Another question which I know there was a lot of discussion going on some time ago but lost all teh information.

So I am about to order my shocks, I would like to know teh spring rates that are best suited:

Locost Fireblade, weight should eb around 440kg, suspension geometry very similar to the book, De Dion tube at the end with the springs 90 degree with teh tube.

Road use and the roads here are generally horrible to say the least

Any more information required please let me know

Hi Phil

My car weighs about the same as yours, although Ive now got modified front damper angles (more upright) compared to book so am better off commenting on how the car was before.
Originally I had 300lb on the front and 175lb on the rears, the only reason I had these was because thats what came with the car when I bought it as a part built project. For road use these were too hard and even on the track it was too stiff as it didnt really allow you to use the dampers properly. I then changed the shock positions when I bought some new dampers, and went down to 225lb at the front and 150lb at the rear, although the fronts were equivalent to 275lb with the old geometry so in effect I dropped 25lb each end.
This still wasnt enough so I have now gone to 200/130 which is better but still I think it could go a bit softer and make it better on road and track. Springs arent really hugely expensive (around £60-80 a set) so you can afford to play around a bit but Id probably start off with maybe 225/125 using standard geometry and see how it is.

cheers
Chris

[Edited on 1/1/05 by ChrisGamlin]

Apology for my ignorance but I want to make sure I am understanding it the right way:
When you specify the value of the spring rates e.g. 250lb, that means the force needed to compress the spring at a specified length which I assume would be when the car is on its wheels; so if that is the case what woudl be teh distance (roughly) between the centres of the mounting lugs of the shocks for the front and the back?

Or I am talking nonsense?

the rating is pounds per inch of compression. Hence 200 pound springs will compress one inch when you hang 200 pounds on them. This might equate to the car dropping 0.66 inches if the shocker is running at an angle. Some springs are progressively wound to be rising rate - ie the more you squeeze the harder it gets. Plus you can get different length springs, and they often have adjustable spring holders so that both the length and preload can be adjusted.

hence, you really want to have a fiddle, and also only tack on the mounting brackets whilst you make sure the ride height is correct!

Yes, I do understand your point, I used to design springs about 5 years ago so I do know how rates are effected over chnage in length. However I want to make sure that I purchase the right shocks for my car. The distance between the centres of the brackets is 300mm when the car would be sitting on its wheels (hopefully), and I have enquired fro some shocks from Protech and they offered me a set with 356mm fully extended length and 260mm compressed.
Thus I suppose that I have to specify the length of the spring as well ensuring that at 300mm, the force is the ones specified e.g. 250lbs right?

assuming you buy the 356mm open springs, and you want the final length to be 300, you need two inches of compression (50 odd mm), hence if you have 200 pound springs, you will need 400 pounds on each to compress them as required. i suspect you wont have 800 pounds weight at the front, so it might be an idea to move the mounting brackets apart.

Hi Phil

The spring poundage I quoted was, as Joel said, a strength of lb/inch linear compression. The spring lengths will be totally independent of that, and will need to be chosen based on the damper length and travel. My car currently has 13" (Nitron NTR) dampers on the front and 14" on the back, as opposed to 12" all round (AVOs) which it had originally. To get 14" dampers on the back you need to use the top bolt on the rear trailing arms as the lower damper mount, rather than having the seperate mount about 2" above on the top of the box section though.
It will vary from damper to damper but I think on the Nitrons I have 8" springs on the rear and 7" on the front dampers.

Chris

Thanks mates, I can now specify the right information to the shock manufacturer hopefully!!

Also, I should have said, the fronts now have different mounting points too in order to fit the 13" dampers. The top damper mounts are now on the side of the chassis rail rather than underneath, which makes the dampers more upright (which is a good thing). I didnt really choose the damper lengths, a friend of mine bought the dampers for his BEC Pheonix and then sold em almost brand new for a good price when he sold his car, so it was a case of buying them and seeing how they would fit. As it is though, they have if anything improved things at the frotn end, and at the back moving the lower mounting was easy and is just as strong now as its ever been as its still in double shear.