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I have an IRS Mantula with 14" wheels - and have experimented with rear tyres. The right size is 205/60 VR 14 - but you won't find a new V rated tyre in that size. The front is 195/60VR14. I have some 205/60VR14 Pirellis with good tread, and I am about to throw them out as I find they really don't balance well and are slippy in the wet - so I suspect the tyres are just a bit old. You can have them for nothing to try the size out - (I am in Cheshire).

I also tried some 205/70VR14 tyres - they REALLY fill the wheel arch - but also tend to jack up the rear and spoil the steering geometry. These are formally approved for the car (a legal requirement in some countries in Europe). They are available new, but on the IRS cars they can rub on the inner wheel arch (they did on mine) - as a result I have a brand new pair of them (20 miles only), and would be delighted if someone with a rigid axle car would take them off my hands (£50 the pair ??).

As mine came with 195/60VR14s front and back, I have the ones that came off the rear as a spare set for the front. I run a normal H rated 205/60R14 rear, and I have told my insurance company (MSM) and they said no problem.

Steve

Assuming you have the correct 205/60 rear tyres I reckon three fingers clearance between top edge of tyre and wheel arch. If you have the wrong sized tyres, like 195/60s then the tyre diameter is 10mm smaller so you need an extra finger at the top to keep the rear of the car at the right height. Or you can drop the overall ride height, but that can make the sump pretty low at the front. For 205/70s the diameter is 32mm larger than the 205/60s. I have photos of each of these somewhere if you are interested.

I had to replace the dampers on mine, but dampers should be roughly in mid-stroke when the car is standing, so don't govern the ride height. It is the spring rate (pounds/inch), spring length and adjustable spring seat height that are the main factors governing the ride height. The damper range should allow full compression without the wheel rubbing, and full extension without the spring unloading. This last item caused me some probs, but helper springs fixed it.

MHS supplied me with new rear Protech dampers and springs. The dampers were 18.25" between centres fully extended, I think 5" travel, maybe 5.5" (it depends how much compression of the rubber bump stop you allow for). The springs were 14" and 200 lb/in rating. When the car was let down onto its wheels the springs compressed 2.45", indicating a sprung corner weight of 485lb (separate thread suggests this car put on weight in the galvanising bath). The trouble is this combination made the car too high. MHS sent me a rebated spring adjuster, but that meant the spring would be unloaded on full extension and still meant I was at the limit of ride height adjustment. I considered a lower spring rating, but it would have taken 150lb/in to get the ride height correct, and that would have been too soft. So I went for a 12" spring at the original rating on normal spring seats, and included a helper spring to get over the fact that it would become unloaded at full extension. That gave me the right ride height and meant I had adjustment in either direction using C-spanner on the spring seat.

My original AVO dampers were completely different as the springs were 12" at 400lb/in rating, so I could use neither spring or damper as patterns (the front springs were 450 lb/in and are now 250 lb/in).

I hope all this helps - I did spend a lot of time trying to get it right, but it was a couple of years ago and I don't remember quite when in all this I discovered the rear tyres were the wrong size (195/60). A helpful soul on here told me the rolling diameter of 205/60 and 195/60 tyres was the same, but the 60 is 60% of the width - so if you change the width, you change the height. So you mustn't believe the advice you get (including mine!) - you have to make your own mind up ! But I did buy a helpful book on Kit car and race car suspension, that I will give you the reference too ig you are interested.

Steve

You can check spring rating by simple measurement. Its all pounds per inch of compression (please don't go metric). Stick a known weight on top and measure the compression. Or stick them on the bathroom scales and add an unknown weight (the scales will tell you how much) and measure the compression.

As the elastic properties of all steel are much the same, an alternative is to calculate the spring rate by measurement of dimensions and turns as indicated by previous poster. I think 250lb/in front and 200lb/in rear was the original spring rate as per previous post.

I can't quote a Gaz part number (mine had far from original dampers when I got it, and I think Gaz dampers are aftermarket anyway) and that would be just the damper not the spring.

I took off Avos front and back as the damping adjusters were seized, and ended up with MHS supplied ally Protech units at the back, and new Avos at the front.

Apart from dimensions you have to decide whether you want aluminium or steel and two way damping adjustment or a separate rebound and compression adjustment. What you needs depend on whether you plan track days, or only want to cruise in comfort. The spring rating is also part of this equations with spine jarring hard springs seeming more usual for race tracks ( though race tracks don't have great big potholes like UK roads).