'87 Caterham spring/damper options

[inchoate]

I'm warming up tie up some loose ends on my car that have been nagging at me for a few years.  I noticed that I've got about 60mm of ground clearance under the sump, which might explain why I keep hitting it on things.  The front springs don't have a lot of adjustment left in them before they become coilbound so I'm thinking it might be time for a replacement/upgrade.  It seems like there might be plenty of resources over in England but I'd rather shop on this side of the pond for logistical reasons.  Any thoughts about who I should call or what I should be looking at?  FWIW this is strictly a road car so I'm looking for a suspension for spirited  driving on roads in less than stellar condition.

[IamScotticus]

I get about 4" with taller tyres.

using 185/ 70 R13.

Is your sump even with the bell housing?  or lower?

 

you might have a need for dampers with more adjustment.

can you put up pictures of the dampers?

[MV8]

Ditto on damper adjustment. If you need springs, I'd remove a spring and measure the free (loose) length, inside diameter, and using a dial caliper (available on ebay or harbor freight for about $15), the wire thickness to determine the spring rate. I expect you will find the springs to be about 10 inches x 2.5 inch ID. Maybe there will be some numbers on the springs you can post also. Assuming the rear is still satisfactory, I'd look at a slightly higher rate or the same rate if the old springs are just sagging and corroded.

There are many sources for springs here.  Probably about $100/pair, shipped from jegs, speedway motors, summit, even ebay, etc. I've used QA1 springs a few times.  

[IamScotticus]

dont neglect to loosen all the A arm suspension bushes any time you raise off the ground, especially for damper service.

If your car hasn't had them refreshed since new, they may be a factor in not maintaining the proper stance.  

Edited May 5, 2022 by IamScotticus

[inchoate]

I'll post some pictures when I get a chance, but I work at a University and were in the middle of finals at the moment.

 

Car has a wet sump that is below the bellhousing. I'm looking into some options on that front but I'll probably keep the wet sump for the time being.

 

The car came with another set of springs but honestly I've never looked too closely at them.  I'm a little wary of putting new/stiffer springs on 35 year old dampers.

 

I was able to find the Caterham part numbers for the original dampers and springs but it turns out that's not all that useful.

Edited May 5, 2022 by inchoate

[MV8]

In that case, with the spring off the damper, measure the fully extended and compressed lengths along with the inside diameter of the eye bushings or bolt shank od. Pics might help too.

Here is one option that may fit your hardware and are around $200 each. I expect your rate to be around 200#/in.

https://www.kineticvehicles.com/Shocks.html

[IamScotticus]

I ask because my wet sump pan  has a bottom even with the BH and has a upward taper to the front.  it is a pretty good skid.

It came on the engine as I bought it. I don't know if it was supplied by Caterham or if it was a custom fabrication, but its a great middle option.  Im sure some oil slapping is going on in there.

If you could find one...

[inchoate]

Here's the front suspension loaded and unloaded.

 

I've got some free time next week so maybe I'll pull a damper off and see what's going on.

[IamScotticus]

I count 15 coils on your springs. 

I have two sets that are 7-8 coils.

I think your springs are an older standard.  there are better options now.

You need to know your front end weight and go spring shopping. 

The pic of my car is loaded.

 

Edited May 6, 2022 by IamScotticus

[MV8]

I made a rate calculator in excel. Provide your wire thickness (0.xxx") and inside diameter of the spring (2.xx"). You need access to one of these for the wire thickness or a really nice person with one in-hand:

https://www.harborfreight.com/6-in-dial-caliper-63730.html

For an educated guess on a few variables and based on your pic I think the rate at ride height (minus driver) is about 130#/inch. I'd fit 20-30#/in more than what is currently installed, whatever that may be.

[GTIspirit]

Slight thread highjack.  What are the typical spring shock specs for a Lotus 7 / Caterham 7 / Birkin 7?

 

What spring length, diameter, and rate?  (2.5" was mentioned above, but Tom at Birkin told me 1.9" ID, which I assume is really 1.88".)

Here's a good overview of all the different combinations available, Eibach Race Springs

 

Then for shocks, what eyelet length and stroke?  Seems like there should be plenty of generic formula car, or circle track options available which should work.   (Circle track is great cost effective performance parts option )

[inchoate]

I dug the extra springs out of the parts disaster.  No clue what they might be but I might just go ahead and chuck them on and see where they get me.

[MV8]

2 hours ago, GTIspirit said:

Slight thread highjack.  What are the typical spring shock specs for a Lotus 7 / Caterham 7 / Birkin 7?

 

What spring length, diameter, and rate?  (2.5" was mentioned above, but Tom at Birkin told me 1.9" ID, which I assume is really 1.88".)

Here's a good overview of all the different combinations available, Eibach Race Springs

 

Then for shocks, what eyelet length and stroke?  Seems like there should be plenty of generic formula car, or circle track options available which should work.   (Circle track is great cost effective performance parts option )

I don't know what is typical on an actual caterham. Easy enough to measure ride height length and how much shaft is left for compression. Should be less than 3 inches needed to add for full extension length due to the motion ratio.

The spring id depends on the damper and seats. Generally not so tight that a 2.5 id won't fit. Triumph springs are bit larger in id but 2.5 id will still fit. I think a typical circle track racing damper has a 5/8 eye but I expect a lotus would have something close to 7/16. A big eye can be adapted with a thicker wall inner sleeve.

[MV8]

35 minutes ago, inchoate said:

I dug the extra springs out of the parts disaster.  No clue what they might be but I might just go ahead and chuck them on and see where they get me.

Using one of the black springs, wrap a string around the wire, use a pencil to place a mark across the string. Tell us the distance between the marks on the string. Measure the id and length with a tape measure or ruler.

[inchoate]

16 minutes ago, MV8 said:

Using one of the black springs, wrap a string around the wire, use a pencil to place a mark across the string. Tell us the distance between the marks on the string. Measure the id and length with a tape measure or ruler.

 

 

Are you asking for a wire diameter?  I've got appropriate tools to measure just about anything.  Don't have a spring dyno though, so I can't give you rates.

 

I'll post up some numbers when I'm home this evening.

[BirkinBernie]

On 5/6/2022 at 5:46 AM, GTIspirit said:

Slight thread highjack.  What are the typical spring shock specs for a Lotus 7 / Caterham 7 / Birkin 7?

 

What spring length, diameter, and rate?  (2.5" was mentioned above, but Tom at Birkin told me 1.9" ID, which I assume is really 1.88".)

Here's a good overview of all the different combinations available, Eibach Race Springs

 

Then for shocks, what eyelet length and stroke?  Seems like there should be plenty of generic formula car, or circle track options available which should work.   (Circle track is great cost effective performance parts option )

Here is a factory drawing of the shock length specs for a solid axle Birkin like mine, manufactured in 2000.  There were many different spring/shock combinations used.  My car has AVO adjustable shocks with 2 1/4 inch ID springs with 7 inch free length.  I bought these from the importer when I built the car (long before Tom's involvement).  The factory supplied shocks and springs were smaller diameter.  TBH, the spring rates and lengths are really not relevant to a Caterham due to differences in the suspension geometry between the cars (spring rate vs wheel rate).  FWIW, my car has Eibach springs, 350 lb/in the front, and 150 lb/in in the rear.  (EDIT: I just discovered that I gave the rear spring rate incorrectly as 200 lb/in.  They are actually 150 lb/in)

Edited May 27, 2022 by BirkinBernie

[MV8]

5 hours ago, inchoate said:

 

Are you asking for a wire diameter?  I've got appropriate tools to measure just about anything.  Don't have a spring dyno though, so I can't give you rates.

 

I'll post up some numbers when I'm home this evening.

Yep. Provide the black spring wire thickness and inside diameter. I will tell you the approximate rate as it sits on the table. The installed rate will be a little higher.

[inchoate]

OK, I threw the Mitutoyos at that spring and here's what I got:

 

8 Coils

Free length:  9.656" or 245mm

Wire diameter .356" which is super close to 9mm

ID: 1.92" at the unmarked end and 1.935" at the end marked with red paint so 48-49mm

OD: a somewhat predictable 2.612 or 66.7mm

 

I arrived at these measurements by using a set of digital calipers and averaging out measurements from several locations.  They're probably not perfect but they should suffice for ballpark numbers. 

Edited May 6, 2022 by inchoate

[MV8]

I get 236#/in. It is interesting that had it been coiled to a 2.5 id (which would be heavier due to being more wire but would not twist as much), the rate would be only 121#/in.

Measure the wire on the car. You should be able to do so without taking anything apart if you let the suspension droop for a bigger gap between the coils.

Edited May 6, 2022 by MV8

[inchoate]

The spring on the car:

 

13 coils

 

a remarkably consistent .357" wire diameter.

2.61" OD

 

Can't give you a free length at the moment but I'll probably pull the dampers next week.