Rear disk conversions?

[Boost All The Dollys]

Does anyone know if the Sprintspeed rear disk conversion is any good?

has anyone heard of any other rear disk conversion that's worth talking about?

[cliftyhanger]

Not a dolly, but I did my own disc conversion on a spitfire.
I used MGF discs and callipers. The discs fitted over the hubs, so space the wheels out 10mm and studs may need to be longer.
I removed the drum backplates, and used them as templates to make a 6mm thick steel plate that would fit in its place.
then clamped the callipers (pads fitted!) to the disc and made a section to join the calliper to the bolted up mounting plate. Welded together. Job done.

If doing again I would use polo/golf callipers as they are much lighter. Bit of grief getting it to fit under 13" wheels, but eventually got there. Except a set of wheels that were riveted together with round-head rivets. They did catch! so not a lot of clearance, but enough.

[Galileo]

Does this help?

viewtopic.php?f=3&t=2157

[Boost All The Dollys]

Not a dolly, but I did my own disc conversion on a spitfire.
I used MGF discs and callipers. The discs fitted over the hubs, so space the wheels out 10mm and studs may need to be longer.
I removed the drum backplates, and used them as templates to make a 6mm thick steel plate that would fit in its place.
then clamped the callipers (pads fitted!) to the disc and made a section to join the calliper to the bolted up mounting plate. Welded together. Job done.

If doing again I would use polo/golf callipers as they are much lighter. Bit of grief getting it to fit under 13" wheels, but eventually got there. Except a set of wheels that were riveted together with round-head rivets. They did catch! so not a lot of clearance, but enough.

sounds good. so the 6mm plate acted as a adaptor and a spacer? the 10mm, was that the difference in the discs and the drums thickness?

would it be an issue to fit them under sprint alloys?

[cliftyhanger]

I can't help with sprint wheels.

The 10mm is the disc thickness (approx) and they sit on the hub, just like modern cars. Drums don't have much thickness at all on the faces, 3-4mm??

The 6mm replaces the drum backplate, or at least did with my spit conversion. May be a little different, but I can't see it being an issue. You need to pull the hub off to do this. Although I have seen conversions where rather than a hole in the centre, it is a big slot, I guess the backplate is cut off, so avoiding removing the hub.

But in all this, think do you need rear discs? they are generally not as efficient as drums, but don't fade in the same way. However, sprint drums would be pretty difficult to get to fade I reckon. Plus points is that sprint brake shoes are horribly expensive, pads for mgf/vw etc generally pretty cheap. Horses for courses. My spit has done 5k last year, couple of trackdays plus the 10CR. Brakes very good and rather better than MGF ones at donnington (ie I could brake rather later when doing the same speeds as the 160's) but the spit is rather lighter and has vented GT6 front brakes (and asbestos pads) Talking of which decent pad material is incredibly important on these old cars. They were designed to be used with asbestos pads that have a higher coefficient of friction compared to the tat most suppliers sell. Mintex 1144 or 1155 are possibly the only easily available alternatives, at a price.

[PaulB]

Hi,
Coincidently, I spent this weekend putting a Sprint Speed system onto my axle. Rob recommends a Fiesta disc, but I used MGF. The MGF rear disc bolts straight onto the Sprint hub, I preferred MGF because the PCD is correct, you just need to remove the hub backplate and machine a coupe of mm of it so it will fit the disc. Even the small screws for attaching the disc to the hub will fit.

It all went together really well and it fits under my 13 inch minilites although I have heard of other people having clearance problems with minilites.

Rob also provides the handbrake cables. I am some way off using it on the road, but it looks good in the garage.

Rear disc 1s.jpg (93.67 KiB) Viewed 1170 times

rear disc 2s.jpg (97.72 KiB) Viewed 1170 times

rear disc 3s.jpg (98.25 KiB) Viewed 1170 times

and here is my front brake

front disc 1s.jpg (222.82 KiB) Viewed 1170 times

[Galileo]

TBH I'm with Cliff on this, I don't really see the benefit of a rear disc conversion. Unlike the just good enough front brakes, the rear is over braked and then some and It's not like you're reducing the unsprung weight with a live axle...

[PaulB]

TBH I'm with Cliff on this, I don't really see the benefit of a rear disc conversion. Unlike the just good enough front brakes, the rear is over braked and then some and It's not like you're reducing the unsprung weight with a live axle...

Depends on what you want the car for. I agree for a road car, but track day or competition car its a different matter.

[iandollysprint]

viewtopic.php?f=19&t=15564&start=270

Sprint speed set up with tracker jack front discs.

Mine is destined for track use. Very pleased with the overall performance, difficult to lock up the wheels and braking performance is massively improved. Do not fit under standard sprint wheels. I ended up with 15*6.5 inch Team Dynamics Monza wheels from an MGF with 185 tyres all round. You can still buy these wheels brand new but need to shave 3-4mm off the inside of them to prevent rubbing on the arches.

[GrahamFountain]

Depends on what you want the car for. I agree for a road car, but track day or competition car it's a different matter.

I believe (and may have mentioned it before, once or twice) that it's very important with a road car to keep the brake balance close to where it was set by Triumph, so you keep the same safety margins, i.e. allowing for variations in the road surface and unequal wear on the tires, etc.: I assume that the point about moving the actual balance back behind the perfect brake balance and consequent loss of control can be taken as read. I also believe it's important to make that point in any discussion of upgrading the rear brakes.

But Triumph set the balance, and the safety margins, for the tires of the time. And I think we've moved on a bit since then. So if you use tires with better grip, and especially if you uprate the front brakes to take advantage of that better grip (so you can decelerate better, and transfer more of the weight of the car on to the front wheels and off the back), then you need to reduce the effort from the back brakes, in order to keep the actual balance the same amount in front of the perfect balance, not increase it.

I know that may seem illogical. But I'm sure those who understand the physics of it will back me up. So if you do use better, grippier tires and better brakes, and keep the brake balance the same, the rear end will, as Galileo puts it, be "over braked and then some". I think I'm in no danger of that with the Firestone Fuel Saver tires I use, though.

For a track car, I'd have thought you'd nearly have to have some means of controlling the brake balance. So I'd also have thought that the big things about rear discs would be the reduced fade and better control of the peddle length, more than increased brake effort as such.

Graham