Spring Weight, How To Find It?

[Veero 1]

I have a set of Bilstein tarmac coilovers on the car on eccentric top mounts. Steering is heavy and the ride on the road is crashy to say the least. I had intended to use the car as more of a track day car but times change so it has become my weekend runabout which will see occasional track use. The current front suspension is far too firm. They were bought off Rob Thomson a fair while ago and I think he had planned on using them on some track weapon so I think the springs are quite stiff.

 

Is there any way I can determine the spring weight somehow? As far as I know there were no written numbers on the coils that I saw before I fitted them.

 

What sort of spring weight would be good for a predominantly road use car with the odd track day for the Bilsteins?

 

Or should I sell up and get a more conventional strut/top mount Bilstein Sprint/Street??line set up for example??

 

Opinions and ideas if you please...

 

Veero

[pug_ham 241 3 Cars]

It might take a bit of reading to find but iirc Adi posted the method to calculate the spring poundage in the pinned topic Suspension And how it works.

 

Graham.

[pete1986 2]

175lb spring seem to be the most popular for road use. Mine are 9" 200lb, I find them a bit crashy on the road so probaly going to change them at some point. Hope that helps

[Veero 1]

Cheers guys. Graham, I had a quick read but couldn't see the way to calculate poundage. Some amazing info there though, way too much to digest in one go.

 

If car springs are the same as springs used on mountain bike shocks then the poundage refers to the force required to compress the spring by an inch? I'm sure I could whip one off the car and if I weigh myself and stand on it could at least come up with a vague appriximation. So if I'm 80 kilos = 172 lbs and I stand on it and it compresses 3/4 inch then that's a 229lb spring (for example).

 

I would like to try some softer ones on the road car before I say categorically I want to sell the coilovers. Any chance someone might fancy a swap?

[benlilly 0]

Wikipedia had this, might be a little easier than balancing (wobbling?)on a spring while a mate measures the length:

 

"The spring rate of a coil spring may be calculated by a simple algebraic equation or it may be measured in a spring testing machine. The spring constant k can be calculated as follows:

 

k=(d^4 *E)/(8*N*D^3)

 

where d is the wire diameter, E is the spring's elastic modulus (e.g., about 30,000,000 lbf/in² or 207 GPa for steel), and N is the number of wraps and D is the diameter of the coil."

 

Or full page here

Wikipedia spring rate page

[pug_ham 241 3 Cars]

Cheers guys. Graham, I had a quick read but couldn't see the way to calculate poundage. Some amazing info there though, way too much to digest in one go.

I've just been through the topic & I couldn't find the post so it must've been in a diffeent topic.

 

Anyway it was similar to what Ben has posted above but iirc you only count the complete number of coils (not including the ones by the strut platforms or top mounts) & the wire thickness.

 

Graham.

[fangio 1]

I've been delving into this lately and experimenting with different springs and dampers.

 

The formula in reality doesn't work that well because the Modulus of Spring Steel varies so much - although it gives a rough guide +/- around 10%.

 

Another part of the problem is that 'the two ends' of the spring are flattened and thus the coils are closer and will be a softer rate.

 

I measured a std 205GTi and a 306GTi spring, which are the same rate BTW. The formula gave 93lbs/in, but, in reality are near to 85.

The 205 spring is 340mm and the 306 is 400mm in length - the extra 60mm is to take account of the extra weight and therefore has more preload dialled in.

 

I've currently got Eibach 7025-120 for a 206 which are 148lbs/in, which I've tried with Challenge Bilsteins (too hard for the road for me) and now with std 306GTi dampers, which is a lot better. I'm gonna give them a go for a while but my back-up plan is to have some custom ones made by 'Faulkners' at the std 205/306 rate but about 370mm length, which accounts for the extra weight of the '6' engine and 'box on a 205, to give a std ride height. They have quoted me £115 for the pair.

 

I have found that you cannot go by other peoples opinions on suspension (for the road) - it is a very personal thing.

 

Hope that helps.

 

Fangio.

[pug_ham 241 3 Cars]

I measured a std 205GTi and a 306GTi spring, which are the same rate BTW. The formula gave 93lbs/in, but, in reality are near to 85.

Interesting, Adi does mention the 205 spring rate to be something like 95lb/in in the topic I linked to above.

 

Shame I can't find the post Adi made about calculating the spring rates because like you say it takes more into account than just the bits mentioned so far.

 

I'm sure he did say that the very ends of the springs aren't included in the calculation just the number of complete active coils between these.

 

I wouldn't take anything from Wikipedia as gospel either without any proper back up, its often been written by internet mechanics.

 

This spring rate calculator might help though.

FORMULA:

 

spring rate =

modulus of spring steel X wire diameter4

--------------------------------------------------------------------

8 X number of active coils X mean coil diameter3

 

modulus of spring steel = 11,250,000 pounds/inch2 = 78,500 newtons/millimeter2

Graham.

[niklas 1]

I'm sure he did say that the very ends of the springs aren't included in the calculation just the number of complete active coils between these.

 

That's correct!

[fangio 1]

Yes. That's right. You discount 1/2 a coil at each end effectively, which is the amount of wind it takes before you get to the 'main' part of the spring.

 

From the formula and available on-line calculators you can find the 'main' spring rate - in the case of 205/306GTi around 93lbs/in.

 

In order to obtain the desired ride height / wishbone angle and therefore the required amount of preload you need to take into account these half coils at each end, which are NOT at the same rate as the main part, but progressively a lower rate (averaging approx. 50% of the main rate).

 

I obtained the weight of the front axle on a weighbridge and discounted the unsprung weight at the front of the car and halved this to obtain a corner weight.

Using a 309GTi spring (310mm free length and by formula 115lbs/in ) fitted to the car I then measured the length of the coil at static deflection ( just sitting there ).

I subtracted this from the free length of the spring to arrive at the amount of spring compression.

 

The corner sprung weight divided by the amount of spring compression gave me the average rate, which is 107lbs/in, some 7.5% lower.

 

Once preloaded onto a damper the lower rated part of the spring can be ignored and the 'main' body of the spring is acting, which would be near to the calculated/formula rate, in this case about 115lbs/in.

 

I am not setting myself up as an expert here - I have researched as best I can and measured and tested my findings in the real world.

 

I am more than ready to be corrected/proved wrong and generally shot down in flames as I'm sure some will do, but going by the general level of knowledge on this subject found in various forums whilst researching this I think I have a reasonable grasp.

 

Puts tin helmet on and hides in trench.

[Veero 1]

Errm getting a wee bit technical. I was just wondering what a track day/tarmac race only spring rate might be (as I suspect these are) and what might be a suitable spring rate would be for fast road, sacraficing a little comfort, but also good handling on track every now and then.

 

So apart from a spring tester (which I don't have access to) is there a practical way I can approximate my spring weight? I'll go measure up tomorrow and see if I can plug the numbers in to the equation there Graham.

 

If what you are saying is right Fangio then surely a 175lb rate spring would be faaaaaar to stiff for the road?

[fangio 1]

Hi Veero,

 

Sorry but, again there is no definitive answer - what to one person feels to be harsh another will say is fine.

 

All I can tell you is that the std 205/306 GTis run about 93lbs/in spring with a stronger than otherwise damper to 'sport' it up a bit.

 

I know people running 200lbs/in springs who find it bearable, but would admit when not driving spiritedly, i.e. poodling around town it can become tiresome. And that's with the dampers turned right down.

 

That's why I thought I'd try about midway and go for 148lbs/in, which I am getting used to and seems, at the moment, to be a good compromise, with the 306 dampers. Whereas the Challenge dampers made them superb at speed over most surfaces but, were hell for me around town.

 

Put it this way, if I stuck my Challenge dampers on and went trackdaying I think that would still give me a reasonable times.

 

Even with 148lbs/in springs I'm noticing 'some' understeer so you should really uprate the rear as well, which gets expensive with TB's at £295, you could possibly get away with harder rear dampers to counter the understeer though.

 

I know how you feel - I've been pulling my hair out. Keep as near to std for a road car is MY advice.

 

Std springs with even sportier than std GTi dampers would give even more control and still be comfortable. I did this on my old 1.9 with KYB Ultra SRs and it was great.

 

Still didn't fully answer the question re ascertaining your spring rate.

 

You need to measure the length of the spring when it's sitting on level ground both sides and take the average.

 

Then measure or, find out the free length of the spring.

 

The difference you divide into the corner weight: std 1.9 is about 540lbs. The resulting figure will be your lb/in.

 

Hope that helps. Paul.

Edited January 22, 2009 by fangio

[Veero 1]

I'm half tempted to whip the coilovers off and put my standard shocks on tbh until I can maybe try someones car with standard springs but uprated dampers. I thought it would be a blast having such a stiff set up for the road but in real terms the roads near me in Gloucester are sheeiiiiiiite. Went for an hour pootle the other day and I spent most of it on pothole lookout and avoidance or hearing the horrible clunk of stiff suspension meeting lumps and bumps. Not fun. Part of it is down to the heavy steering too and the eccentric top mounts which of course don't have rubber pads like standard mounts.

 

I agree about personal opinion though. I think I want something somewhere in between standard and what I currently have. Perhaps the tarmac inserts are overdamped for my purposes.

 

306 GTi dampers presumably run on standard top mounts or could they be used with eccentrics? I'd quite like to go for a slight, maybe 20-25mm drop to match the rear and just lower the COG slightly over standard ride height without compromising too much on ride comfort. The lively fun car I remember is starting to turn into a chore to drive. Who wants to buy a set of Billy tarmac coilovers ??

[RossD 43]

I'm not sure of the Eibach 7001 spring rate but I have those along with the Challenge dampers all round, ride is firm but very comfortable..... It was crashy and horrible to start with until I found that the strut top nuts were working loose, tightening those transformed the car!

I think Sandy may have measured the spring rate of these somewhere and written it in the forum.

[Veero 1]

Yeah I had that to start with but they are tight now. Still crashy, maybe it's just the roads round 'ere are proper crap.

 

If I was to go for some 306 GTi struts would it be a case of buying a new pair for the Gti6 and mating them to 205 top mounts, spring pans and bearings?

[pug_ham 241 3 Cars]

306 GTi dampers presumably run on standard top mounts or could they be used with eccentrics?

I don't think there is any point fitting eccentric mounts to otherwise standard dampers. To start with the top of the strut piston / rod would need machining to get enough length for it to fit through the mount but even then with standard sized springs there isn't any space for much adjustment.

 

If I was to go for some 306 GTi struts would it be a case of buying a new pair for the Gti6 and mating them to 205 top mounts, spring pans and bearings?

Yes, 306 Gti-6 dampers (new or s/h) with the 205 top mounts, spring pan, top rubber & bearings fit together with no problem afaik.

 

Graham.

[SPGTi 2]

Easy way to measure the springrate is to use a pillar drill and a set of bathroom scales. Use the pillar drill to compress the spring, see the force required on the scales and calculate.

 

Steve

[Veero 1]

Unfortunately I have neither...

 

Yeah Graham after I left that reply I realised that the angle would be all wrong with the standard struts.

 

Right I might just get a new set of struts and put them on my standard mounts. Is fitting drop springs ok? With the car jacked up do the springs come off the pans?

 

Any ideas what a set of Bilstein Tarmac coilovers on Compbrake eccentric top mounts (2000 miles use) be worth?

[205Rallee 59]

Unfortunately I have neither...

 

Yeah Graham after I left that reply I realised that the angle would be all wrong with the standard struts.

 

Right I might just get a new set of struts and put them on my standard mounts. Is fitting drop springs ok? With the car jacked up do the springs come off the pans?

 

Any ideas what a set of Bilstein Tarmac coilovers on Compbrake eccentric top mounts (2000 miles use) be worth?

 

Don't ditch the bilsteins! If they are the 300/200 inserts with the long top spindle, stick a set of Skip Browns dual rate tarmac springs and the proper fixed PTS spherical joint top mounts and they should give a good initial ride, with very smooth steering (the PTS top mount) and no crashiness at all. Once you've got the right set up, along with a properly sorted rear end, you'd never want to go back to a standard set up.

[Veero 1]

Don't ditch the bilsteins! If they are the 300/200 inserts with the long top spindle, stick a set of Skip Browns dual rate tarmac springs and the proper fixed PTS spherical joint top mounts and they should give a good initial ride, with very smooth steering (the PTS top mount) and no crashiness at all. Once you've got the right set up, along with a properly sorted rear end, you'd never want to go back to a standard set up.

 

Ok where could I get such items? Also I don't remember seing any markings on the coilovers or inserts at all, how can I tell that they might be the ones you've described there?

[welshpug 1,626]

both you would get from Skip Brown Cars, they have a website now!!!

 

http://www.skipbrowncars.com/

[Veero 1]

both you would get from Skip Brown Cars, they have a website now!!!

 

http://www.skipbrowncars.com/

 

Lol such a useful website... I guess it will have to wait until I can phone them in business hours then...

[205Rallee 59]

Lol such a useful website... I guess it will have to wait until I can phone them in business hours then...

 

 

Yes there web site is ...not a site more a page with a telephone number. They are very helpful and that's where I got my mounts and bilsteins from originally. Be prepared the mounts might be expensive, but the skf bearings in mine have lasted ten years with nothing but a good covering of grease and er that's it.

 

Take their advice on the spring rate and you shouln't go wrong. Mine have a splodges of yellow and red paint on them if that helps...

 

Lol such a useful website... I guess it will have to wait until I can phone them in business hours then...

 

 

Yes there web site is ...not a site more a page with a telephone number. They are very helpful and that's where I got my mounts and bilsteins from originally. Be prepared the mounts might be expensive, but the skf bearings in mine have lasted ten years with nothing but a good covering of grease and er that's it.

 

Take their advice on the spring rate and you shouln't go wrong. Mine have a splodges of yellow and red paint on them if that helps...

[Rom 0]

Ive not run Billies myself, but on GAZ i found 225 lb too hard for the majority of my road use.

I still have the springs, your more than welcome to try them, as they may be softer than tarmac spec ones ? There 2.25 in ID.

 

From what i read, billies are very good coilovers. In general thier reputation is always good, but in particular, for the 205 they seem to be 'the' choice. There are different kinds / inserts etc i think though.

There something im looking to change to at some point.

 

Oh i should add...im on 180 lb springs now. And there still pretty hard. If the calculations are right, thats double standard stiffness.

Track only boys run anything from 225 upwards i think.

Edited January 25, 2009 by Rom

[Veero 1]

Sorry 205Rallee I wasn't down talking SBC, just the website has no info on. Pretty busy this week too, dunno when I will be able to call them.

 

I would like to keep the billies if I can sort the spring rate out and make it more comfortable for the road but not if it's going to cost me hundreds for springs and new top mounts. Getting a Mazda Bongo in 2 weeks and that has taken all my money