Talk to me about Final Drives......

[shaunroche]

I mean diff ratios, not the journey to the 'Crem!

My Sprint is finally going on the rolling road next week to have the carbs re-jetted etc. and I was asked by the chap what diff was in it. When I said it was standard he couldn't believe it!

Now, rather than change the full diff he was talking about changing the crown wheel and pinion - he and his mate used to rally a sprint in the 70's & 80's and he thought they fitted a cw&p from something like a Standard vanguard which was something like 4:1?

So, what different final drive options do I have to alter the acceleration etc. that people generally fit?

Additionally, what options do I have re gears....have people fitted different ratio gears, or even fitted something like a ford five speed?

It's not something I've looked into really until now so I'd be interested in your thoughts chaps!

[Mad Mart]

The usual trick is to fit a 4.1:1 CW&P from a Triumph 2000 (manual).

[sprint95m]

Additionally, what options do I have re gears....have people fitted different ratio gears, or even fitted something like a ford five speed?

I fitted a type 9 Sierra gearbox to an 1850 Dolomite.
(Indeed I wrote an article which was published in Dolly Mixture)

The gear spacing is way better than a Sprint. The 'box is also a bit lighter.


My intention is to fit a T9 to my Sprint.
I have one from a Capri 2.8 for this task.

You need an adapter plate (from Canley Classics) and the bellhousing from an 1850 'box.
The clutch plate is for an 1850HL (or Transit V4 ).
Obviously you need a bespoke prop (but that is not too difficult).


My schedule for repairing the Sprint is about two months behind where I wanted to be
but hopefully will have a painted rolling shell by the end of the summer
with mechanicals to be fitted over the winter.



Ian.

[Carledo]

It really depends on what you want to do with the car Shaun. The original diff ratio is 3.45 which gives a decent top speed and, with your horsepower improvements, it should still be pretty quick getting there! Lowering the final drive to 4.11 is a big trade off, acceleration will improve, but will mean that you are snatching gears much more often and top speed AND fuel consumption will suffer accordingly. If you ONLY intend to use it on track, this is not a biggie, but for a road/track car, i'd stick to the stock ratio (but with an LSD) Since the slant is not a particularly high revving engine, a low axle ratio could be rather annoying.

If I were you, I'd drive it around a bit first and see how it feels before committing to something that could make your driving experience a whole lot less enjoyable.

Steve

[gmsclassics]

The car I’ve raced uses a 4.1:1 ratio. However for the reasons Steve states, I wouldn’t consider using anything other than standard in a Sprint predominantly used on the road. You really need to have changed the Sprint from one with good torque to one that develops a lot of power high up using an STR139 cam capable of revving to 7000 plus.

That said 3.77 and 4.1 ratios did exist in early 2000 saloons, manual and auto, but I found that many of these had received transplants of 3.54 axles from later cars during their long life. Thus each diff I looked at had to be measured.

3J Driveline do I believe make an LSD for a TR6 which should fit. At the same time consider changing the half shafts to a much thicker diameter ones. Standard ones with wide sticky tyres can bend and let oil out through the seal into the wheel bearing and eventually onto the tyre!

At some stage converting a Capri 3 litre rear axle to fit is something to consider. I cut the bracketry off a standard Sprint axle and welded it in place on the Ford one. These already have 9” rear drums. You then get access to umpteen diff ratios, cheaper LSDs, thicker half shafts and even floating axles, plus off the shelf brake shoes with better linings.

Geoff

[sprint95m]

We need to be careful here,
New Zealand assembled cars were not necessarily the same as UK specs.

UK specification for the big six differentials:

4.1:1 2000 manual all mark one and mark 2 (1963 -75)
3.7:1 2000 auto (all years) 2000TC manual (from 1975)
3.45:1 all 2.5 and 2500.

The 3.7:1 was also used in (both manual and automatic) Stags




Dolomite Sprints used the same back shoes as 3 litre Capris in the UK.
(If you have rivetted on linings it is possible to get linings from a whole variety of cars.)



Ian.

[shaunroche]

Excellent!! Proper can of worms opened here!

Firstly, many thanks for all the replies fellas, it all makes for good reading!

The car is currently road legal purely so that any mods I carry out can be tested straight away rather than having all the rigamarole and waiting to get at a track.

The car is a track and sprint car predominantly and the rounds in the series I'm competing in (Revington TR Sprint & Hill Climb Championship) are mainly circuits with the exception of Curborough and Harewood Hill Climb. Therefore as you say Steve, I guess it needs to have a decent top end that the std diff gives.

To be honest, I could only get it up to about 105mph at Goodwood recently but that might be more down to not having the correct jets in the carbs - it's getting properly set up on a rolling road at the weekend so we'll see about that one.

How the whole question came up was that it's really obvious that the car needs an LSD first and foremost and then the conversation degenerated into talking about final drive ratios and 'boxes etc......

Looking online at Ford axles, I'm very surprised at how much cheaper stuff for Ford's is....It sounds very much like a better option is to go for the Capri back axle (to get an LSD), particularly as Geoff says, it sounds easier and cheaper to go down the Ford rather than Triumph route....

I've spotted this on eBay......

https://rover.ebay.com/rover/0/0/0?mpre ... 2752837835

What width is a Sprint back axle if a Ford one is 52" ish?

The car is definitely not slow, I'm very pleased with it...it's certainly quicker than I am! I'm intrigued though Geoff that you say you were running a 4.1:1 diff....this wouldn't give much of a top end would it with a std ratio o/d box?

[shaunroche]

I fitted a type 9 Sierra gearbox to an 1850 Dolomite.
(Indeed I wrote an article which was published in Dolly Mixture)

Tell me more Ian, do you have a pdf or scan of it please?
Just been doing some calcs and I like the sound of that!

[Carledo]

What width is a Sprint back axle if a Ford one is 52" ish?

I'll measure a Sprint axle in the morning for you Shaun!

Steve

[shaunroche]

What width is a Sprint back axle if a Ford one is 52" ish?

I'll measure a Sprint axle in the morning for you Shaun!

Steve

Good work that man!

[sprint95m]

I fitted a type 9 Sierra gearbox to an 1850 Dolomite.
(Indeed I wrote an article which was published in Dolly Mixture)

Tell me more Ian, do you have a pdf or scan of it please?
Just been doing some calcs and I like the sound of that!

I don't have it on PDF. When I get a chance I will look for the Dolly Mixture....


Ian.

[gmsclassics]

With the tracks I raced on, the gearing was more about harnessing the power to accelerate rather than top speed per se, and I used to find top speed was more limited by the brick like aerodynamics. I tried a standard, non overdrive 'box, standard overdrive and a ST geared close ratio 'box. Latter was great but due to the high first, I usually lost places off the start line. The standard OD 'box gave the best race position as with a lightened flywheel, good tyres and a small twin plate clutch I could make up anything up to half a dozen places before the first corner if everything went well. That 'box has overdrive on 2, 3,and 4 gears to maximise the power out of corners (2nd OD is fine but only to select in the corner and then put the power down). Effective top speed was in 4th (not overdrive) at 7000-7500 rpm. Engine had block brace fitted.

I found the LSD was worth at least 2 seconds a lap, especially where there are slow tight corners. Those were made worse as best lap times were achieved by setting the suspension and steering geometry up for very fast corners, not the slow ones. Front and rear adjustable tie bars are fitted.

Ford axle from memory is just over half an inch narrower, but I used spacers and long M10 stds (NS380) to position the wheel in the centre of the arch to allow the largest tyres I could fit in the space (rolled arch edges, I wasn't allowed to fit bubble arch for my race series). Tyres were 195.60x14. Bigger the footprint the faster you can corner. Ford axles come with two flange sizes and the Sprint prop bolts straight onto the smaller. Bigger problem is the 4" stud pattern. After re-drilling the Ford hubs to the Sprint PCD, I ended up buying stonger half shafts with separate hubs from 3J Driveline. I waited until they made another batch of hubs and they drilled the blanks to the Sprint PCD specially for me. In hindsight, keeping the rear Ford PCD, fitting Ford PCD front hubs (using stub axles from Rob at Sprintspeed) would open up access to a wide range of wheel sizes and makes a lot of sense. Two sets perhaps, road tyres to the track and change wheels to sticky rubber just while racing? I was warned against using 40 plus year old alloys on the racetrack as the modern sticky tyres would put much higher stresses on them. The 14" ROH wheels have been fine.

We measured and photographed the Sprint axle, cut these brackets off very carefully. The after grinding the brackets off the Ford axle, we simply postioned and welded the Sprint ones on. We also had another Sprint axle so we could make doubly sure positioning was perfect. Relatively straightforward.

There are videos on YouTube (search Dolomite Sprint No 20) and some others from the series (AES) from other cars. As I'm selling the car, there are photos on https://www.trademe.co.nz/Browse/Listin ... 1505414459

Happy to answer any other queries.

Geoff

[shaunroche]

With the tracks I raced on, the gearing was more about harnessing the power to accelerate rather than top speed per se....

Geoff

Cheers Geoff, love that level of information as it gives me a good baseline to work from.

Thanks for taking the time to answer, really glad I opened this topic now!

Understand the bit about old wheels but I can get bespoke minilite type wheels made up quite cheaply so the offset and pcd wouldn't really be a problem.

[shaunroche]

Question - I understand that the different axles across the Dolomite range are different due to the bolt on diffs etc. but if I wanted to transfer the bracketry off say an 1850 to transplant onto a Ford axle, would that be possible or must it be a Sprint axle?

[xvivalve]

The bracketry is the same on all axles.

I've measured a Sprint axle in the back garden here and from outside of drum to outside of drum it measures just over 53"...