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MG MGB Technical - Reducing clutch pedal pressure

Since fitting a radial, roller bearing clutch release system to our rally GT the pedal pressure required has increased significantly and my driver finds it very painful to drive since having had a none too successful foot operation. Has anyone any experience/ideas for reducing the pedal pressure without having to take the engine out again!!

One suggestion I have had is to reduce the master cylinder bore size, which should be possible as the pedal travel has been significantly reduced and is in fact limited with a stop. Does anyone know of a smaller diameter master cylinder that would fit the B? In the radial release setup there is no slave cylinder.
Graham Gilmore

According to my information the MGB master is 19mm bore and the V8 17.8mm. BHA5217 in the Parts Catalogue, couldn't see any of those at any of the usual suspects but elsewhere showed the part number as GMC 1011, which Paul Depper lists - http://tinyurl.com/9suldr
Paul Hunt

Graham
I don't know if this will help or not and I can't remember exactly what it was on but on either an early Toyota Landcruiser or similar Nissan Patrol, one of them had a tiny little power booster like a brake booster but probably only about 70- 80mm diameter on the clutch. With the motor off (no vac.) the clutch is very heavy but with it going your Grandmother could handle it with ease.
Just a thought Cheers Willy
WilliamRevit

Graham, is the required pedal movement still the same as with the original slave cylinder? If it's a bit less you could drill the hole for the clevis pin a bit closer to the pedal pivot and increase the leverage of the system. Similar to what was done in this article
http://www.britishv8.org/Articles/Budget-Brake-Mod.htm
Bill Young

Thanks for the ideas chaps.

Using the V8 m/cyl would only reduce the effort required by about 14% and I want to at least halve it. That means reducing the bore to around 13mm. Maybe I can get one of the specialist reconditioners to sleeve the cylinder, modify a piston for me and supply some rubbers.

I had thought about using a brake servo mounted in the footwell but again would have to modify it to reduce the mechanical advantage gained.

Modifying the clutch pedal arrangement is an idea that I had not considered before but will do so now.

Thanks for the ideas, anymore would be appreciated.
Graham Gilmore

For a race car fitting a servo might be ok, but I wonder if there would be enough "feel" in the pedal with a servo to feather the clutch as we usually do on street driven cars. Seems to work ok for brakes, those seem to not be quite as sensitive to the "feel" as the clutch as far as I've seen. Also servos rely on engine vacuum for power and with high lift cams and a lot of overlap sometimes vacuum is on the low side, add in heel and toe braking along with throttle blipping and you have the recipe for low assist on the shifts. I think I'd look at a slightly larger bore master cylinder and alter the pedal ratio.
Bill Young

If you reduce the area of the master cylinder by half, then you'll only move half as much fluid per pedal stroke meaning that you'll need to move the pedal twice as far to effect the same movement at the slave cylinder. You'd have to do your sums very well, and get the set-up just perfect, to make sure that you got full engagement at the top of the pedal travel and full disengagement at the bottom of the pedal travel. If it works at all!

Why does the roller bearing make the clutch more difficult? I cannot see how that has an effect on the force required to disengage the clutch. Did you fit an 'uprated' clutch at the same time?

Neil
Neil

I agree with your comment Neil about doubling the travel but I reckon that is OK. At the moment the operating regime is right at the top, ie you don't have to push the pedal very far to disengage. I have already fitted a stop to prevent the pedal travelling too far and over-centering the diaphragm.

Yes, it is a Helix competition clutch operated by a Cambridge Motorsport radial clutch assembly which I now believe has a larger piston area than the slave cylinder and without the mechanical advantage of the release arm, hence the problem.
Graham Gilmore

sounds all right .., but what did the factory use in grx307d
jez

Jez, Bill Price told me that they always used the standard carbon release bearing in the comp. cars. That was in the days when they made good quality ones in the UK........
Graham Gilmore

This seems to me to be a hydraulic release bearing i.e. the 'slave' is fitted inside the bell-housing, and so is completely different to the standard slave, release arm and release bearing arrangement, which is why the pedal pressure and throw have changed so much. A roller bearing used with the conventional slave and release arm won't affect pressure or throw. Although personally I would never want to put the hydraulics in the bell-housing. And never use a roller bearing again in place of a plain one, come to that.
Paul Hunt

I was not keen on having the slave cylinder inside the bell housing either but I am afraid that replacing the carbon bearing every 3,000 miles or less was no longer an option after doing it twice. So far the roller bearing has done over 5,000 miles in UK and Europe. At the time that I did this in 2007 the roller bearing for the standard clutch arm was not available. I have just fitted the MGOC one to our Roadster.
Graham Gilmore

I don't know why your carbon bearings lasted so little time. In my experience they last at least 30,000 miles, and probably longer. I always slip the car into neutral and let the pedal up if I'm stopped for more than a couple of seconds, but then so do lots of drivers of old cars. The modern habit of sitting at red lights with the car in gear and the clutch disengaged is death to a carbon bearing.
Mike Howlett

Mike, I do that too in every car I drive now, not just the MG. Taking it out of gear at the lights I mean.

Unfortunately all the auto drivers we have these days are what I call creepers. They always stop short at the lights then creep forwards bit by bit (and then are usually slow to go once the lights actually change). I always end up with a huge gap in front of me.

Still a bit of space to accelerate into isn't a bad thing!

3000 miles for a release bearing does sound very short?

Simon

Simon Jansen

According to their website, the system is supposed to mean "lighter and smoother control for competition clutches". Does that imply something amiss in the set-up if you've got a 'heavier' clutch?

It also says that periodic maintenance is required. And as it is in the bellhousing, then that's an engine-out job!

If you are breaking release bearings every 3,000m and this axial one is making things heavier, I'm wondering if there is something else wrong. Do you ride the clutch? Was the old release fork bent?

N
Neil

I think I recall this topic from before now Graham mentions they are only lasting 3k, although I thought that related to competition. If you are talking road use then there is definitely something very wrong, there are tens of thousands of us running around with carbon and no problems - in the main. There *was* a spate of carbon release bearing failures but this was due to an 'improved' design feature where the carbon ring was pinned into the casting instead of being bonded (see http://www.mgb-stuff.org.uk/wn_clutchframe.htm and click on 'Release Bearing'). Thankfully they have reverted to bonded and seem OK again. Because of the doubts over the carbon I did look recently for a roller type to fit to a 4-cylinder but couldn't see it from the MGOC, but see it now, I wonder if I missed it or they were NLA and are A again.

By changing to a completely different mechanism at the slave end then unless the effective bore of the new system is the same as the old then the throw and pedal pressure *will* change. Having to limit the pedal travel to prevent 'over-throw', and also finding the pedal pressure is now much higher, is exactly what one would expect when the effective bore of the new slave is smaller than that of the old. Hence the search for a smaller bore master, or reducing the spacing between pedal and master push-rod pivot points. You would only get lighter and smoother control - without changing anything else - if the new slave effective bore were *more* than the old. But too much more and the clutch won't release properly giving drag and difficulty changing gear.
Paul Hunt

That's my point. Having a roller bearing per se will not decrease the force needed to disengage the clutch. It 'may' reduce frictional losses which you get when you move a stationary part (the release bearing) into contact with a rotating part (the clutch cover) but we're talking infinitesimal - does your engine slow much when you disengage the clutch?

So the only way to reduce the load is to increase the leverage, either mechanically or hydraulically. With a bigger bore slave cyliner, you will have to press the clutch pedal more to effect the same movement of the slave cyliner - and voila, more 'control' and less force required at the pedal. Quite why more 'control' would be an advantage in a competition car, I don't know - I suppose a harsher clutch would seem less harsh, but the harshness is only being reduced by increasing the amount of slip and that's going to lessen the life of the plate. It might lengthen the life of the gearbox I suppose.

N
Neil

Engine slowing when you push the clutch pedal down is an interesting thing. I'd seen a number of claims of this as part of a discussion on whether there was less load on the starter cranking with the clutch down or up (as if it matters). Some talk of it reducing by several hundred rpm, hardly something one would miss. It is something I've never noticed, and indeed neither of mine change the idle speed at all. Both have roller bearings though (standard on the V8) so I wondered if that was it. But tried a friends carbon-bearing 4-cylinder and that is the same, with the old clutch and the new. Also interestingly when doing the clutch we had to change the slave as well as that started leaking, and it is now so light compared to the old (and mine) that initially we thought it wasn't working but it is.
Paul Hunt

My engine doesn't slow when idling, but the cranking speed 'used to be' slower with the clutch pressed. But that was before I fitted a modern high-spec 12V battery instead of the two 6Vs.

N
Neil

Graham
Your choice of an internal slave cyl. is fine. Quite a lot of modern cars have gone that way. Personaly I wouldn't reduce the master cyl. to anything less than 19mm. Going smaller than this greatly reduces the life of the rubber cups in there. Even if you fitted a small booster as suggested earlier ,the pedal pressure might be OK but you will still have the short pedal travel problem. Bill's suggestion of moving the pushrod closer to the pedal pivot is the go for you with what you have. I fitted a 25mm brake m/cyl to a B once without a brake booster and fiddled around with pushrod position on the pedal and ended up about 3/8 in. closer. Best brakes ever.
Willy
WilliamRevit

A lot of interesting points coming out here folks, thanks. Just to re-cap.

The car is mainly used for historic road rallying and in the UK that includes around 10 driving tests per day with each test requiring 3 or 4 fast start/stops and hence a lot of clutch loads. Also when driving the regularity road sections one is often changing gear to maintain the set average speed.

The last two carbon bearings went after around 3,000 miles of such work but the roller bearing has lasted well over 5,000 miles which include 2,000 miles on Norwegian dirt roads on a rally. A friend also experienced the same 3,000 mile failure on his GT under exactly the same conditions.

The clutch pressure plate is a Helix competition clutch, supplied by Cambridge Motorsport at the same time as the co-axial roller bearing and is inherently stronger resulting in a higher force requirement.

Looking at all the ideas it would seem that moving the push rod connecting point down around 0.5” may be the 'simplest' one to try, initially without modifying its mounting points initially and just 'angling' the rod down. This would give around a 25% reduction in pedal pressure requirement. If the driver finds that acceptable I can then move the master cylinder mountings as per Bill Young's technique.

Thanks again for all the ideas and comments.

By the way Paul, the MGOC roller bearing is advertised again in the latest Enjoying MG.
Graham Gilmore

Graham - noticed it myself last night. But after fitting one myself, and having it squeaking just as it starts to take up or finally release pressure, I'd be in no hurry to fit another. I won't even leave that one in if I ever have to change the clutch again, but then mine is only road use.
Paul Hunt

I read the article about modifying the 'brake' pedal and, although it is cheap, it is quite an involved job unless everything is already apart... you'd have to take the whole lot apart, do some welding (properly!!!) and then put it all back together and rebleed. A whole day in a home garage I reckon. Plus several weeks to get the ar back out of the brakes - particularly if you have a remote servo!!!

It occured to me that that another way of checking whether the extra leverage was sufficient to help the driver would be to extend the pedal downwards. The extension would be quite a lot as there is say a 5:1 ratio on the pedal already, but it would be easier to bodge something for a test than to modify the other end (which requires a piece to be welded in and the pushrod hole to be enlarged). My car is in the garage, and it's too cold to go out there(!) but I think that you could make a 'test' extension with a piece of quarter plate (maybe even plywood) held to the pedal with some heavy duty tape, wire wraps or jubilee clips. It might extend too low, but Graham you said that you are not using all of the travel anyway. And as it is the clutch, then you don't have to actually drive anywhere to test it so there is no danger if the contraption comes apart during the test... And it would only take you half an hour! It's not a safe permanent solution, but it could tell you if increasing the leverage is the way to go for your driver.

FWIW

Neil
Neil22

Agreed Neil22, a mock-up would be a good idea and I have been thinking along those lines also, maybe by just bolting a length of angle iron to the pedal.

Also agree that it is too cold for the garage at the moment - it is snowing quite heavily here as I type.....
Graham Gilmore

This thread was discussed between 23/12/2008 and 05/01/2009

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