With Metro 4 pots and 8.4" ventilated discs on the front and MGF calipers with 8.4" solid Mini discs on the back will I need an adjustable valve to get the balance right?

Also, would it be needed with the same fronts but with drums on the back?

Last question - would the valve be needed in the front or back brake line? I presume it's the back but I need to ask that because I haven't a clue about Rossa mid engined weight transfer under braking but I presume that it will be less than in a Mini, for example.

I believe that IVA/MOT tests are pretty strict on this (and rightly so) and had planned to fit an adjustable valve while I was fitting the servo but it would have to be well hidden because they're not allowed and it would be better not to have it if it's not needed.

Well in my view it may be close to being ok from the start, it should be very close.

Metro calipers have simular piston area to Mini calipers, and the MGF rear calipers are smaller diameter so will have less effect, from memory around 38mm diameter for the MGF and 52mm for the Mini. Hopefully one of the Coupe guys can comment as Im sure Nigel? may have this setup.

Neil

I can't comment on a numbers point of view as my Coupe was already registered when I made the change so apart from the MOT requirements it's never had the braking efforts measured but I think the Metro turbo fronts MGF rears and Mini single line master cylinder will be close to right from the word go. Wet or dry I can lock the front wheels with a hard shove or if I go daft I can lock all four wheels. You don't specify the master cylinder as a twin circuit one will often have different size pistons for front and rear for this very reason so might add a new factor into the mix. Of course every car is different so if your weight distribution is different to mine it might be enough to throw this out.

This is one of my bug-bears with IVA as they don't allow an adjustable bias valve at all. SVA allowed it but you had to make it permanently set so a lever type was the answer which you set at the test and then drilled through the lever and inserted a roll-pin to lock it. I don't see why this had to be disallowed.

Personally I would be hiding one in a box somewhere. Doesn't matter that the tester suspects it's there - so long as the box isn't designed to be opened without tools I don't think he is allowed to ask you to open it (check on this). Disguise it anyway and the question never comes up. It might look a little suspect if you disappear into the car for a few minutes after failing the balance test and then it passes though so try to get it right from the word go and be subtle if it needs adjusting. If you've got wieghts and wheelbase I think the manual has the formula so find a friendly MOT station that you can take the car to and get it on their rollers for some testing.

Yes it will be the rear line that needs it as you want less effort on the rear wheels since weight transfers forwards under braking.

Iain

Thanks for the replies. My master cylinder is a Metro dual circuit one which I believe is from a car with a servo as standard - I used this one because it's shorter than most and has enough bonnet clearance. My servo is an unusual dual circuit one (there's a photo in the Projects forum) and has separate cylinders for front and rear. It came with no info atall - not even a bracket! - and I don't know the characteristics of each cylinder so this is what made me think about fitting a bias valve. Alan at Rossabitz has the same servo and says that it works well so the balance must be near enough - it comes back to what you say, Iain, that it's fine for MOT testing but IVA is a bit stricter. I will have to weigh the car anyway and get the axle weights so I suppose a bit of testing will be in order, my problem being finding somebody in this part of the world who knows what I'm talking about If I can see a way to hide it I think I might be as well to fit a bias valve when I'm at it. Never a dull minute

Some of the Metro masters had two pistons the same size and a limiting device in the line to the rears it screwed directly into the master and just looks like a big pipe union. Problem with that device is that it's fixed and designed for the weight and other braking system characteristics of a Metro.

The Rover Metros had a similar system but went for a diagonal split so each circuit from the master fed a front brake and had a T-piece which fed the opposite side rear through a similar (maybe identical) device. On the Rover they sit vertically on the bulkhead just off from the centre of the car.

I fitted one of those to my Robin Hood when it failed it's SVA due to too much rear braking effort and it did the trick. Thing is on the first test I'd fitted the inertia valve from the Sierra but they took no account of it and that it wouldn't do squat on the rollers but in the real world it would prevent the rear wheels from locking first. I was told that if such a device was fitted they were supposed to ignore the readings for the balance part of the test and take it out into the yard, stand on the brakes and observe which wheels locked first. Tester said "no" when I asked about this and I didn't have any documentation to hand and didn't want to wind him up as the fail list was small. Might be worth investigating chapter and verse on IVA with respect to this and maybe going fore-armed since you should be able to find a car-park where some testing could be done by yourself to make sure it behaves correctly even if the numbers don't add up.

Iain

http://i134.photobucket.com/albums/q89/ ... CF0738.jpg

Knew I'd find a picture if I looked hard enough. That master appears to have three outlets though.

http://i26.photobucket.com/albums/c119/ ... C00838.jpg

And the Rover ones - nice of the person who put the picture on the interweb to remove the engine so we can see them.

Iain

The IVA test does allow an automatic inertia operated pressure limiting valve as fitted to the later Minis.

You may be able to get one of those to do the job.

http://www.minimania.com/part/13H7757/I ... ini-Cooper

My master cylinder is identical to the one in your first link, Iain, and you're right in saying that it has three outlets - mine has a bleed nipple fitted to blank off the one where the limiter should be. However, as you say, it's designed for the weight characteristics of the Metro and the Rossa is a bit different. Inertia systems are permitted as gtmdriver points out but I'm leaning towards something that I can control ie an adjustable valve and if I go down that route I think I know where I can fit it discreetly........ Having said that, I hadn't really considered the inertia valves probably because I know little about them but I know a bit more now! The pile of parts in my garage is growing fast so I'll need to start doing something soon..

Interesting - I've never seen a Mini with an inertia valve (well I might have but I haven't noticed it).

As I said I had one on the Hood when it went for it's SVA test but the tester only tested it on the brake rollers so no inertia equals it not doing anything and a fail. I don't think this was right but....

The sierra valve has a ball-bearing that rolls forwards under braking and cuts off the pressure to the rears at the point it moves. The valve points upwards and towards the front of the car so that it takes a bit of deceleration before the ball moves. In theory this should be almost independant of other factors since it's purely down to the weight (or rather inertia) of that ball versus the rate of deceleration (OK as the car dips at the front the slope decreases). To set the point at which it operates you can tweak the angle that the valve points upwards - more towards the horizontal and the earlier it will operate.

I do wonder whether that would be considered a dis-allowed adjustment on the modern IVA test - i.e. have slots for the bolts that hold it or a couple of sets of holes. I'd like to have it in black and white that the IVA test has provision for testing an inertia device if I was going down that route.

Iain

I see what you mean. It's a pity that somebody could be put to more expense and hassle due to the interpretation of the regs by a particular tester but I suppose it's human nature and can't be helped.