Spring rates what do you have

[KiwiBirkin]

Hi guys im running 175lbs springs on the rear of my Birkin and 250lbs on the front, the car weighs 1278lbs with a full tank of gas and 1477lbs with me in it. I have the shocks set to 3 clicks from full soft on the rear and it still feels a bit hard too me over bumps so i was just wondering what others were running in there Caterhams/Birkins/Westfields (all similar sized cars)

 

I was considering dropping the rear to 125lbs or 150lbs springs and then if it was too soft making the rider harder by using the clicks on the GAZ shocks as there is 30 levels of adjustment.

 

From what I have read the original S3 lotus only had 90lbs springs on the rear.

 

Cheers Stephen

[bsimon]

Caterham typically used either 150 or 170 fronts and 180 rears. After the mid 90's, the rear dampers were lengthened to use a 90/180 progressive rate spring.

 

My car, of similar weight to yours, had 170s on the front and 180s on the rear. There were some 250 and 300 springs available for the VX race cars, but I've heard they are a bit harsh for road use.

 

My car always felt a bit wishy-washy so I changed the fronts to 225s and retained the original 180s on the rear. This mixture, along with the fat VX front ARB, seems rather planted without rattling your teeth. I am using a rear ARB as well.

 

Not very scientific, but I was happy with the results.

[pi7ot]

I'm using US market Birkin rates of 300F 150R.

The ROW Birkins seem to run softer rates.

There was a bit of debate regarding the Higher front rates on US cars being responsible for bending the front lower wishbones. In the end though suspension travel, or relative lack of was a contributing factor.

 

Fiddling with damper settings definitely affects the handling balance, as does a 2 psi change in tyre pressures.

 

 

m

[bsimon]

300 does seem a bit harsh, however there may be differences in the spring inclination geometry between Birkin and Caterham that makes the higher rate necessary.

 

Adjustable dampers would be nice. They're positioned relatively high on my short list of upgrades.

[SkinnyG]

My Locost is 1275 wet, with a solid axle. I calculated my spring rates to 2.4cps suspension frequency, which came out as 320lbs front, 140lbs rear. It's FIRM, but not punishing, and handles REALLY well.

 

You could send me some numbers, and I could crunch them for you.

[11Budlite]

My 2001 Birkin originally came with 300# front and 150# rear. When I installed the rear trailing arm kit with heim joints I went with 175# rears on the recommendation of Woody Harris. I find the rear suspension to be much more compliant even with the stiffer springs. I also have my shocks set on the soft side with the ride height set at about 3 1/2" clearance to the oil pan due to some of the rough roads I drive on. Knock on wood, I haven't bottomed out in years!

[slomove]

I changed the stock 250 lbs springs on my Birkin to 325 lbs long time ago. Much better handling and no comfort disadvantage. I remember that Woody Harris had 400 lbs front springs on his Birkinsport race cars.

 

I would not necessarily compare with Sevens of other brands. Different suspension geometries (shock/spring angle and connection point leverage) may require different spring rates.

 

Gert

[KiwiBirkin]

Cheers for the feed back guys im going to try the 125lbs springs as I have them on order now and they are on NZ $75 a pair so if there too soft i will try 150lbs next.

 

I talked to Fraser Cars this morning which is a NZ manufacture of Lotus 7s and they are the same in dimensions as the Birkin and have a very similar geometry to the Birkin and they recommend 110lbs rears and 225lbs fronts for all their road going cars with a small amount of track use.

 

I then got my friend with a Fraser to take me for a ride this afternoon and his car was much nice in the rear end at soaking up the bumps where as mine is a little bit harsher and feels like it jumps over some bumps, he has 125lbs rear/225lbs front and Gaz shocks set to 12 clicks rear and 15 clicks front the one thing I did really notice was how his car squatted in the rear when he planted his foot down on the GO pedal.

 

Any way I will try the springs and see how it goes.

[KiwiBirkin]

Well after replacing my rear springs from the 175lbs that I ordered with my new shocks to 150lbs springs I have notice a world of difference in the way the car handles the bumps on the roads and also the ride quality.

 

I now have about 10 clicks on the rear GAZ shocks after some experimenting with my wife & me in the car, we are now getting a really nice ride on the NZ roads which are far from smooth and also the car seems to be handling the corners better, I did notice a small bit of roll in the rear until I put some more clicks on the damper but now its really handling well and the ride quality is fantastic and smooth it such a massive improvement im really pleased.