Front spring rate

[kram350kram]

Any recommendation on front spring rates for a TCS. Want something a bit stiffer than stock. Thanks for any input.

[BDA]

A while ago, CertifiedLotus declared that 10” 150# fronts and 14” 115# rears were perfect (I believe I got the length of the rears correct). I switched from 8" 250# fronts and 13" 130# rears. I liked the ride height of the stiffer springs but while it wasn't really a harsh ride, it was a little tiresome, especially on a long trip. I like the softer springs better but they are too long in my opinion.

The workshop manual says the ground clearance should be 6” measured at the closing plate and the 10” springs are too long to get the car down there even with my Protech shocks spring perches  adjusted all the way down. The front is higher than I had it with the 250# springs but not as high as the TCSs came from the factory so it's not terrible. The rear is also higher than when I had the stiffer but shorter springs even though the spring perches are all the way down too. Richard at Banks recommends the rear 1/2" higher than the front. When I had the stiffer springs, I adjusted the front so the closing plate had 6" of ground clearance (per manual). Then I measured the ride height behind the front wheels. I raised the rear so that the ride height measured in front of the rear wheels was 1/2" higher.

I would suggest 8” 150# fronts an 10” 115# rears (assuming you have adjustable spring perches). The reason for the 10” 115#s is because I couldn't find 12” 115#s.

edit: I pretty much rewrote the post to include more information than when I originally tapped it out on my iPad.

[kram350kram]

Thanks BDA. I have coilover shocks. Would you go with a 7" spring for more drop if you had to do over? I want to get the 175-13" tire level with the fender lip. That really only gives a few inches of shock travel though. How much shock travel do you have?

[BDA]

No. if I change my springs again, I will go with my recommendation: 8” 150# fronts and 10” 115# rears (if I could find them, I’d prefer 12” 115#).

I reread my post and it might not be clear. I currently have 10” 160# fronts (Dave Bean didn’t have the 150s I was after) and 14” 115# rears. I like them fine but I’d rather have a lower ride height. I think if you’re used to the stock TCS ride height, you’d like the way my car sits so it’s not terrible but I’d prefer it to be an inch or so lower.

I wouldn’t restrict my search to the usual suspects (DBE, rd ent., Lotus Supplies, etc.). There are several hot rod/racing outfits that sells a pretty wide selection of springs.

[kram350kram]

I had 10" springs and way to tall. As an experiment I cut them down to 6" and like the ride height with the adjuster nut in the middle of the shock, lower control arms level with the ground.  I think 7 or 8 " with helper springs might do it. Merlin Motorsports in the UK seems like the go to place? Thanks

[BDA]

There are lots of places on this side of the pond. Do a DuckDuckGo (or Google if you must) search and you’ll find at least a dozen places.

[EuropaTC]

Strangely enough, I have 250lb fronts, 8 working coils and a free length of 228mm, so roughly 10". I like it but then again I've always liked firmly damped/sprung cars so maybe that's just me.

On adjustable Protech dampers with the perch right down the car looked almost like an S2 it was so low. In the end I raised it slightly but it's still a touch lower than standard all round. No recent pics, the one shown was a few years ago when I'd stopped tinkering with it.

When I did mine I did a lot of research into what others had done, what Lotus did originally and where I thought I wanted to be.  One thing that I ended up with was to retain the same F/R suspension frequencies to what Lotus had originally. So what I'm saying is that I wouldn't change the front without looking at the rear as well.

A small change, say 10-20lbs, won't be drastic because the coil works at an angle and so reduces the effective spring rate. It's different at the rear because it's practically vertical so you get the full rate as the effective rate. But if you retain the OEM rears (75lb) and go to 150lb front you'll be altering the F/R balance quite a bit.

On my dimensions/calcs, OEM frequencies were 1.2F, 1.4R, a ratio of 0.92. With 150/75 it's 1.4F, 1.4R, an equal match. My original thoughts were for an equal ratio from what I read on the internet, but I didn't like the car so ended up with 250/160 which roughly translates to the 0.92 ratio Lotus had.

You might like the equal F/R frequencies but I didn't so all I'd say is that if you're going to change the front, be prepared to change the rears as well.

Brian

[Kendo]

I thought suspension frequencies as a suspension design input came along well after the Europa. Anyone know? That said, even if true, Lotus were wizards at suspensions, so they might have arrived at the same answer by other means.

[kram350kram]

Europa TC, what diameter are the front tires on your car? I will be changing the rear shocks too, so thanks for the tech info.

[EuropaTC]

Europa TC, what diameter are the front tires on your car? I will be changing the rear shocks too, so thanks for the tech info.

They are 15" wheels but with 195/50 tyres, which comes out to pretty much the same as the old 13" 175/70's that I had previously.

[EuropaTC]

I thought suspension frequencies as a suspension design input came along well after the Europa. Anyone know? That said, even if true, Lotus were wizards at suspensions, so they might have arrived at the same answer by other means.

My first brush with suspension calcs was around ''82/'83 with a book by Alan Staniforth, designer of the Terrapin cars, and it was his book got me into the maths of the subject. He was around the same age as Chapman and competitive in the same 50s/60s/70s era and beyond. He both calculated and used his "string computer" for design so it wouldn't surprise me to find Lotus did similar, if only for their race cars ?

But given the Europa was originally a cheap replacement for the 7, I'd suspect it was mostly "seat of pants" development with a collection of springs and dampers until they got a decent set-up.

The big plus for us today is that we can put numbers on what they did back then and see how it relates to what we do today. For example, I read a lot about the idea that you should have identical F/R rates and tried it, but for me it didn't work. Reverting to similar ratios that Lotus used makes the car feel better for me, just stiffer all round. I doubt I drive significantly faster, I just prefer the ride.

[Kendo]

Knowing nothing about suspension frequency, I went Googling. They mention one approach being a "flat ride" with rear stiffer than front. Front hits the bump sooner, so a "flat ride" makes both front and rear settle at about the same time. Maybe that's what you were feeling with the "seat of pants" indicator.

[BDA]

I thought I'd show the difference in the way my car stands 250/130 vs 160/115.

The first picture is when the car had 8" 250# front springs and 13" 130# rear springs. The spring perches were raised a very small amount (on the order of 1/4" to 3/8") front and rear, I don't remember precisely.

The second picture is as the car is now with 10" 160# front springs and 14" 115# rear springs. The spring perches are at the bottom of all the shocks.