Spitfire 2 questions about my Spit

[simpzimmer]

I have two questions about my 79 Spit. First of all, a couple of people have said that there might be a way to convert my rear suspension to coil springs. If so, is this a good idea? And how would a person go about doing this?
Second question. My rear-end has started whining. I'm figuring the crown wheel and gear will have to be replaced sometime. Victoria British lists two options. One is the 3.89 and the other is the 3.63. Will both of these gears work in my differential? Is there a big difference?

 

[Andrew Mace]

Anything is possible, but I see no good reason to make major modifications to the rear suspension unless you're looking to do some particular sort of competition driving. IF you've any problems with the current setup, they're best fixed with new components as needed, including possibly the readily available heavy-duty rear swing spring.

As for the differential, you might find it cheaper to find a good used one rather than rebuild yours IF it really needs it. As for the two options, 3.89:1 was normally fitted to US-spec. cars from 1971 on. 3.63:1 was fitted to the home market Spitfire 1500s, which had more power from less emissions spec's and dual SU carbs. You could probably use the 3.63:1 here to reduce cruising RPMs without suffering too much on acceleration (think cheap overdrive!), but you'd need to recalibrate your speedometer or get one (from the UK) suitable for that ratio.

 

[simpzimmer]

Thanks for the reply. About the suspension, a couple of people have let on like it would be such a better system of suspension that I thought it was at least worth asking someone on here about. Also, the differential gear ratio is just me not knowing. I've only had my Spit for two years and I've still got a lot to learn about the mechanics. I'm getting there, though.

 

[guzzul]

I agree with Andy on both counts. The Spitfire rear suspension has its pro's and cons for sure, but with a decent spring, shocks and bushings, it works well for this small car. Conversion to coils would seem to be a pretty major conversion and would seem to completely change the rear axle design.

As they age, pretty well all the rear ends howl a bit, some more than others. Often it is not the ring and pinion gear, but rather the differential stub axle bearings or pinion bearing. The shims that set the 'lash' between the ring and pinion also wear as do the sun and planets gears, and all these can generate noise. The crown and pinion are quite expensive, and as Andy notes you may be better off just getting a proper rebuild, although these are expensive too (about $1,000 the last I checked).

 

[skucera]

The rear leaf spring in a Spitfire also acts as the upper suspension arm. You would need to design a replacement for the this upper arm. Assuming you wanted to use the rear shock mount to be a new spring perch for your new coil springs, you would need to beef this up too because it is designed only to take a portion of the vehicle's weight, not all of it.

Really, since the rear suspension design hasn't been a problem even for competition, so why redesign it? Even new in the 1970's Spitfires could do 0.85+ G's on a skid-pad, and they have won SCCA H Production classes almost every year since the late 1960's, including 2007. The swing spring rear suspension on your 1979 is a very good design, much improved from the early Spitfire rear suspension that my old Mk. 1 had. Also, this rear suspension design is still very similar to the Corvette's rear suspension design, except that on the Corvette the rear spring is under the drive-shafts instead of above. Also, the Corvette uses a fiberglass spring to save weight (too bad someone hasn't engineered a lighter replacement for the Spitfire's steel leaf spring, eh?).

As has been mentioned several times, differentials wear and can be rebuilt. It is expensive.

Scott