IVA Brake Fail
Discussion
Arden took my Minari for its retest, brake efficiencies are ok - not with much to spare AIUI, but ok - but it has failed on the brake distribution ratio - too much braking effort at the rear.
I suspect that part of the problem is that the Minari is FWD and pretty light, so there's not much weight at the back. Arden are at a bit of a loss and they aren't getting much feedback from the tester at Yeading. (Apparently they have a good relationship with the chap at Leighton Buzzard, but the test centre there is out of action.)
There is a compensating valve in the circuit (the one from the Alfa donor) but AIUI, Arden have adjusted it so that it is always fully open, so I don't think that is helping the distribution test, but it probably is helping the efficiency.
As I see it, there are several things that we could try, for example:
Adjust the bias valve so that it does its job, while taking care not to affect the overall efficiency too much - could be a tricky setting to find though!
Could try just throwing some weight in the back - spare wheel, toolbox etc, might do the trick - not sure just how much this would help though.
Last one I can think of is the tyres - the ones on the car are a really old mix and match set that were on the wheels when I bought them. Probably won't help with the distribution test, but might help with the overall efficiency.
Does anybody have a feel for how much a fresh set of tyres would help? I guess the downside would be that they would need to have a reasonable run to get the release agent off.
Ah well, I'm sure we'll figure something out!
TIA
Does anybody have a feel for what will help?
I suspect that part of the problem is that the Minari is FWD and pretty light, so there's not much weight at the back. Arden are at a bit of a loss and they aren't getting much feedback from the tester at Yeading. (Apparently they have a good relationship with the chap at Leighton Buzzard, but the test centre there is out of action.)
There is a compensating valve in the circuit (the one from the Alfa donor) but AIUI, Arden have adjusted it so that it is always fully open, so I don't think that is helping the distribution test, but it probably is helping the efficiency.
As I see it, there are several things that we could try, for example:
Adjust the bias valve so that it does its job, while taking care not to affect the overall efficiency too much - could be a tricky setting to find though!
Could try just throwing some weight in the back - spare wheel, toolbox etc, might do the trick - not sure just how much this would help though.
Last one I can think of is the tyres - the ones on the car are a really old mix and match set that were on the wheels when I bought them. Probably won't help with the distribution test, but might help with the overall efficiency.
Does anybody have a feel for how much a fresh set of tyres would help? I guess the downside would be that they would need to have a reasonable run to get the release agent off.
Ah well, I'm sure we'll figure something out!
TIA
Does anybody have a feel for what will help?
Defcon5 said:
How is it actually tested?
Not sure exactly. AFAIK, it's done on a brake test machine like the ones at the MOT tests. They measure the brake force distribution between front and rear and also the weight transfer. As long as the proportion of weight on the rear exceeds the proportion of braking effort on the rear, the conclusion is that the rear wheels won't lock up first, so the car passes.gtmdriver said:
Did you re-con the brakes when you built the car? It seems strange that they are so marginal as regards overall efficiency.
Rear drums have had a new wheel cylinder recently - I think they put new shoes on too. I'm pretty sure it's had new discs and pads on the front but, because of the protracted nature of the build, these are probably over ten years old now 
otherwise master cylinder ok, braided hoses - all should be ok. Maybe just need a bit more bedding in, but this is why I wonder about the tyres - are they the weak link?Arthur Jackson said:
Do VOSA have correct weight figures for the car?
Interesting that you should say this - there was a mistake and I have now provided the correct paperwork, but AIUI the weights used in the test are the as weighed data from the day - possibly including the driver.@Martin - I don't get why you think new tyres would make things worse ... old tyres (and I do mean old - I bought the wheels second hand in 2008 and most of the tyres are types that hadn't been made for a while back then!) will break traction before new ones would, so I would think it would help - once they are run in.
I guess my original question was how much do tyres contribute to brake efficiency?
@rdodger & oem, yes, that does seem to be the logical thing to do, as long as it doesn't affect the overall efficiency too much.
I guess my original question was how much do tyres contribute to brake efficiency?
@rdodger & oem, yes, that does seem to be the logical thing to do, as long as it doesn't affect the overall efficiency too much.
Frankthered said:
Interesting that you should say this - there was a mistake and I have now provided the correct paperwork, but AIUI the weights used in the test are the as weighed data from the day - possibly including the driver.
Could you then just make the rear brakes worse for the test? Mega cheap pads, bit of WD40 here and there, so the fronts lock first?My first move would be to check the whole system for air, partial seizure of any of the hydraulics and contamination of the friction material.
The brakes should be easily good enough to allow you to use the limiting valve to balance the rear brakes without bringing them below the required efficiency level.
The brakes should be easily good enough to allow you to use the limiting valve to balance the rear brakes without bringing them below the required efficiency level.
Frankthered said:
@Martin - I don't get why you think new tyres would make things worse ... old tyres (and I do mean old - I bought the wheels second hand in 2008 and most of the tyres are types that hadn't been made for a while back then!) will break traction before new ones would, so I would think it would help - once they are run in.
I guess my original question was how much do tyres contribute to brake efficiency?
@rdodger & oem, yes, that does seem to be the logical thing to do, as long as it doesn't affect the overall efficiency too much.
getting myself confused lolI guess my original question was how much do tyres contribute to brake efficiency?
@rdodger & oem, yes, that does seem to be the logical thing to do, as long as it doesn't affect the overall efficiency too much.
arden are an mot centre so adjust brake bias on their brake testing nachine and see what can be done
martin
The tyres will make NO difference in the test. This is because on the brake tester, the tyres do not reach limiting friction. If the tyres aren't slipping on the test brake, whatever you do to them will make no difference. They won't be slipping, since the surface is akin to mega-sandpaper.
In almost any vehicle with disk front and drum rear brakes, there has to be a rear brake limiter. This is because disk brakes work at much higher line pressures than drum brakes.
My suggestion is to increase the effect of the rear limiting valve, so that the rear brake effort is reduced.
In almost any vehicle with disk front and drum rear brakes, there has to be a rear brake limiter. This is because disk brakes work at much higher line pressures than drum brakes.
My suggestion is to increase the effect of the rear limiting valve, so that the rear brake effort is reduced.
Paul Drawmer said:
My suggestion is to increase the effect of the rear limiting valve, so that the rear brake effort is reduced.
Don't even think about half-arsed bodge solutions (like temporarily imcreasing the weight over the back) just to get you through the IVA test. The regulations on brake distribution are there for a reason - a car that has a tendency to lock its rear brakes first is outright dangerous - and brake bias valves are fitted for a reason.
Re brake bias valves, the fitment of these has had more milage on various forums than poss any other 2 subjects ,I run a RH S7 with out the seirra donor valve and have had no probs in 8/9 years ,not a track day car but driven with certain amount of thrutch other s have a valve /2 m/c with adj fitted and say they wouldn't do any thing else,I suppose it's down to personal pref and the components used and the condition of them .In your case I think my first post suggestion should suffice but also consider scrubbing the front disc pads and disc and bedding them to increase efficiency HTH BTW its a disc and drum set up on my car ,std drums and linings and Hi Spec 4 pots and 260mm discs with fast road pads
Edited by one eyed mick on Monday 16th May 09:24
one eyed mick said:
...I suppose it's down to personal pref
Basic brake balance (which is what the IVA tests - not fine tuning) is not down to personal preference. If it's far enough out of kilter to fail the IVA, then it's potentially dangerous. As in 'oops, I'm facing the wrong way in the path of an oncoming artic on a bend'-type dangerous.Those who know me on this forum will know that I'm hardly a fan of nanny-state safety legislation in general, or the IVA in particular, but there are some basic areas where idiots (and the innocent public) need protecting from themselves, and brake balance is one of them.
Sam_68 said:
Basic brake balance (which is what the IVA tests - not fine tuning) is not down to personal preference. If it's far enough out of kilter to fail the IVA, then it's potentially dangerous. As in 'oops, I'm facing the wrong way in the path of an oncoming artic on a bend'-type dangerous.
Those who know me on this forum will know that I'm hardly a fan of nanny-state safety legislation in general, or the IVA in particular, but there are some basic areas where idiots (and the innocent public) need protecting from themselves, and brake balance is one of them.
Sam have you got any ideas of your own on how to help the OP which are not half arsed solutions?Those who know me on this forum will know that I'm hardly a fan of nanny-state safety legislation in general, or the IVA in particular, but there are some basic areas where idiots (and the innocent public) need protecting from themselves, and brake balance is one of them.
thescamper said:
Sam have you got any ideas of your own on how to help the OP which are not half arsed solutions?
As above; Paul Drawmer suggested it first, but correctly adjusting the bias valve would be favourite....oh, and if it's based on the AlfaSud arrangement (ie. inboard front discs) rather than the later outboard Alfa 33 set-up, then make sure the front calipers are in top-notch condition; they're pretty mediocre at the best of times.
One-Eyed-Mick probably hasn't had any problems because a 'Seven' inherently has a much more rear-biased weight distribution than his Sierra donor. The Alfasud/Alfa 33 engine hangs out in front of the gearbox at the front and the rear wheels are basically just there to stop its arse dragging on the ground, and the Minari's weight distribution won't be all that different to the donor, so will still require reduction of the rear braking effort if you want to prevent the rears locking up first.
Edited by Sam_68 on Monday 16th May 13:03
Paul Drawmer said:
akershly (blush) it was monovisioned michael who said it first.
How did you get to know my Sunday name ? not many people know it have we met other than on here? Far from embarrased or upset I've been monocular for 42 years this It doesn't faze me at all it was all down to a guy on a m/bike and a Triplex windscreen which exploded when his shoulder hit it at closing speed of 80/90 mph 1 peice cut me inthe centre of my right eye se la vie !.thanks for pointing out my solution ! bye for nowGassing Station | Kit Cars | Top of Page | What's New | My Stuff