The Triumph Dolomite Club - Discussion Forum

Regarding the Vauxhall Valve, I think Steve previously mentioned it was a MK3 Cavalier 2.0 one he used.

So I think it would be one of these. But I'm not sure which one.

I can find one on ebay here but it's a set of two from Italy so quite expensive and might be the wrong one.

There is also this one for a MK2 cav. I'm not sure if this one would work.

And a set of MK2 cav ones here.

I previously bought one from Ebay but it was the wrong one, and no longer listed on ebay. The fittings were such that I'd have needed a bunch of adaptors to get it to work. I struggled to find the right one so just bought a brake bias valve I haven't fitted yet. If I can find the right Vauxhall valve I will probably fit that.

GrahamFountain wrote: ↑Sun Aug 20, 2023 12:39 pm

Carledo wrote: ↑Sat Aug 12, 2023 10:44 pm
I've never tried to suggest that the LSV or some reasonable facsimile is not needed on a standard Sprint (or any Dolomite running a Sprint axle) It most certainly DOES and I have the brown trousers to prove it. The TJ kit bypasses the need, but then the car isn't standard any more.

I get what you mean that increasing the relative effort of the front brakes is, in a sense, the same as reducing the effort from the back and will make it less likely that the rear wheels will lock first.

However, there are a couple of other issues:

Firstly, if the increased effort from the front brakes allows for more deceleration from the greater overall braking effort, that has the reverse effect, i.e. it tends to increase the likelihood that the rear wheels will lock first. Or at least, it tends to cancel the effect of moving the brake balance from bigger front brakes. That's because, at a higher deceleration, there's a greater transfer of weight off the back wheels onto the front, and (through Amonton's 1st law of friction) the increased downforce on the front wheels increases the brake force needed to make them lock, but reduces the brake force needed to make the rear wheels lock. And that double effect can exceed the effect of changing the brake balance; especially if the full potential of the improved front brakes can be turned into greater deceleration.

The other issue is that the LSV also compensates for any change in the height of the centre of gravity of the car and load. That matters because the higher off the road the CoG is, the greater the transfer of weight from the front to the back under braking, because the weight of the car acts through that CoG and, with that being higher up, has more leverage. Hence, raising the CoG increases the tendency for the back wheels to lock in the same way that moving it backwards reduces it. That means the LSV limits the tendency for the back wheels to lock over a large range of circumstances; where upgrading the front brakes is a simple one for all fix.

But, given the point about how unreliable the LSV is, how does the Vauxhall pressure reducing valve work?

I understand the TR7's (and SD1's) brake bias valve, which does much more than just reducing the pressure to the back wheels, and see how that should stop the back wheels locking first in an extreme braking event, more or less whatever you did to grip, front brakes, and CoG. They are a bit expensive at £150 or so new. But, because it's self-contained, and does not connect to the suspension, just working off fluid flows, it should be a lot more reliable than the Dolomite's LSV.

Graham

I agree with everything you've said above. I can't fault your theory or your logic.

There's only one simple thing that trumps it all, and that's the fact that, in practice, it WORKS perfectly with a TJ kit and no LSV!

The Carledo, as originally built, had standard Sprint brakes, all new and perfect condition, but sans the LSV. I left it off because the one on the axle donor car was, you guessed it, seized solid! Under normal driving conditions this didn't cause any issues (Remember the 2 door is even lighter at the rear than a normal Sprint) HOWEVER, under extreme braking and particularly in wet conditions it WOULD lock the rears first. Which led to a few "moments" that I couldn't let go!

So I initially sought an LSV, but, not being able to find one for any price, sensible or otherwise, I looked elsewhere and in my large collection of Vaux bits, found a pressure control valve from the back end of a (disc rear) 2.0 Cavalier. This I fitted comfortably and it solved the issue. The rear brakes still produced adequate numbers on the MOT rollers but the premature locking was a thing of the past. I returned to driving the car with my usual leaden right foot.

About a year in to driving it, I partook of my first trackday (Prodrive it was, at Triumphfest in 2013) and found out just how bad the brakes were when pushed a bit too hard! Which, in turn, led to me fitting my first set of TJs. Being the lazy git that I am I left the Vauxhall PLV in place, so it's not a fair test. I have, however, since transferred all the Carledo's running gear and brakes into the Grey 1500 FWD shell but didn't transfer the PLV and it's fine, even in snow, the fronts lock first.

The Dolomega also has ( uprated) TJs and no LSV and that's perfectly happy too, even with the even bigger 256mm discs and Puma pads and calipers that I built specially for it.

Between the 3 cars, I have probably 20,000 miles of use, under all conditions from tootling to the shops to full out track days and probably dozens of heavy stops. Since I got them right (ie on the Carledo) i've not had any cause to doubt they are going to stop me competently under any forseeable circumstances. I'm 100% confident!

I've not taken a Vauxhall PLV apart to see what's inside, it looks possible to do so though. But what I imagine, given it's small size and linear design, is not much more than a taper seat piston and a spring, sort of like bleed screw in reverse, with a spring instead of a thread, the spring holding the piston back away from the seat and letting fluid pressure pass around it, until a certain predetermined pressure is reached that lets the seat seal off any more pressure from going in to the rear brakes. The Cav, being a diagonal split dual circuit with rear discs, has 2 of these valves, one each for each rear caliper, but I only used a single one in the front to rear brake line as I was only dealing with drums. By a stroke of good fortune, it seems to do the job just right.

I'm not at all familiar with the TR7/SD1 bias valve, got any pics?

Steve

I understand and appreciate that kind of practical engineering.

The possible problem - and I admit it's only possible - is that you can only ever be sure it's right in the conditions you've tested. So, for example, if you increase the grip from the tires by going wider and or to softer compound and or you fit a roof rack to carry all the stuff SWMBO wants to take for a week by some other seaside, and or you carry four above average weight Americans, etc., etc., you don't know if it's still safe. And even if you test in all the possible conditions that you will ever drive in, you can't say for sure that it's safe for all those anyone else might. Still, it is probably not an issue. But you never know.

N.B. I know you can test the condition of 4 above average weight Americans and, in the right circumstances, the only possible cost is the loss of said Americans and the car. Some might consider that a fair cost overall, but I deny that I'm in any way Listerine.

But I work (or I will when I get back) in a regime where we have to be able to show something is safe; for stuff like this, < 10^-8 failures per operational hour.

However, as I remember, the thing that stops it being possible to work out what the perfect brake balance is (where the front and rear wheels lock at the same deceleration), and seeing if you've a credible safety margin relative to that, is that it's not reasonably possible to work out where the CoG of the car is for any given load. In which case, being certain of safety in all possible conditions want's something other than setting a fixed brake balance and hoping it covers everything the real world might conspire to throw at it.

I wonder if that's where the Triumph engineers were when they threw the Sprint together - I still say that because production numbers for the Sprint are such a close match to the FIA's changing requirements, between 1974 and 1980, to maintain Group-1 homologation status, it's a virtual certainty that was its only role. In which case, there'll be a lot of "Sod it, just do what works for now for bits like these.

So, if the LSV does what's needed, but not reliably, maybe it's worth looking at other alternatives; especially if one of them would work with both standard and uprated front brakes. Of course, I would never ever admit that any solution could ever possibly work with uprated rear brakes.

So here's the drawing and description of the TR7 pressure reducing valve from the TR7 Sprint ROM.



And how it plumbs in with the dual circuit master.



I'm not sure that it's going to be quite as effective as a working Sprint LSV. But I've never had one fail on the long list of TR7s I've had. I'm sure Robsport have a bin full of SH ones.

Graham

A TR7 has different weight, different brakes in both make and size (both ends I think) and a load of other variables.

I think that bias valve, as I understand it, is clever and does adapt somewhat to changing conditions but it still works pretty much like the Vauxhall one, just a bit more complex! Whether it will still work as well on a Sprint as it does on a TR7, I suspect you will let me know eventually! I might actually look at one for the next (220+horse) incarnation of the Carledo!

My test conditions usually include the use of 195/50/15 Toyo Proxes, but the ex 1500 has Sprint rims with 175/65/13s which are very old, very hard and due to be replaced with 185/70/13 Toyos any time now. I'm a little circumspect about how hard I push these cos i'm aware they are a weakness my normal driving style can ill afford. But i've not had any issues so far (but only a couple thousand miles in this iteration) I also have some considerable suspension upgrades, so it's an entire package i've put together that works so harmoniously. Different strokes for different folks, I can only really say what has worked for me on my own personal cars.

As for the 4 oversized Yanks, If I had them in MY car, i'd have bigger problems than potential prem lockup - like the exhaust scraping the ground!

Steve

PS, I've been thinking for some time of seeing if I can fit 1500/1850 rear brakes to a Sprint axle, that'd solve it for good!

Carledo wrote: ↑Sun Aug 20, 2023 10:09 pm A TR7 has different weight, different brakes in both make and size (both ends I think) and a load of other variables.

True, but the problem with the 8-valve TR7 is that the front brakes are deeply underwhelming - as I remember, the same as an HA Viva, but with servo. So it should still have the same issue that the relative power of the back brakes is excessive - the servo won't change the ratio.

Also, thinking about what it does in limiting the pressure to the back brakes until the front calipers have had the fluid they need to clench when the pedal stops moving. That should work towards preventing rear lock-up whatever the sizes of the brakes - actual or relative. It should, at least, be better than just applying a dose of hope.

Unfortunately, it wouldn't help me anyway, as I have single circuit brakes. But it doesn't look that difficult to fit one on the back of the front suspension tower, where it is in the TR7, for those of you that have dual circuit ones.



Graham

GrahamFountain wrote: ↑Mon Aug 21, 2023 10:52 am

Carledo wrote: ↑Sun Aug 20, 2023 10:09 pm A TR7 has different weight, different brakes in both make and size (both ends I think) and a load of other variables.

True, but the problem with the 8-valve TR7 is that the front brakes are deeply underwhelming - as I remember, the same as an HA Viva, but with servo. So it should still have the same issue that the relative power of the back brakes is excessive - the servo won't change the ratio.

Also, thinking about what it does in limiting the pressure to the back brakes until the front calipers have had the fluid they need to clench when the pedal stops moving. That should work towards preventing rear lock-up whatever the sizes of the brakes - actual or relative. It should, at least, be better than just applying a dose of hope.

Unfortunately, it wouldn't help me anyway, as I have single circuit brakes. But it doesn't look that difficult to fit one on the back of the front suspension tower, where it is in the TR7, for those of you that have dual circuit ones.



Graham

Well you'd need a dual circuit Dolomite to use it in the first place (why not FIT a dual master to your TR7?) but I have those by choice anyway. The perfect mounting point is that on the bulkhead previously occupied by the PDWA which wouldn't be usable in conjunction with this bias valve anyway, even if I hadn't long since thrown it away for being useless!

On my other pet hobby horse of equalizing pipe run lengths, from that TR7 pic, it looks like the pipe to the N/S/F flexi goes all the way round the front of the car to get there whereas the O/S/F pipe is very short, like roughly 6ft vs 1ft.

This, in my humble opinion, is a recipe for emergency stop disaster, the 6/1 ratio being only slightly better than the roughly 8/1 ratio of the original racing Capris that started the whole equal length pipe thing back in the early 70s. For comparison, the standard Dolomite front pipe ratio is only about 1.3/1 and seems tolerable under any emergency braking conditions i've been able to generate.

I've modified several Capris over a long career in fast cars, relocating the 3 way for the front brakes from the O/S flitch to the centre of the bulkhead and fitting equal length pipes from the 3 way to each front flexi and I guarantee it works! With the original mismatched setup, under emergency stop conditions, (or any racing brake application) with the first hard application, a Capri will snatch hard right, (MkI Triumph 2000s also had this issue, ask Chris Witor!) possibly hard enough to snatch the wheel from your hands if you aren't holding it in a death grip. Under ordinary road driving conditions, it's not an issue, it's only when you need it most that it bites you.

Which in turn is why I make an equal length rear pipe mod for the Dolomite,with a 3 way in the centre of the axle. The "changing ends" problem is obviously most noticeable on a Sprint with a seized LSV, but it CAN affect any Dolomite if you brake hard enough, especially in the wet, or with old, hard tyres (or, heaven forfend, BOTH)

Steve

Carledo wrote: ↑Mon Aug 21, 2023 2:15 pm Well you'd need a dual circuit Dolomite to use it in the first place (why not FIT a dual master to your TR7?) but I have those by choice anyway. The perfect mounting point is that on the bulkhead previously occupied by the PDWA which wouldn't be usable in conjunction with this bias valve anyway, even if I hadn't long since thrown it away for being useless!

On my other pet hobby horse of equalizing pipe run lengths, from that TR7 pic, it looks like the pipe to the N/S/F flexi goes all the way round the front of the car to get there whereas the O/S/F pipe is very short, like roughly 6ft vs 1ft.

This, in my humble opinion, is a recipe for emergency stop disaster, the 6/1 ratio being only slightly better than the roughly 8/1 ratio of the original racing Capris that started the whole equal length pipe thing back in the early 70s. For comparison, the standard Dolomite front pipe ratio is only about 1.3/1 and seems tolerable under any emergency braking conditions i've been able to generate.

I've modified several Capris over a long career in fast cars, relocating the 3 way for the front brakes from the O/S flitch to the centre of the bulkhead and fitting equal length pipes from the 3 way to each front flexi and I guarantee it works! With the original mismatched setup, under emergency stop conditions, (or any racing brake application) with the first hard application, a Capri will snatch hard right, (MkI Triumph 2000s also had this issue, ask Chris Witor!) possibly hard enough to snatch the wheel from your hands if you aren't holding it in a death grip. Under ordinary road driving conditions, it's not an issue, it's only when you need it most that it bites you.

Which in turn is why I make an equal length rear pipe mod for the Dolomite,with a 3 way in the centre of the axle. The "changing ends" problem is obviously most noticeable on a Sprint with a seized LSV, but it CAN affect any Dolomite if you brake hard enough, especially in the wet, or with old, hard tyres (or, heaven forfend, BOTH)

Steve

It's the Dolomite Sprint that only has single circuit brakes, being a 73 build one - I'm pretty sure all the production TR7/8s had dual circuit brakes.

I always felt like my old Ford Crapi needed wipers on the side windows, so you could see where you were going in the wet. But that wasn't simply a problem under braking. And to be fair, I only bought it for its reg.: ATR 716V.

I do know that swapping of ends does happen in wet/icy conditions, as the only times I've spun a TR7/8 have been once on black ice, and once on something that fell out of the back end of a horse/cow and then got rained on a lot. But I've never had a TR7 do that to me, even with wider tires and bigger front brakes. I have, however, got TR7s into four wheel, straight-line slides often enough. That suggests to me that the problem is something that the TR7's pressure limiting device didn't affect.

From a purely theoretical perspective, I struggle to see how the back wheels actually could lock first in the wet. Basically, which lock first is determined simply by the ratio of the weights on the front and back wheels and the ratio of the brake efforts between the front and back wheels - if the ratio of the efforts is greater than the ratio of the weights, the front wheels always lock first. So, if there's low grip, the front wheels will always lock first before they can cause the deceleration needed to transfer enough weight off the back wheels and onto the front for the weight ratio to exceed the brake effort ratio, front to back. Or, if the back wheels can lock first at the low maximum deceleration you have in the wet, simply because they are over-powerful, they would lock first at the same low deceleration in the dry, i.e. at so much less than the maximum possible from the front brakes that they'd be doing it all the time.

I get how a delay between the brakes at the front will have interesting effects on the steering; especially when one front wheel locks well before the other. Does it pull one way when the brakes first grip, then the other as one wheel loses traction? But doesn't there still then have to be something to make the back wheels lose traction for that to become a spin?

Graham