Sprint brake upgrades

[GrahamFountain]

Technically, it's not theory, it's modelling; specifically, the application of the laws of physics (as derived by Newton, Amontons, Coulomb, &co) to a simplified (fixed CofG) model of a motor vehicle in a specific condition (braking at the limit of grip in a straight line with a given coefficient of friction).

And yes, it is good, because even with having to guess the location of the CofG, it still shows where you get the best return on your investment in upgrading the brakes, and what general effect an upgrade will do to the safety margins that were designed in by Triumph. Hence, it lets those who want to do something like a proper engineering job do so.



This shows the inputs to and outputs from the model for a specific location of the CofG. The important points are how much lower the maximum brake force can be at the back before they will lock first in the best conditions for grip that apply, and how flat that curve is. It is only correct for the estimated position of the CofG, but that's an input variable, and the model will give the answers for any value chosen.

That the maximum front brake force has to be so much larger than the rear (to avoid the rears locking first) means that upgrading the front brakes will always give you a lot more return than upgrading the back ones. That the curve describing the safe limit on the rear brakes is so flat means that upgrading the fronts does not mean upgrading the backs is any safer. Indeed, as it does turn down towards the higher levels of grip, it shows that upgrading the front brakes and fitting grippier tiers, makes it less safe to upgrade the back brakes. It's not clear whether "significant upgrade" includes removing the LSV or not, which I admit is a cheap upgrade, but no one has given what change in hydraulic pressure that gives at the back.

But, remember, the original point was that advising people to significantly upgrade the back brakes by fitting rear disks on a publicly visible forum is risky. Assuming they do manage to get significantly more brake force (and that's not a foregone conclusion), it's a costly way to do it, compared to upgrading the fronts, and it erodes the safety margins that Triumph set when they designed the car.

Yes, you probably can erode those, a bit; especially for a well maintained car, etc. However, if you don't have any understanding of the physics, you don't actually know what you're doing, so you have no idea what the effects are, and that's never going to be good engineering - even if it is fun. And, if what we're discussing is a daily drive and general dad's taxi, it's just not safe to go upgrading without, at the very least, an understanding of the fundamental principles: some of you do; some of you don't, or so it appears. But none of you address the issue of safety.

Whereas, based on my understanding of the physics and with evidence from the model, I'm very happy to state that if you upgrade the front brakes by a modest amount and stay with fairly normal range road tires that won't have a huge CofF (even when they are worn to slicks), you won't have much effect on the safety margin that Triumph set for the safe bias of the brakes. The corollary being, if you do upgrade the rear brakes, you do erode that margin, especially at the higher levels of grip, and you do risk the rear wheels locking first. That's probably only going the happen in a full blown emergency stop. But isn't that exactly when you really, really want it all to work as it should?

So it's the issue of advising the general public to follow your lead and quite possibly go beyond what you've tested and found safe for you specific cars that I rail against most. I assume those who do go beyond what is safe don't necessarily report that – for one reason or another. I just hope they do it far enough away that they are not a danger to me and mine.

Graham

[GrahamFountain]

The ratio of forces on the front and rear suspensions has nothing to do with where the braking is done. It is entirely a consequence of the mass of the car, where the CofG is, the stiffness of the suspensions, and how much deceleration is applied. Unless you changed the suspension, moved the CofG, or changed the mass of the car, if it dived more, you were stopping at a greater g value.

I will, however, acknowledge that there is a small stiffening effect on the dolomite's rear suspension, because of the effect from putting the rear axle links in greater tension, when you do more braking at the back. I looked at that when the same question came up over the TR7 and concluded it wasn't big enough to make a discernible difference. Graham

So whats your point?

My point, in that post, was that the amount of dive isn't controlled, to any significant extent, by where you apply the brake force, front or rear.

The dive is caused by the force couple formed between the resolved brake force applied to the road surface and all acting in that plane and the reaction to the deceleration that acts through the centre of gravity, which must be some way above the road. One couple can only be countered by another, which can only be the vertical forces on the front and rear suspension. So those vertical forces on the suspension are only determined by the total brake force and the position of the CofG, not where on the road that brake force is applied. And, for a given suspension setup, the dive is determined almost entirely by those vertical forces.

If you're still not convinced, I can draw you a picture, but it will take a day or two.

Graham

[GrahamFountain]

The stiffer front springs fitted by Ian will limit the weight transfer to the front meaning the rear brake upgrade will be effective.

Not exactly: The stiffer springs will only limit the dive by changing the response of the suspension to the vertical forces on it, i.e. those that counter the couple between the resolved brake force acting at the road surface and the reaction to the deceleration acting through the CofG.

There will be a secondary effect, in that reducing the dive will change how the CofG itself physically moves relative the points of contact between tires and road due to the front springs compressing, etc. But that, in turn, is determined by how far the CofG is from the roll centre and if it's in front or behind it. And I've a strong suspicion that the CofG and roll centre won't be far apart, as there would be some rather odd effects on the handling if they were. In any case, how much the suspension moves is small in comparison to the length of the wheelbase. As the movement of the CofG must be much less than the dive at the front, it will be totally insignificant when expressed as a fraction of the wheelbase

In which case, the down forces on the wheels, which are the same as the vertical forces on the suspensions, won't change much and the limits on how much force you can apply at the back without eroding the safety margin in the brake bias won't change either.

But if someone can identify how much less the CofG physically moves with the stiffened suspension, at or around 0.8 to 0.9 g deceleration, it would only be a case of adjusting the input values and the model will show what difference that has on how you can change the rear brake force keeping the same safety margin.

Graham

[Carledo]

I have to admit to being an experimental physicist rather than a theoretical one!
And I also understand that my car is not a good guide on setting up other peoples since a) it only has 2 doors and a short tail as well as having weight removed from the rear and b) it has a Vauxhall engine and gearbox which is lighter and sits further back in the car. What effect this has on the CofG i'm not sure but there is bound to be some. Add in stiffer, lowered suspension and 195/50/15 rubber and you have a car which, while of the same general design as a stock Sprint, is not going to react in the same way.
As I said in an earlier post, I was forced to control the output of the (standard) Sprint rear brakes on my car when it only had normal Sprint front brakes and no LSV. The solution I chose was a pressure limiter valve from a Vauxhall Cavalier 2.0 ltr. I don't know (and have not, TBH, tried to find out) what pressure this cuts in at but the car it came off had disc rear brakes and 2 of these valves (1 each side) so those single discs with tiny pads were way too much for the Cav which is longer and considerably heavier than a Dolly and this may be indicative.
Since I fitted this valve, I have had no more trouble with premature locking of the rear wheels and brake performance on the MOT rollers is more than adequate. I now have TJ brakes on the front but I haven't removed the valve since I don't see any point in messing with a system that works! (plus i'm a bit lazy)
So, as far as I am concerned, anything better than Sprint drum brakes on the rear is a waste of time, money and effort, BUT, this only applies to MY car!

Steve

[Edin Dundee]

Some of this interesting and useful to us and some if it belongs in a text book.

[GrahamFountain]

Some of this interesting and useful to us and some if it belongs in a text book.

Thanks for that, Bill. And I'm glad to see it's "and..." not, "but some of it it belongs in a text book."

Graham

[iandollysprint]

I dont need a picture thanks. It would not help.
Can you answer my question on what, with all your input, you would suggest as being the best set up? I assume you have practical experience here? This is after all a forum for constructive input.

[Carledo]

I dont need a picture thanks. It would not help.
Can you answer my question on what, with all your input, you would suggest as being the best set up? I assume you have practical experience here? This is after all a forum for constructive input.

If your setup works for you Ian, I would stick to it! All the theory in the world can't match real life experience!

Steve

[Carledo]

Braking VERY hard from somewhere close to 100mph for quarry corner at Castle Coombe. Down to about 60mph here and just turning in! Even under these extreme circs the o/s/r wheel did not lock!



Steve

[trackerjack]

Here is my old sprint dicing at Quarry with a Nissan Skyline, one of the fastest cars around!
It was on the Castle Combe site as an example of a slow car being driven fast (sprint) and a fast car not so fast as it should be, however at the end of a lap I could still spit on him.
My car was fitted with my "Trackerjack" front brakes and a removed load sensor but standard rears.
Go round Combe as a passenger with Carledo or Mad Mart and you WILL experience the brakes used to their max.
AS stated before theory is fine but there are so many variables its bonkers to try and nail it to an exact science.
Tyres/pressures/wear/age.
Springs?
Shockers and their settings.
Suspension mounts/standard/poly/worn.

Sorry to say it and I dont wish to argue but this is typical of some forum threads that spirol away from a simple question from someone who just want to know what to do.

Now sure enough someone will point out that the track is not the only place the cars are used, maybe but mine got used for everything such as towing commuting both winter and summer in all weathers.

The fact that the car in question is a Triumph and produced by BL says all you need to know, a company known for failure and I have read that one brake manufacturer actually refused to have anything to do with the Dolomite as the brakes were so poor.
If you consider that the car started as a 1300 and not expected to do speeds over a 100 and over time morphed into a road burner of close to 120 mph and acceleration to match still with the same brakes albeit with a sodding great servo to hide the shortcoming.
Do you really trust the engineers/bean counters?

Thse same engineers made TR5/6 with diffs that fall out and front suspension that fall off the chassis.
A sixpot engine that can drop thrust washers within 40k from new.
Marina and Spitfire with collapsing fron suspension etc.
These days they would be in court all the time.

[Mahesh]

Chris, many thanks for starting this thread.

I must admit that reading through, it cannot be denied that the physics and the real world physical are both worth knowing, if only not to spend obscene amounts of money because of manufacturer claims, and a balance can be struck depending on what items are selected as opposed to a kit.

I'm of the personal opinion that you can never have too much braking power irrespective of how you drive or where, your leg adjusts as a regulator pretty quickly, the limiting factor being how much.

[GrahamFountain]

I'm of the personal opinion that you can never have too much braking power irrespective of how you drive or where, your leg adjusts as a regulator pretty quickly, the limiting factor being how much.

Well there is always a physical limit from the tires on how much force you can apply with a brake. If you have more brake force than the down force on the wheels times the maximum coefficient of friction the tire gives on the driest road (probably about 0.9 to 1.0), the wheel must lock. In which case, any more brake force than that is a waste of money and unsprung weight, because you can never, ever use that force. The only thing bigger brakes than that will do is move the point at which the pedal locks the wheel up off the floor. You can get much the same effect with a bigger servo, which is liable to be cheaper.

The model gives that value for each brake in Newtons. But the problem is, it's not so easy to get a comparable value by calculation, unless you know the maximum hydraulic pressure, either from your own or from some sort of British Standard leg (would it have a kite mark). You would have to choose whether you want that figure for the hardest you can possibly press in an emergency, or for the maximum you're comfortable with. If you did know that, it'd be easy to work out for a disc and not quite so easy for a drum. There are ways to measure it, but I don't have access to any.

Graham

[Dolly racer 33]

Chris, many thanks for starting this thread.

I must admit that reading through, it cannot be denied that the physics and the real world physical are both worth knowing, if only not to spend obscene amounts of money because of manufacturer claims, and a balance can be struck depending on what items are selected as opposed to a kit.

I'm of the personal opinion that you can never have too much braking power irrespective of how you drive or where, your leg adjusts as a regulator pretty quickly, the limiting factor being how much.

Cannot agree with this. My race Tomcat was built and developed by Rover Sport for the 95/6 Dunlop Turbo race series, these cars ran in slicks in period, ran Mintex 1155 front pads, 1144 rears on a standard rear disc/calliper and an AP racing set up up front. If I run Toyo 888's or slicks I can brake very late and controlled into a corner. If I run a tyre with less grip, the brakes are a nightmare with no feedback, they lock up as soon as you apply any pressure to the pedal, I have to use a much softer pad to overcome this.

Every set up is different, the only way to ensure you keep the balance built into the cars by Triumph is to keep with the original size discs & pads, just make them vented. This is what BL/Triumoh did in period because they had to keep original wheels, master cylinders & rear brakes etc.

[iandollysprint]

Some nice photos above.

If anyone wants to have a ride with me in mine please come along. I am really pleased with my set up for the road and it is far from standard, but if it helps for others to make some judgements on what they want all well and good. Whether that be engine, suspension, seats, brakes, tyres alloys etc.

I dont have photos in action though! Not yet...

[GrahamFountain]

If I run a tyre with less grip, the brakes are a nightmare with no feedback, they lock up as soon as you apply any pressure to the pedal, I have to use a much softer pad to overcome this.

That's related to what I mean about its being the tires that determine what you can do with the brakes. But I do think there's at least some room for improvement in the effort the front brakes can give on a doly sprint. Not, much perhaps, but a bit. Maybe I'll change my mind when and if the servo ever comes back from reconditioning, because I've always felt the effort from standard TR7 front brakes is adequate when the servo works properly - but they're crap when it don't.

The issue of vented discs is different. I would guess it depends on how often you see brake fade, and how much it matters to you to avoid it. Me, I don't see it at all with the doly. I have seen it on the TR7 Sprints, even with TR8 and the big Princess brakes. But I didn't see it often enough to bother with vented discs: only seen it on drive outs from Drivers and Register 'do's really. I did buy some vented discs and adaptors once, for M16s, but never got around to fitting them. I've moved house 3 times since then, and they're lost somewhere now.

Graham