular (Wheeler & Davies) Sebring set-up the clutch MC has to be filed a bit on the
fitment flange for it to sit alongside, but offset from, the MC alloy spacer.
Its lower nut and bolt fixing is a 'B' to fit so I make a custom fitting that is a press fit in the lower hole (see pic to follow)
|Custom fitting for lower clutch MC hole.|
|Well don't worry about the archives... I'm pretty sure what your asking isn't in there.
I have another option you might consider that won't cost much more then your going to invest
Several specialty company's make what is know as a triple master cylinder braking system it uses one master for the front, one for the back and one for the clutch
The front and rear masters cylinders use a bias bar that connects the 2 together... you can get any size masters you want for any of the 3 masters you need so you can mix and match to coordinate with your curent existing slaves
These are very high end masters even at the economy scale and you can buy these as a complete kit with new peddles or in limited form to make your own creation to save a few dollars
I have a complete kit from tilton, and I LOVE IT going on 12 years and over never touched it scence I installed it
Check web sites for Jags.com and summit racing.com they will have several to look at online
Btw ... If you make your own kit with the current peddle box and peddles you have to attach the clutch master to the out side of the box ... that IS in the archives several times look for photos from an old friend of ours and my mentor named BOB YOUNG he has a number of photos and did an extensive amount of work for redeveloping the peddle box for a triplle master cyinder set up before he passed on, maybe 4 to 5 years ago
|thank you guys
what I am looking to achieve is period appropriate modifications to the car.
it was originally modified in the mid to late 60's and so thats what I am aiming towards rather than modern upgrades
With the current 7/8 MC and 7/8 salve on the 9 spring clutch the force from the driver seemed excessive. Now having stripped it all apart i've discovered the clutch fork was bent and modified (extended), as was the slave push rod, these have been rectified back to normal so it may be operating on a better lever angle now. If i dropped the clutch MC to 3/4 with a 7/8 slave that would be softer but run the risk of too much slave push rod travel?
as for brakes happy to go 3/4 MC 3/4 rear cylinders but would dropping to a 5/8 MC again be advantageous in any way? I note the sebrings ran 5/8 MC on the brake side but believe also 5/8 rear MCs on riley 1.5 drums?
|For the brakes I think your plan for a 3/4" m/c with 3/4" rear cylinders will work very well.
The 7/8" rear cylinders were always too big, and can lock up the rears too easily - same problem on the Minor (where the 7/8" cylinder came from). The original rear brake proportion was 37%, and I would think it would be similar for a Frogeye. Changing the front to standard 8.25" diameter discs, makes very little difference to the brake proportions, except that with the grip from modern tyres, the brake pedal can be pressed harder, which just increases the problem with the oversized rear cylinders.
The Minor also went down to 3/4" rear cylinders, combined with a smaller master cylinder bore, whilst increasing the drum size and retaining the same front cylinder size. The balance went from 63%/37% to 74%/26%, and the pedal pressure reduced by around 15% for the same brake performance.
The original or later 3/4" rear cylinders work fine without the 'residual pressure' valve in the master cylinder - presumably it has already been removed?
Are you planning to fit the later 3/4" twin-piston midget rear cylinders, or the later 3/4" Minor ones, which is a straight swap with the existing 7/8" cylinder - assuming it is still the original design?
You could get one bore of a dual master cylinder sleeved so that the brake side was 3/4" and the clutch 7/8". I was lucky and found one that someone had converted (use it on my MkII 948cc Sprite with front discs and 3/4" rear wheel cylinders using standard backplates - following Alan's very helpful advice on using 1098cc Morris Minor wheel cylinders - thanks Alan!)
There is a mention of a company in the following article that may be able to sleeve a master cylinder for you: http://www.gerardsgarage.com/Garage/Tech/DBrake/TandemMC_Part1e.htm & http://www.pastparts.co.uk/index.php?route=information/information&information_id=8
Useful info here too, particularly on the different dual master cylinder pushrod lengths: http://www.gerardsgarage.com/Garage/Tech/DbrakeUg.htm
If I were buying a new dual master cylinder it would be a genuine AP Caparo one, for example from Powertrack Brakes Ltd, http://www.powertrackbrakes.co.uk, not the TRW ones that others have reported problems with.
Also worth looking at Coasting Ltd for parts: http://www.coastings.co.uk
For seperate Girling master cylinders in a Sebring style pedal box I would want genuine ones, for example from Merlin Motorsport or Raceparts. Another option would be to use Wilwood equivalents in Sebring pedal boxes.
Guess you have seen the info and drawings on Sebring Sprite website for pedal boxes, including pedal differences?
|The MM rear cylinders are much more expensive than Midget ones. The MM are a direct fit. The Midget cylinders need a later backplate. So if you can source the backplates at a reasonable price, the .75 Midget cylinders are the way to go.|
This thread was discussed between 26/02/2018 and 01/03/2018
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.