Thread: Parabolic leaf springs

Originally Posted by isuzurover

Did you read this bit?



Come on clarkie - you know you want to build a leafer that is more capable than all the coillers offroad, rather than just put a 110 chassis uynder it

What springs do you have fitted at the moment?

Need to Ask DaveS3, but i think he said Wagon springs with a leaf or 2 removed... and i know its got Mil hangers..
I will have to look at them to see and count the leaves

What springs do you have fitted at the moment?

Did you use the leaf thicknesses from here?


I happen to have my Stage 1 supplement at work here, and it says ths stage 1 should have the standard 109" 11 leaf fronts, and the 8+2 leaf station wagon rears - HOWEVER - most Stage 1's I have seen have had the Heavy Duty rears (8x7.1mm leaves).

So if you have standard rears, you should have a rate of either 270lb/in or 368lb/in. IME the LR specs seem pretty good, so what I usually do is use the SF as a fitting factor to match the calculated rate to the one in the manual. For 8/9 leaf rears I get an SF of 1.13.

A rate of about 270lb/in in the rear is a good compromise between comfort/articulation and load carrying capacity. If you don't ever intend to carry a heavy load in the rear you could go a bit lighter, but not a huge amount. If you have the 8-leaf springs though, pulling 2 out brings you down around 270.

If you have 11-leaf fronts, removing 2 leaves brings you down around 220lb/in (from memory), which works well, but for a V8 you could probably pull out 3.

Originally Posted by stirlsilver

For the front it seems like I'll have to remove 4!!! of the 4.5mm leaves to get it down to 200lb/in.

So the font packs would have:
6.5mm x 2
4.5mm x 2

that's a very thin front pack!
What are your thoughts ben??

Do you have std or aftermarket springs? If std - This is a good reason to use the Manuell values, not measured ones, as they are more accurate!!! There weren't any leaves of those thicknesses in the stock spring packs. The main leak should be 5.15mm and the others should be 4.36mm (or do you have aftermarket springs?)

If you have std springs, I get the following (k = r = spring rate) value at the bottom is the sum of both leaf thicknesses:
Original:
k E n w t l SF
38.5 3.05E+07 1 2.5 0.203 36.25 1.08
234.5 3.05E+07 10 2.5 0.172 36.25 1.08
273.1

3 leaves removed:
38.6 3.05E+07 1 2.5 0.203 36.25 1.08
164.2 3.05E+07 7 2.5 0.172 36.25 1.08
202.8

If you indeed have 6.5 and 4.5 mm leaves - then you are right, 2 leaves of each would give:
k E n w t l SF
154.5 3.05E+07 2 2.5 0.255905512 36.25 1.08
51.3 3.05E+07 2 2.5 0.177165354 36.25 1.08
205.9

I measured the leaf thicknesses off the packs on the car. Couldn't use verneer calipers so the best accuracy I could get to was 0.5mm (I appreciate that the thickness has a large impact (^3)) so you would want to get those values as accurately as possible.

The leaves are definitely not orignial as I bought them new about one and a half years ago. So as a result I don't have the luxury you did of knowing the specifications of the packs.

The actual results I got was 315lb/in on the front and 295lb/in for the rear (I rounded). At the end of the day this is just an approximation, and by using a SF of 1.1 (lowest) it should understate the spring rates.

When I purchased the packs they (before I pulled any out) they came with:
Rear
7mm x 9

Front
6.5mm x 3
4.5mm x 6

Originally Posted by stirlsilver

I measured the leaf thicknesses off the packs on the car. Couldn't use verneer calipers so the best accuracy I could get to was 0.5mm (I appreciate that the thickness has a large impact (^3)) so you would want to get those values as accurately as possible.

The leaves are definitely not orignial as I bought them new about one and a half years ago. So as a result I don't have the luxury you did of knowing the specifications of the packs.

The actual results I got was 315lb/in on the front and 295lb/in for the rear (I rounded). At the end of the day this is just an approximation, and by using a SF of 1.1 (lowest) it should understate the spring rates.

When I purchased the packs they (before I pulled any out) they came with:
Rear
7mm x 9

Front
6.5mm x 3
4.5mm x 6

Fair enough. So your rears may be the 7.1 mm extra heavy duty - but 0.1 mm doesn't make too much difference. I would go with 5-6 leaves.

You can double check the measurements with verniers around the centre bolt hole when you pull the packs apart.

If those rear measurements are right, then 4 leaves should do the job, or 5 if you want to make sure, or have a dual battery/winch.

EDIT - when doing the calcs, the best SF values I found are 1.08 front and 1.13 Rear.

I've just put in a 16 gallon tank to replace the leaky 10 gal and noted among other issues that the spring hanger to mount the front of the rear spring set is cosy with the tank and the shackle bolt wont go anywhere till the tank is lowered. Looked like it would have been the case for the 10 gal too or it would have been very close. Question then ... is it just a fact of life that the tank needs to come down to change rear springs on an 88?

If so I suppose I should hold off fitting the second 16 gal on the passengers side until the Minister for War and Finance approves the business case for the RM parabolics ... and at the moment her solution to the rough ride is to go in her Impreza instead.

Hi Ruski,

Yes the tank has to come out to do the rear springs. I have just finished putting new parabolics on the back of mine tonight. Fronts should go in tomorrow. My personal experience with RM parabolics is don't bother, they are not worth the price. Mine had them on when I got it. The receipts show them to be less than 2 years old and the car had not been driven much in that time and they were badly collapsed. Drivers side rear particularly so the car sat at a real lean. The difference between drivers and passengers side was 70mm so I am talking a lot of lean. I also talked to the distributors and the set on mine were not the only ones to have given up very quickly.

I have some standard springs that I was going to put in but I could not get excited about pulling spring packs apart and such like so I got some Britpart parabolics from Sydney. Price was ok so we will see how they go. Sitting them next to the RM they sat at least 50mm higher when upside down on the ground and the drivers side RM was worse by at least 25mm. I haven't tightened everything up yet and the fronts are still the RMs but it now sits nice and level and quite a bit higher.

TimJ.

Timj - were your parabolics the correct way around - the original springs are stronger on the drivers side and if put in on the passenger side the car would lean to the drivers side.

Are RM parabolics the same??

Garry

Hi Gary,

To the best of my knowledge the parabolics are not sided in the same way the standard ones are. The dramatic difference in height between the old and new ones tends to suggest they have sagged badly as well. I did think about just swapping them over but there was too much difference and I wanted some extra clearance as well having put some big dents in the cross member under the gearbox last time I was 4wding. I believe that the RM parabolics were the best available at one time but that they had some problems with their supplier and quality may have suffered. Not sure if they are back to the best again by now but I didn't want to test it.

Tim.