Lotus Elan

Elan springs rear-front - different lengths & wire diameter

PostPost by: TurbineHeli » Sat Dec 15, 2012 12:31 am

Hi guys,

My understanding from many years ago is that driver side springs as fitted at the factory were always 1/2" longer to accommodate the weight of the driver.
Perhaps one of you can confirm?

Thanks,

AMA
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PostPost by: au-yt » Thu Oct 02, 2014 6:27 am

Rather than start a new discussion best to pick up where this left off.
Along with changing the 40 year old lotocones as discussed else where, I am changing the quite rusty rear springs I couldn't live with the saggy R/H side. I measured up the old and they were close to the manual dimensions with the Right hand side being 1/4 lower and softer.
I ordered new springs and when they arrived they were same wire, same full coils but an inch longer. :?: what the.
I enquired as to reason and was told that they were made to the factory last revision of the manual.
I checked the rates using drill press with adjustable bed, bathroom scales and ruler
and as you would imagine are stiffer. which is no issue given there age.

After all this babble what specs does every one have in the manuals that are out there?
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PostPost by: rgh0 » Thu Oct 02, 2014 7:54 am

I don't know of any revision to rear springs - specs in my manual 8.7 coils, wire dia 0.4 inch, length free 14.71 inches, rate 67.5 lb/inch.

Front springs had an optional longer spring 16.75 inches versus standard 16.08 inches I think this was around head light height in some markets. other front spring specs in my manual 19.6 coils, 0.324 inch wire dia, rate 75 lb/inch

Spring free length or age does not affect stiffness, just number of coils, wire diameter and spring diameter. The actual spring steel bulk modulus has an affect but this varies only slightly for different types of steel.

Where did you get your springs from?

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PostPost by: au-yt » Thu Oct 02, 2014 9:32 am

Hi Rohan
I got them from Steve at Lotus Marques in Melbourne. He sent me this spec info which he says is last revision to the spec. Humm..

Rear Spring Specifications Elan (standard spring)
Lotus part number A026D0006Z (see section ?D? of the Elan workshop manual)
Number of coils = 10.8 (after-market 11 turns)
Internal diameter = 3.480 inches (88.39 mm)
External diameter = 4.300 inches (109.22 mm)
Wire diameter = 0.410 inches (10.41 mm)
Free length = 16.00 inches (406.4 mm) (after-market 410mm)
Fitted length = 9.250 inches (235 mm)
Spring rate = 75 lbs/inch

Graeme
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PostPost by: rgh0 » Thu Oct 02, 2014 11:42 am

Steve is making a modified spring probably to suit available now metric wire diameters in Australia. I need to check his design but the spring rate and free length which are the critical outcomes do not match the lotus standard Elan design published in all versions of the workshop manual as far as I know..I have never seen a Lotus manual with different rear spring data. The higher rate does not really hurt with modern tyres but the longer free length plus higher rate will result in a higher than standard ride height unless you have adjustable platforms. The later cars where heavier so a change is not out of the question and this would restore the ride height. However My 68 S4 coupe had springs to the design in the workshop manual

As Gary observed in the earlier email trail there appeared to be some undocumented change in the springs he had collected over the years and this may be what Steve is basing his design on ?????? Maybe he has some information that was published by Lotus in service notes or the like but is not commonly known - he tends to keep that stuff close to his chest.

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PostPost by: au-yt » Thu Oct 02, 2014 9:48 pm

Hi Rohan
I am chasing Steve for a copy of this information or service bulletin.
I did find this picture which is what mine will sit like with the new springs and to be honest I dont mind the extra height.
I remember years ago with My S2 it would drag its bottom with a load of two people and luggage.
Cheers
Graeme
Lotus-Elan-Sprint.jpg and


As against this, and this side was the high side

IMG_0778.JPG and


IMG_0500.JPG and
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PostPost by: rgh0 » Fri Oct 03, 2014 7:00 am

Hi Graeme
Out of interest I have been doing some calculations to try to reconcile the spring data

Nominated Lotus Manual spring rate 67.5 lb/in. Calculated rate from spring dimensions data 71 lb/in. This calculation is based on a steel spring bulk modulus of 11500000 psi Lotus may have used a slightly different number to get their data (or just tested a bunch of springs). In reality the difference at 5% is not significant.

Lotus Marques nominated rate 75 lb/in. Calculated rate from Lotus Marques provided spring dimensions 64 lb/in, this difference at 15% is significant. It appears the rate is actually similar or a little lower than the original Lotus spring design and not the higher quoted 75 lb/in number ( unless that is a typo and 65 lb/in was really meant).

Thus the longer Lotus Marques spring at 16 inches versus the 14.7 inches of the Lotus original standard design would produce a higher ride height for the same load by about 1inch (allowing for the spring angle from vertical) as its rate is about the same. Actual ride height and installed spring length will vary depending on car weight and passenger / luggage load assumed in arriving at the installed length.

You measured the spring rate for the springs you removed and the new Lotus Marques springs I would be interested in the data of what was the actual rate you measured. I would also be interested in the free length of the springs you removed.

You said the springs supplied by Lotus marques had the same number of coils and same wire diameter as your old springs but from the workshop manual data versus the supplied Lotus marques data this should not be the case. However potentially Lotus did change to a spring with more coils and thicker wire at some stage as supplied by Lotus Marques, which keeps the same spring rate and they also increased the free length to increase the ride height ( or compensate for increasing weight in the later cars). These changes in spring free length and to wire diameter and number of coils may have taken place at different times. So there may be a mixture of spring lengths / number of coils / wire diameter out these as installed by Lotus.

I also note there is a cryptic and unexplained note in the Dave Bean catalogue spring section saying -"do not use early springs on S3's or S4's or late springs on S1's or S2's". This may be just referring to the front springs where there is a documented change in the parts manual at chassis number 9297 ( this number is actually well into the S4 production some time in 1969 ?) or to changes made in both front and rear springs at an earlier time - who knows???


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PostPost by: au-yt » Fri Oct 03, 2014 10:45 am

Hi Rohan
I have done some measure measurements as follows
The original springs are 10.1mm wire
The new springs are 10.35mm wire
The way I arrived at the rate was to use a drill press. and I measured the length at 60Ks
Old springs at 13" = 32.3 Kgs at 12" =60kgs
New spring 13 3/8" = 60 kgs only one measurement in fear of braking the drill press!!
There were two lengths which may substantiate the earlier comment about one being longer than the other but only 5mm this spring was also softer by a small amount and it was on the drivers side.
Graeme

IMG_1034.JPG and


IMG_1037.JPG and


IMG_1038.JPG and


IMG_1040.JPG and
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PostPost by: rgh0 » Fri Oct 03, 2014 11:14 am

Your measured spring rates for both new and old springs are about the same around 55 lb/in. i.e. both springs have approx. 2.5 inch deflection for 132 lb / 60 kg load. This is lower than the calculated spring rates of around 65 to 70 lb/in. However your bathroom scale may be under reading. I have found I needed to put a rigid plate on scales like you were using to get a more accurate number as the spring alone in the centre deflects the scale top plate and affects the scales accuracy.

The new springs will give you a ride height about 1 inch higher than the old ones due to their longer free length based on your test data and this is in line with the calculated data.

It is interesting that your Sprint like my S4 has original springs to the design in the manual which suggests that Lotus never changed the rear spring specification during production as both the manual and parts list suggest.
Though Lotus may have changed the replacement spring design after production ceased due to low ride height complaints when supplying replacements and this was recorded in a Service note somewhere.

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PostPost by: au-yt » Fri Oct 03, 2014 11:27 am

Your right about the scales accuracy ( although I hope they are close I like my weight on these :) ) , the measurements really should have been at the ride height as the new springs have a longer base coil, so that may be the reason the rate was different. As well as the extra inch there will be another 5mm as the original Lotocones have pushed up that far, for want of a better description.

I will let you know latter tomorrow as I will put the R/H side back together and see how the car sits.
Cheers
Graeme
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PostPost by: au-yt » Sun Oct 05, 2014 1:02 am

Springs fitted and with the help of one wife and 20Kgs in the boot everything is now bolted up and tight.
The Result I am quite happy with, the car now sits up and looks fresh and handles much better, sound odd that different spring and new Lotocones would make such a difference.

I actually am happy with how its sitting the extra height with two up according to the others on the run, looks "normal"

130K run today with two other Elan's a 912 Porsche and a MX5.
IMG_1042.JPG and


IMG_1049.JPG and


Regards
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