C-Type Replica Spring Rates

Hello all,

Newbie here looking to pick the brains of all you experienced builders out there. Just acquired a CERA C-Type rep (long story). Its a great piece of kit but the back end seems somewhat saggy and regularly bottoms out. It’s a live axle set-up with a single coil-over on each side. I think the starting point is to replace the old coil-overs and go from there. I’ve identified the required shocks but I’m having trouble with getting the right springs. The ones on the car have no markings so I’ve no idea about the correct spring rates. I’ve had 150lb/in suggested but these are nowhere near as beefy as the ones currently on the car.

Any ideas?

Thanks in advance

Rateone

Hi…The single rear coilover each side on my Realm C type is 650lb…its not a live axel but used cut down xj6 driveshafts and lower wishbones and xj6 dif(standard Realm setup)…Steve

Thanks Steve,

That’s a real help. Mine has trailing arms and what looks like it might be a MkII unit.

Cheers

Dave

You sure about that Steve? That is four times as hard as my D (from memory). At 650 lb/in it would be impossible to bounce the corner. I’ve met you and you’re not even baby elephant-sized.

Hi Peter…Jumbo here​:joy::joy: here is the rear coil over 650 the fronts i think have 450 425 on them(cant get to them to check…it was discussed on one of my C group chats…i thought i remember it being the spring rate…im no expert and a call to Adrian at Realm would give a definate answer…or a post on tne JDC replica facebook site…All the best…Steve ps looking at the SNG site the coil spring for the adjustable platform shock on tne E type is rated 325lb and you use 2 each side on an E so 650 sounds right…possibly

I think there’s a logic to both and I’m no expert.

Without arguing over mass vs weight, if a 2400 lb car weighed the same at each corner the tyres would support 600 lb each and if they were 600 lb springs they’d settle slightly less than an inch (because some of the weight is unsprung) while the ride height would drop by slightly more (because of suspension geometry). Let’s guess the two variances cancel out and so the body did drop by an inch. Two fat ladies and a tank of fuel might add another 600 (none of it unsprung) so the car would drop by more than an inch etc. G forces on dips and rises would alter the effective load and the dampers stop it bouncing around. The actual stroke of the damper piston is not huge.

My single coil-over per side solid axle has much more unsprung weight and far less sprung. The axle is too heavy for me to carry but the alloy rear end is not. In my case, I used dampers with threaded spring seats to set ride height and from memory the Demon Tweeks catalogue had nothing like 650 lb coil/-over springs.

I figured that a full 15 gallon (Imp) tank will add 100 lbs and myself 200 lbs, so an extra inch drop beyond static ride height (sharing 300 lb between 2 springs). I haven’t driven it yet but it all looks/feels right on 2 springs of about 150 lb/in at the back. The front is TB and being much lighter the car will feel a bit stiffer on standard bars, due to lower forces.

Hi Peter. …sounds like you know what your talking about…so why does an E type ride ok with twin 325lb coil overs each side…?..i thought becaus the C is very low you need a spring rate that dosnt allow an easy up/down movement…a friends Realm has rears with 450 on them and he bottems out quite a bit so wants a stiffer spring. …Steve …PS…Renault Twingo up to 650lb rear spring https://www.kamracing.co.uk/car-tuning/renault/renault-twingo-mk2-rs/suspension/renault-twingo-mk2-gaz-gha-coilovers-renaultsport-rs133-cup.html

Could be right Steve and just because it sits right and looks right it might be too soft. The E is not only a much heavier steel car, it has to work with two or more, plus their luggage, I guess?

The way I’ve got mine, the compression and rebound damping are adjustable, the springs are not coil-bound at full bump and the progressive rubber bump-stops start supplementing the spring after an inch or two in either direction, which is effectively the same as stiffening the spring. My dampers are probably less tilted than yours, which would need a stronger spring to compensate.

With luck, someone who knows what they are talking about will chip in :slight_smile:

Thanks for the guidance. That’s all really useful info. I’ll look into getting a new set of 650lb/in coils.

Cheers

Dave

Hi David,

Do you have the spring length and ID?

Regards
Keith
Tempero D

Hi Keith,

The springs are pretty chunky. They’re 9 coils with the external diameter of the coils being 80mm. The diameter of the spring wire is 11mm (including the plastic coat). It’s difficult to say how long they are fully extended as I haven’t taken them off the shock but I would estimate 230mm (c.9"), might go to 254 (10").

Cheers

David.

Remember that rate is a property of a spring over it’s range of movement. A 150lb spring can support one corner of a 2400lb car just as well as a 600lbs spring. The only difference is the 150lb spring will move 4 inches instead of one. If you juggle the spring seat height/ride height, damper rate and travel etc,you could make either work but the ride would feel different.

Do you really want a setup where you have to double the weight of the car to compress the suspension one inch???

Hi David,

Given your use of metric I am assuming you are not in the USA, Myanmar or Liberia! A friend gave me a pair of coil springs (400 lb/in, 9" long and 2.25"/60mm ID and around 83mm OD) when cleaning up his garage prior to moving, that he bought for a C replica he was working on for a friend. But for some reason didn’t use them.I think they were for the front which fits the comments made on the thread. Was thinking if you were relatively local (North East USA) then it could be useful to try them and see what happens.
One other option with the caveat that I have no idea if it would work/give you an idea of what spring rate would work, is at add a spring rubber which would increase the spring rate.
Digging around on-line I found this website. If your springs aren’t marked with the spring rate you may be able to calculate the rate Compression Spring Calculations - Quality Spring, Affordable Prices
I plugged in the numbers for the springs I have and came out to at 399 which was close enough to 400. And is mm or in.

Regards
Keith

Hi Keith,

Thank for that. I’m in the UK but I’ve had a play on the same website. Got similar values to you. It’s a tough one. As Peter says it’s a trade off between spring rate, travel, damper rate, etc. Don’t want the car dragging it’s backside on the ground over every bump but equally I don’t want to lose my teeth either. Nor do I want to spend money amassing a spring collection.

David