OK - I've done a search, couldn't find what I want to know, so it's time for a new-old question...

I used to run 275 lb/in springs at the front, 180 lb/in at the back. Was too harsh, and still the back end bottomed-out on bad bumps. The back end was sorted by fitting correct-length shocks with 150 lb/in springs, and now it's a lot smoother and rarely bottoms-out.

Trouble is, when the car goes over a bump it feels like the front end is bouncing up due to the high spring rate, leaving the back spring with too much to do when it reaches the same bump... it's as if the front isn't taking its share of the bump absorption and throwing the whole car onto the back. It's hard to explain, but that's what it feels like!

For info, my front suspension has the front shocks/springs about 5 to 10 degrees more upright than the book design.

I'd be interested to know what spring rates other people are using front and back - preferably in a similar car - CEC with an all-up weight of around 600kg, entirely for road use. My gut feeling is that I should go down to 200 - 220 lb/in springs, but I'm open for advice...

All info gratefully received...

David

David, have you tried playing with the damper settings? I thought I had a few problems with my spring rates and stiffened up the dampers close to max and the problems went away.

Might be worth a try.

275 is very hard. Some good info in this thread (mainly about BEC's).
http://www.locostbuilders.co.uk/viewthread.php?tid=64364

For a CEC you might want to try 240's first.

If I set my dampers to max I tend to lose my fillings!

Generally I run at the minimum damping setting, or maybe a click or two up - I get no oscillation, just the reaction to the bump, then immediately steady.

David,

Try this calculator

http://www.racingaspirations.com/wheelfrequency.php

It is based on the calcultions found in Alan Stanifirths suspension design books.

I have used it with good results in the past.

I would aim for a 115 cpm for a sporty but reasonable ride on the road.

You can weigh your unsprung weight by jacking the car up, undoing the top shock mount and resting the wheel on the missus's bathroom scales.

Doug.

iank - thanks, it sort-of reinforces my view about the front being over-sprung. The x-flow isn't a hugely heavy engine, although double a bike engine's weight, so the BEC experiences are probably not too far wrong.

Doug - that's an interesting link! I'll have to have a play over the next few days. Perhaps I'll go out and buy a super-cheap set of scales though...

[Edited on 1/8/07 by David Jenkins]

I have 320 in front, 140 in rear. Also a 5/8" front sway bar.

BUT, the wheel rates are around 140 - a better number to compare. My front track is as wide as the rear (longer than book arms), so I need higher spring rates to compensate.

G

Anyone able to tell me the formula for working out the effective rate for a spring mounted at an angle?

I've just been out to measure mine - it's at 75 degrees (i.e. 15 degrees from vertical).

Stick your old rear ones on the front.

I was recommended 180 r and 220 f

If yours are more upright 180 might be enough. It would cost nothing to try.

Atb
Paul G

Unfortunately I sold them to someone a while ago, during The Great Garage Clear-out...

quote:

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Originally posted by David Jenkins
Anyone able to tell me the formula for working out the effective rate for a spring mounted at an angle?

I've just been out to measure mine - it's at 75 degrees (i.e. 15 degrees from vertical).

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Does this mean you want the corner weight gauge again David? Can take to Pauls on the 11th if you do.

Quite possibly... I'll let you know before the day!

Sounds more like dampers to me. The instantaneous loads on hitting a bump should easily overpower a spring unless it is massively too heavy. A damper set too stiff however would restrict the spring and make it feel too stiff.

I could be wrong but worth considering!!

The front dampers are currently set to minimum damping!

Hi am i right in thinking you car is a live axle rear.

If so you are right in the ballpark with 150lb on the rear for a car with driver and passenger. At the front given the angle of the front dampers again 275 is in the ball park and will not be giving extreme problems as TangoMan said above ^^ .

You need to be looking at the dampers or stiff wishbones . Set front dampers to min and see if you can move or bounce the front of the car manuley by hand. If no or little movment then try slackening all front suspesion pivot bolts and try moving. It should be possible to bounce the car manuely by hand whith dampers set to min if not possible dampers are valved incorectly for your application.

cheers matt

Thanks Matt. I will admit that my front suspension doesn't have much bounce-ability, so I'll investigate the things you've suggested. I don't think that I can afford new dampers just now though!

In the mean time, I've had a play with that wheel frequency calculations page that Doug suggested, with the following results:

Spring rate(actual rate due to slope) > cpm value

275 (256) = 124 cpm
200 (186) = 106 cpm
180 (167) = 100 cpm

Advice on that page (from Stanniforth) suggests

60 - 80 cpm Comfortable road car.
80 - 100 cpm Sports road car.
100 - 125 cpm Racing cars without wings or ground effects.

So going on those figures I'd be better off with 180 or 200 springs?

The plot thickens...

cheers to all,
David

[Edited on 2/8/07 by David Jenkins]

Really for Matt's benefit...

Took the nosecone off so I could get a grip of the chassis - plenty of bouceability in the front suspension!

I'm able to bounce it up and down without difficulty, and it stabilises immediately when I stop pushing.

David,

If you remove your springs from your coilovers, you can measure your motion ratios directly, i.e., what vertical movement in the wheel causes a 1" deflection in the shock. This would settle the issue of how your spring rate relates to wheel rate. While you're at it, you can measure your unsprung weight very exactly as well.

Low-mass vehicles are very sensitive to changes in payload, so you'll want to do your prognostications with a half tank of fuel and weights in the seats representing a typical payload. Older Sevens have had very high rear wheel frequencies, up to 150 cpm, I think mainly to keep the rear end from bottoming out with the car loaded to the gunwhales.

If you lighten up your front springs a great deal, you may notice a tendency to oversteer. I did some calculations that convinced me that the Locost would benefit from a front sway bar (like the Lotus 7 had). Builders who have added bars typically report good success with them.

FWIW, I've always been intrigued by the service offered by these fellows:

http://www.racing-car-technology.com.au/

Pete

David, as a point of reference, my Vauxhall XE car is 560kg total weight, within a spit of 50-50 distribution, so prob very similar to yours weight wise.

I run 140lb springs on the back and 275lb on the front, and I think they are not far off. Maybe a touch solid on the front but I know I have some bind in the bushes so I won't change anything till that is sorted this winter.

I also, like you, run my dampers (protech std off the shelf ones from MNR) pretty much on minimum. I run 3 clicks on the front and 1 click on the back.

Any stiffer is far too overdamped!

PS I am with the previous poster on sway (antiroll) bars too.

I think mine could benefit from a very light one on the front, which then might take some of the antiroll work off the main springs and let me run lighter ones without leaning onto a front corner so hard.

(having said that, and bear in mind I am no slouch behind the wheel, getting to the limit of grip on my Yoko A048R's on the road is not something that happens often, I think you need a seat with shoulder supports for that malarkey as its very difficult to keep your upper body in position, and its also unlikely with passengers in the car as they scream (or in my mum's case, get black dots in her eyes and think she is going to pass out) long before that!!!