Rear disc brakes

[SprintV8]

Thank you for your input I do appreciate it.
Can’t say I agree with everything but everybody has got there own views and opinions.

I’m still going for the rear discs on mine.
Just prefer to be over braked as I would like to do some track days when finished.
There also a new set of Toyos R888 waiting patiently to be fitted.

[GlenM]

Phil,

I have come to the same conclusion.

I am going for rear discs on my CA18 DET project. I have a Trakerjack vented disc kit for the front but I could eventually upgrade to a vented kit with alloy 4 pot calipers. I am also going to be fitting R888s but 225mm wide, so the tyre grip should be much better than the standard 175mm Sprint tyres.

Glen.

[cleverusername]

whats the beneift in fitting rear discs apart from the lkleyhood of the rears locking up easer which personally I don't see as a benefit, if your brakes are not good enough this is down to one of two things, you're too used to a modern car with better brake balance etc OR there is actaully something wrong with your brakes such as old pads, weak fluid, sticky caliper or poor adjustment all of which can by sorted in a few hours

The problem is people assume that disks are automatically better than drums, so fitting them will improve things. Whereas in reality, you need to understand how something works before you can successfully upgrade it.

It is a bit like people who think they can get more power out of an engine by bolting on a bigger carb and randomly hacking bits off their cylinder head with a dremel. Who are then shocked when they put their car on a rolling road and find it makes no difference or has made the situation worse.

[cliftyhanger]

My 2p worth
On my spitfire I did convert to rear discs. Partly becuase the availability of decent rear linings for spitfires and even larger GT6s is "difficult" (although there are places that will reline your shoes in quality material) , but the handbrake on spitfires is notoriously weak.
I did the job well enough, but on reflection I should have used a lighter ali rear caliper (VW, available with a few different piston sizes) The MGF disc is easy to use though.

On my Toledo, and in future my dolly (once I get the sprint axle fitted) I will stick to drums. Simple and effective, and plenty big enough.
I am hopeful that once my OEM shoes do eventually wear out, I can get some quality linings.

So the Spitfire has shortcomings from the factory (much in the same way dolly front brakes are really too small, why they didn't use GT6 sized discs is a question that has never been answered) but the opposite is true of sprint drums, they are too big (although back to using bigger front discs/calipers, that would have meant the who system would have been in balance)

[new to this]

whats the beneift in fitting rear discs apart from the lkleyhood of the rears locking up easer which personally I don't see as a benefit, if your brakes are not good enough this is down to one of two things, you're too used to a modern car with better brake balance etc OR there is actaully something wrong with your brakes such as old pads, weak fluid, sticky caliper or poor adjustment all of which can by sorted in a few hours

The problem is people assume that disks are automatically better than drums, so fitting them will improve things. Whereas in reality, you need to understand how something works before you can successfully upgrade it.

It is a bit like people who think they can get more power out of an engine by bolting on a bigger carb and randomly hacking bits off their cylinder head with a dremel. Who are then shocked when they put their car on a rolling road and find it makes no difference or has made the situation worse.

Thats aways going to happen when people push the boundary, there must be a few heads that have been ported badly, but unless you make mistakes how you learn what works

[Carledo]

I'm leaving this here, as i'm obviously flogging a dead horse!

It's your time and money guys, do what you think best and good luck to you!

Steve

[Carledo]

..............much in the same way dolly front brakes are really too small, why they didn't use GT6 sized discs is a question that has never been answered..........

The Dolomite and GT6 share a common hub bearing, it would be simplicity itself to fit the GT6 hub, disc and caliper. It may even fit under a Dolomite steel wheel, I haven't tried. It WON'T however fit under the bulky and clumsy Sprint alloy and the Sprint is the model that needs it most. But I think the answer that has never been given is to do with wheel ET. The Dolomite has a much higher wheel ET than a GT6, 35 vs the GT6 at circa 20, This difference is probably enough to make the Dolomite wheel foul the GT6 caliper. If you've ever tried fitting Sprint wheels to a Spitfire (or any of the other small chassis cars) you will know, as I do, that you need to fit longer studs and a minimum 10mm spacer to make it work.

In any case, the Dolomite brake problem is not really about stopping power, the brakes are good enough to stop the car if well maintained. The problem is fade from sustained use and the cure for that is vented discs, something the GT6 discs aren't!

A more important question never answered, is why the wordwide motor industry adopted smaller and smaller road wheels during the 60s and 70s! The size of the wheel is the prime limiting factor on the size of the brakes that may be fitted. Had the factory equipped the Dolomite with rims even 1" bigger, the problem wouldn't even exist! I have my oversize 256mm vents on the Dolomega and they fit comfortably within a 14" fake minilite.

Steve

[xvivalve]

A more important question never answered, is why the wordwide motor industry adopted smaller and smaller road wheels during the 60s and 70s!

Performance. Greater initial acceleration is possible with a smaller wheel...at the expense of top end speed

[Carledo]

A more important question never answered, is why the wordwide motor industry adopted smaller and smaller road wheels during the 60s and 70s!

Performance. Greater initial acceleration is possible with a smaller wheel...at the expense of top end speed

This is logical - and at the same time makes no sense at all! Manufacturers have always made this adjustment by varying the final drive ratio, where unlimited variations are possible. All Dolomites have 13" wheels but there are 5 different diff ratios to make the best of different power outputs.

If I had to guess, i'd say ride height is a more likely culprit. The smaller the wheel the lower the car can get and the "sleeker" it looks to the prospective customer. Especially back in the days when low profile tyres were unheard of. The 175/70s on the Sprint were amongst the earliest low profiles fitted to a production car.

Steve

[cliftyhanger]

Yes, I reckon wheel size was fashion.
The prewar cars had huge wheels, and as moncoques and cars (especially the Mini) became more modern, a smaller wheel looked "better"

And now we are going the other way, huge wheels with rubber bands. (I have just got a newish modern car, ride was harsh and potholes a nightmare with the 16" wheels and 55 profile tyres. A change to 15" 65 profile is much better)

Back to brake size, the GT6 discs even as non-vented, will cope much better with braking as the larger disc copes better, plus the brakes are are by design better. IIRC Sprints "better" brakes were a bodge, using different pad material and a bigger servo.

And I too will be using 256mm vented discs when I fit the sprint running gear to my 1850.

[new to this]

Yes, I reckon wheel size was fashion.
The prewar cars had huge wheels, and as moncoques and cars (especially the Mini) became more modern, a smaller wheel looked "better"

And now we are going the other way, huge wheels with rubber bands. (I have just got a newish modern car, ride was harsh and potholes a nightmare with the 16" wheels and 55 profile tyres. A change to 15" 65 profile is much better)

Back to brake size, the GT6 discs even as non-vented, will cope much better with braking as the larger disc copes better, plus the brakes are are by design better. IIRC Sprints "better" brakes were a bodge, using different pad material and a bigger servo.

And I too will be using 256mm vented discs when I fit the sprint running gear to my 1850.

You wont be able to fit 256mm discs inside Sprint wheels 13 inch you would need 14 or 15 inch wheels, im using 260mm disc but im using 14 inch wheels

Dave

[Carledo]

Yes, I reckon wheel size was fashion.
The prewar cars had huge wheels, and as moncoques and cars (especially the Mini) became more modern, a smaller wheel looked "better"

And now we are going the other way, huge wheels with rubber bands. (I have just got a newish modern car, ride was harsh and potholes a nightmare with the 16" wheels and 55 profile tyres. A change to 15" 65 profile is much better)

Back to brake size, the GT6 discs even as non-vented, will cope much better with braking as the larger disc copes better, plus the brakes are are by design better. IIRC Sprints "better" brakes were a bodge, using different pad material and a bigger servo.

And I too will be using 256mm vented discs when I fit the sprint running gear to my 1850.

A little tip I picked up when I owned a GT6, The pads from a 75 Saab 99 fit the calipers perfectly, have a harder lining and also the lining material covers the entire metal backplate of the pad vs the standard GT6 pad where the lining only covers aound 75%. It did make a difference!

I only discovered this because SWMBO had a 75 Saab 99 at the time I had the GT6!

You need late Puma calipers and carriers to go with the 256mm discs which are from some sort of Passat. I have a part number somewhere for the discs.

Different lining material and a bigger servo is the total of upgrades to make Sprint brakes better, poor work by the boffins IMO.

The Sprint's oversize rear brakes have always been explained to me as being a by-product of the boys in the back room "stealing" the axle from the TR4. It's only recently i've bothered to think about this and my conclusion is there might be a grain of truth somewhere, but more likely it's another urban myth. Because 1) the TR4 is cart sprung and the Sprint has 4 links and coilovers, 2) the TR4 rear track is narrower by some 2.5", not including the differences in wheel ET, Sprint 35, TR4, a mere 5, 3) the TR4 had manually adjustable rear brakes, 4) the TR4 has 4.5" PCD wheels and the Dolomite has 3.75".

So I think the diff case and diff itself may have been stolen (or at least poached) from the TR, but it would have needed different, longer axle tubes with different mounts, longer half shafts with different hubs and studs, a redesigned back plate, different shoes and a different drum.

With all that taken into consideration, surely it would have made more sense to make the axle tube ends accept the 1850 rear brakes, which would have been a better match? I consider it more likely that someone, somewhere, made a mistake in their arithmatic and it wasn't discovered untill the tooling was all made and prototypes were on the road. Rather than an expensive retool they just slapped the LSV on it as a sort of afterthought!

Steve

[GrahamFountain]

I'm leaving this here, as i'm obviously flogging a dead horse!

It's your time and money guys, do what you think best and good luck to you!

Steve

Yep, it's far too much like a combination of sadism, necrophilia, and bestiality.

But why does no one seem to appreciate that it's the tires that limit how quickly you can stop?

The brakes, i.e. calipers and drums, are just one of several things that determine how hard and far you have to press the pedal to stop in the minimum distance/time set by the tires and the road surface.

And why are the dangers in having the back wheels lock first, and what affects the risk of that happening, also nearly always ignored?

Some of the things that affect that risk are a bit less than obvious, like increasing the grip from the tires; especially with bigger front brakes as well. But that increasing the effort from the back brakes increases the risk the back wheels will lock first can't be that hard to see, can it?

Graham

[sprint95m]

I'm leaving this here, as i'm obviously flogging a dead horse!

It's your time and money guys, do what you think best and good luck to you!

Steve

You are lucky Steve,
as a forum moderator this is "The Hotel California" for me




Ian

[sprint95m]

The Dolomite and GT6 share a common hub bearing, it would be simplicity itself to fit the GT6 hub, disc and caliper.
Steve

Not sure Steve......
Many years ago Tinweevil posted a photo of a GT6 stub axle next to a Dolomite one, the former is longer.
Would the disc still be aligned with the caliper on a Dolomite upright?


Ian