Sprint rear brake bias valve.

[pfg49]

My V8 Dolomite is based on a 1980 1500 HL, and I presume doesn't have a rear brake bias valve. I've bought a Sprint axle which I've nearly finished overhauling, is it necessary to fit the Sprint bias valve?

[GlenM]

If you uprate the front brakes (which will be essential with a V8 fitted), probably not. The LSV was fitted to stop the large rear brakes locking up with the feeble standard front disc set-up.

[soe8m]

If you uprate the front brakes (which will be essential with a V8 fitted), probably not. The LSV was fitted to stop the large rear brakes locking up with the feeble standard front disc set-up.

Locking rearbrakes is not related to the size/stopping power of the front brakes.

Jeroen

[GrahamFountain]

If you uprate the front brakes (which will be essential with a V8 fitted), probably not. The LSV was fitted to stop the large rear brakes locking up with the feeble standard front disc set-up.

It's certainly not as simple as re-balancing the brakes by increasing the front to match an increase in the rear; especially where there's an overall increase in brake effort that causes more weight to the transferred onto the front wheels under breaking, making it more likely that the rear wheels will lock first. It's for that reason that Triumphtune used to recommend that you reduce the back brake effort on TR7s if you increased the fronts very much at all.

But that load sensing valve device on the Sprint axle is a marvel in preventing the rear wheels from locking - as long as it works. There's a full description of how it works in the BL Repair Operations Manual that is well worth a read.

Graham

[GlenM]

My reply was based on the general consensus that when fitting the uprated TJ front brakes, it is not necessary to retain the LSV, therefore this is likely to be the case with any similar uprated front brake set-up. Hence my use of 'probably'.

[Carledo]

My reply was based on the general consensus that when fitting the uprated TJ front brakes, it is not necessary to retain the LSV, therefore this is likely to be the case with any similar uprated front brake set-up. Hence my use of 'probably'.

I'm with Glen (and TJ Jon and others) Theory can take a hike in the face of practical experience! If you upgrade the front brakes with a TJ kit or similar, you don't need the LSV. I'm currently on my 10th TJ install and I now routinely delete the LSV as part of the job.

Oh, and guess what? I've yet to remove a WORKING LSV! They've ALL been seized solid! Even the ones on cars in regular use!

What DOES seem to help, is using the smaller bore 1850 rear wheel cylinders. But that is more to do with curing the extra pedal length induced by using the bigger piston Ford calipers of the TJ convo.

Steve

[GrahamFountain]

I'm not against upgrading the front brakes per se, I'm just cautious about upgrading the back ones without an understanding of the possible consequences, especially where the total braking effort is increased.

And I don't think it's well understood that the total braking effort is not related to the efficiency or efficacy of the brakes. It is determined, primarily, by the tires, especially width and compound. I also think it's not generally well understood that increasing the grip changes the brake balance in a bad way, i.e. make it more likely the rear wheels will lock first, because of the greater transfer of weight under braking - which is greatest on a clean, dry road with well worn tires. Do we need to discuss, once more, the reasons why locking the back wheels first is not a good thing?

The only things that upgrading the brakes does on its own is to make it easier (require less pressure on the pedal) to reach the limit imposed by the grip of the tires, and increase the number of times you can do that before the brakes fade. But I don't underestimate the psychological effects of reducing the effort on the pedal for any given deceleration. I'm just not one who needs the brakes to be easier to press or one who drives in a way where brake fade is an issue.

Graham

[bazyerma]

I'm not against upgrading the front brakes per se, I'm just cautious about upgrading the back ones without an understanding of the possible consequences, especially where the total braking effort is increased.

And I don't think it's well understood that the total braking effort is not related to the efficiency or efficacy of the brakes. It is determined, primarily, by the tires, especially width and compound. I also think it's not generally well understood that increasing the grip changes the brake balance in a bad way, i.e. make it more likely the rear wheels will lock first, because of the greater transfer of weight under braking - which is greatest on a clean, dry road with well worn tires. Do we need to discuss, once more, the reasons why locking the back wheels first is not a good thing?

The only things that upgrading the brakes does on its own is to make it easier (require less pressure on the pedal) to reach the limit imposed by the grip of the tires, and increase the number of times you can do that before the brakes fade. But I don't underestimate the psychological effects of reducing the effort on the pedal for any given deceleration. I'm just not one who needs the brakes to be easier to press or one who drives in a way where brake fade is an issue.

Graham

Thats fighting talk

[soe8m]

This time I agree fully with Graham. Steve, by reducing the rear cilinder diameter you are moving the balance to the front, reducing the rear brakes so a continue reduced rear stopping power not to lock up, also when you can have more in dry or loaded conditions controlled by a valve.

Jeroen

[Carledo]

The OP is only upgrading the rear brakes by virtue of necessity, this being the need for a stronger Sprint axle in a V8 powered 1500 shell. So he's basically taking the car to stock Sprint spec, which would normally need an LSV to control the rear brakes and stop them over-riding the fronts. IMO premature rear lockup is NOT something I ever wish to experience again, it's a guaranteed brown trouser moment in the making!

Nor do I have a quarrel with the LSV or it's necessity on a stock Sprint. Or rather, I wouildn't have, if the wretched thing worked! My experience with them has not been good, though they don't fail in the sense that they would deprive you of all rear brake effort, they seize in a more or less central position when the entire point is that they should self adjust to take account of the loads imposed, greater or lesser axle loads from passengers and luggage in one direction and weight transfer from heavy braking in the other.

Since I DO have a quarrel with the Sprint's front brakes being insufficient to cope with the rest of the car's performance, especially if driven in a spirited fashion, I unashamedly champion the TJ conversion. A side effect of fitting the conversion is to regain a natural balance between the front and rear brakes that obviates the need for the LSV ( A factory fitted bodge to cover their penny pinching use of the too-large TR4 rear drums on the Sprint) thus, from my point of view, killing 2 birds with one stone, a win-win!

Graham and I have discussed, elsewhere and at some length, the pros and cons of uprating the front brakes, I see no need to rehash old opinions. All I will say here about the TJs is that, from my point of view they aren't THAT much bigger or better than the stock ones, nor would I wish or need them to be. I once fitted a servo to my GT6, it stayed on a week and came off again, you CAN have too much of a good thing. The primary reason I fit the TJ conversion is for the VENTED disc aspect which dramatically reduces heat induced fade, the REAL Achilles' heel of the stock system. But the fact that the TJs ARE just that bit better than the stock brakes is enough to restore the balance disturbed by the factory's cost cutting. I don't see a downside, The TJ conversion is worth around 5 secs a lap at Castle Coombe and means that I have brakes for a full day's sessions, rather than just the first 3 laps!

Steve

[GlenM]

If you uprate the front brakes (which will be essential with a V8 fitted), probably not. The LSV was fitted to stop the large rear brakes locking up with the feeble standard front disc set-up.

It's for that reason that Triumphtune used to recommend that you reduce the back brake effort on TR7s if you increased the fronts very much at all.

You are incorrect there, Triumphtune did not advocate reducing the back brake effort. From the Triumphtune catalogue in relation to fitting the four pot front calipers - "Again use the smaller rear wheel cylinders to balance the braking". The smaller diameter cylinder will actually move further for the same amount of fluid displaced, although it will require more pedal effort. The catalogue also states that this wheel cylinder is used as an alternative to the wider shoes fitted to TR8s, which were NLA many years ago, which implies an that they offer a similar increase in rear braking effort.

[GrahamFountain]

If you uprate the front brakes (which will be essential with a V8 fitted), probably not. The LSV was fitted to stop the large rear brakes locking up with the feeble standard front disc set-up.

It's for that reason that Triumphtune used to recommend that you reduce the back brake effort on TR7s if you increased the fronts very much at all.

You are incorrect there, Triumphtune did not advocate reducing the back brake effort. From the Triumphtune catalogue in relation to fitting the four pot front calipers - "Again use the smaller rear wheel cylinders to balance the braking". The smaller diameter cylinder will actually move further for the same amount of fluid displaced, although it will require more pedal effort. The catalogue also states that this wheel cylinder is used as an alternative to the wider shoes fitted to TR8s, which were NLA many years ago, which implies an that they offer a similar increase in rear braking effort.

It's not how far it moves, it's how much force it applies to the shoes for the same pressure on the pedal. Given the same hydraulic pressure from the master cylinder, i.e. the same pedal effort, the smaller diameter cylinder applies less pressure to the shoes and less braking torque to the wheel and less decelerating force between the tire and the road and less likelihood of locking the wheel with more overall deceleration (if you manage to get more effort from the front brakes because you've got more grip or because you weren't strong enough to get to the limit set by the tires before the upgrade).

But, to say the same as TT in reverse, it uses less fluid in moving the same distance. That does, as Steve says, compensate for the bigger effective diameter callipers on the front brakes tending to lengthen the pedal.

Graham

[GlenM]

As Steve said, this has all been debated before, so no point going over it all again.

[GrahamFountain]

The fact (not opinion, not theory, not speculation, but fact) is that the maximum deceleration the brakes can give you is limited simply by the force between the road and tire at which it stops rolling and transitions to sliding friction. That's because sliding friction provides a lower force than rolling friction for a given downforce on the wheel. Another fact is that this limit is determined simply by the tire and the road surface it's on and the weight on the wheels at that time, not the force on disk or the ratio between the force on the pedal and the force on the disk.

But my issue was about the things that change whether the front wheels lock first, as they should. And that's something that can change in a more complex way than may always be obvious. That it can change as a result of increasing the power of the back brakes (or reducing the front ones) may be obvious. But then there must be an issue that the problem itself is not obvious. Otherwise why would there be folks who think switching a road car to rear disks without considering the need to re-balance the brakes, i.e. turn back down the effort from the back brakes they just spent so much money increasing?

It's also a possible problem that some see dive on the front suspension as causing the weight transfer, not a response to it. The way the downforces on the front and rear brakes change as you decelerate is determined only by the height of the center of gravity of the car and the total amount of decelerating force between the road and the tires. It's not affected by how much of that decelerating force comes from the front or the back, only the total. So fitting bigger brakes at the back will not reduce the dive, only increase the risk of the back wheels locking first and the possible laundry problem that Steve mentioned.

But one of the less obvious things that change the risk of the rear wheels locking first is that if you increase the grip from the tires. In that case, you can decelerate more quickly, so more weight is transferred onto the front wheels and you reduce the likelihood that the front wheels will lock. But, perhaps counterintuitively, even though you've increase the grip at the back, you also reduce the weight on the back wheels at the limit, and increase the likelihood that the rear wheels will lock first. However, if you've increased the grip by much, you probably have to increase the brakes to get to the new limit at a reasonable pedal pressure, and increasing just the front brakes is enough to get you into that risk situation - but only with more grip.

So, ignoring changes in grip and to back brakes, upgrading the fronts is only about how hard you have to press to get to that point where the front wheels lock. And that is purely a matter of personal preference.

My opinion is that the brakes on the Sprint take me as near to locking the front wheels as I want them to for the amount of force I'm used to applying. Also, I don't go even to that point except in more extreme situations, so brake fade is not an issue. If I went racing, I'd have a different opinion, but I don't.

I don't think I can do better here than to quote Jon Tilson:

As I have explained and will repeat here one more time, standard Dolomite brakes are quite capable of slowing the car in a continuous emergency stop from 90 mph to zero maybe twice in relatively quick succession, ergo they are demonstrably adequate for normal road use. Brakes used to be engineered so that lock up on a dry road could be achieved with pedal pressures ranging from 60 to 100 lbf at the peddle. This is the sort of effort that a normal woman driver can just about achieve in emergency at the top end of the range.

The problems come or are compounded because of bad maintenance allowing partially seized callipers and discs to go thin, or even fitting el cheapo brake pads from Moprod etc. I have personally transformed the brakes of numerous dolomites (some of them owned by forum members) by servicing calipers and fitting decent pads. I sometimes shudder to think what people are actually driving about with and wonder if a lot of the bad press and hot air generated about dolomite brakes on various forums is down to ignorance of what the barkes really should be like.

Now when on track or rallying or regularly needing to whiz up and down mountain passes with a full load of passengers I quite accept that the standard setup will cause some fade. In these cases by all means upgrade if you want to.

But please dont subject normal users to wrong advice and make them spend money they don't need to and will see little benefit from when a 20 quid set of caliper seals and some proper pads will see them stop as they should.

Jonners

I'd only add that giving the servo a coat of looking at never hurts.

Graham

[Carledo]

My opinion is that the brakes on the Sprint take me as near to locking the front wheels as I want them to for the amount of force I'm used to applying. Also, I don't go even to that point except in more extreme situations, so brake fade is not an issue. If I went racing, I'd have a different opinion, but I don't.

As I have explained and will repeat here one more time, standard Dolomite brakes are quite capable of slowing the car in a continuous emergency stop from 90 mph to zero maybe twice in relatively quick succession, ergo they are demonstrably adequate for normal road use.

I'd only add that giving the servo a coat of looking at never hurts.

Graham

In some ways, I don't disagree with you or Jonners (a man I had a lot of time for despite our frequent friendly clashes)

If you don't push the car to it's limits, the brakes are adequate (love that word, damning with faint praise never got simpler)

At least, they WERE adequate in 1973. Nowadays, every not-so-hot hatch has bigger vented discs, rear discs and a more powerful servo than any Dolomite, with ABS to overrule the less sensitive driver's panicky right foot. With general driving standards (IMO) at an all time low and showing no signs of improving, almost anything made in the last 30 years, stops better than our cars. With more and more idiots out there with more power and better brakes, it seems to ME to be sheer self (and car) preservation to upgrade accordingly. Since ABS is not really a viable option, I have to rely on skill and hard won experience to keep me out of trouble when stopping hard. I have I think, engineered my brakes to make this intuitive, rather than requiring excessive concentration.

One somewhat counter intuitive thing i've done, is to replace the Sprint servo on the Carledo with a smaller version from an early 1850. Whilst I did this initially from operational necessity (the Sprint servo I had failed and I couldn't get another quickly) It has remained in this state the last 5 years or so because I found it felt and worked better that way! Admittedly the Carledo is VERY light but I found it a tad TOO easy to lock it up with the Sprint servo, the smaller 1850 offering gave more feel under my foot (a personal taste thing as Graham so rightly says) but it works for me. It's only just possible to lock it up in the dry (by seriously ignoring the car's warnings) and when it does, only the fronts lock. Which is as it should be!

Over the years i've mixed and matched, cherry picked from different models and modified pretty much all the braking system on the Carledo AND the Dolomega till nothing remains of the original Toledo brakes on the Carledo and only the rear shoes on the Dolomega are standard for a 78 Sprint. (OK, the pedals remain stock if being pedantic)

I'm pretty sure i've now got it right on both very different cars, effectively unlimited time and budget helped, along with a good measure of hindsight. TBH, i've probably been a bit OTT in my pursuit of braking perfection, my only justification is that i'd like to go fast AND keep my car in one piece!

Steve