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I want to refurb a pair of front struts from my spares stash -the strut tubes are in good order and I would like to fit new standard road springs but the only ones I can find have 8 coils (red-red marks). I have an old spare pair, as well as those in use on the car- they all have only 6 coils!
Info. received says these `new` 8 coil `red-red` versions are about the right free length but what is the spring rate me wonders. Haynes is my only source of info. in this area and it notes 88-93lbs. as standard- depending on year/comm. nos.
Any additional input/ experiences would help here. I`m in a quandary as to what best to do!
Thanks in advance

As a rough rule of thumb and trying to minimise the many variables involved….A coil spring is a helically wound torsion bar, that is to say that the wire/rod twists as the coil compresses or expands, this twist is what provides the spring force.

If the free length of the coil is the same, the outside coil diameter is the same, the wire/rod cross section diameter is the same and the metallurgy of the wire/rod is the same….then the wire/rod length will be longer to accommodate those two extra turns and therefore the 8 turn spring will be softer.

But of course there are other factors, such as variable spring rate springs….this effect can be achieved by a few ways, varying the heat treatment along the length of the spring, tapering the wire/rod used to make the spring, and tapering the outside coil diameter of the spring.

All of the above will only apply to new, unused springs, as springs will soften with use over time, the more compression/ expansion cycles a spring goes through over time the softer it will become.

Aren’t you glad that I simplified the whole thing for you?

Dealing with new springs the only practical way to know is get the actual factory specs for the springs in question.
Dealing with used springs, then the only practical way to know is have them tested by someone who has the equipment to test them.

Or just use the suck it and see method of putting the bloody things on the car
The latter is recommended for all of those who’ll be using the car in a relatively normal fashion… only those who are chasing precious tenths of a second off their lap times are likely to need to get all scientific about it.

I think I can only be semi helpful. When I got my car, it would drag (on the Californian Catalytic converter cage which hung lower than the frame) on every speed bump regardless how carefully I went over. I replaced the front springs with what were termed stand weight springs. They were red marked springs as I remember, don't know the number of coils but it stopped the dragging except for the center speed bump in London Drugs parking lot (don't go that way). The car is stiff enough (I used hard black poly on front suspension), there is minimal body lean, wouldn't want anything harder. They also brought the front end up so the car sits horizontal.

Old standard springs may well have sagged over time so if you are rebuilding the struts its worth fitting new springs at the same time. I would get them from someone who is a TR7 specialist who knows our cars.
Personally I really like the 200lb springs I got from Steve Wilcox, I know they are not standard but they do not feel at all hard when driving and give a nice ride.

Don't you have companies who retemper springs in the UK?

I have never replaced a spring in my life. I take them to a spring works, & get them retempered as new. Occasionally on truck or trailer springs I have added an extra second leaf to increase spring weight carrying capacity. I raise or lower the ride height by the static length the retempering is set to.

My 7 has stock springs, with 35% stiffer anti role bars. This gives better grip & cornering, without silly rock hard spring rates. For what it is worth, this is the way I set up my Formula 2 Brabham Cosworth back in the day. Soft springs & stiff roll bars. It won every race, or the F2 class in F1 races in every race it started in the 20 months I raced it.

Hasbeen

According to my ageing TT manual front springs with RR marking are 200 lbs and with a fitted length of 6,50" (compared with 7,10" for standard springs)

Just an update
Thanks for inputs to date. So far I have no further info. on the `8 coil red/red` front springs that are said to be `standard` but their spring rate remains unstated/unknown whereas the originals were 6 coil and approx. 100lbs rate. I will devise a spring tester (I have a plan) and test it on some spare old springs I have, and then... ....?