Lotus Elan

Rear Suspension Geometry

PostPost by: summerinmaine » Tue Jul 20, 2010 4:17 am

I'm spending some time going through the entire set-up on my S2 S/E. I have a Spyder frame and rear suspension conversion with CV joint axles, and one thing that seems a bit odd is that the axle shafts are at at a significant angular displacement while at rest. The shafts are angled at about 30 degrees between the hubs and the differential. Is this normal? I would have expected that the differential output shafts should be generally aligned with the wheel hubs under normal loading conditions, in order to avoid unnecessary working/loading of the CV joints during rotation.
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PostPost by: rgh0 » Tue Jul 20, 2010 5:39 am

At normal ride height with standard suspension the drive shaft angle is a few degrees down from the diff to the wheel hub. There is an accurate drawing in the workshop manual that gives all the relevant dimensions.

If you have a Spyder chassis with the Spyder twin wishbone rear suspension or the orginal Lotus rear suspension the ride height and thus the drive shaft angle should not have changed.

If you have a 30 degree angle then your ride height sounds to high

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PostPost by: Galwaylotus » Tue Jul 20, 2010 7:58 am

What's your overall wheel (tyre) diameter? Is it close to factory spec? If you're running low profile tyres then the ride height will be affected and needs to be taken into consideration when setting up.
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PostPost by: bcmc33 » Tue Jul 20, 2010 9:47 am

summerinmaine wrote:I'm spending some time going through the entire set-up on my S2 S/E. I have a Spyder frame and rear suspension conversion with CV joint axles, and one thing that seems a bit odd is that the axle shafts are at at a significant angular displacement while at rest. The shafts are angled at about 30 degrees between the hubs and the differential. Is this normal? I would have expected that the differential output shafts should be generally aligned with the wheel hubs under normal loading conditions, in order to avoid unnecessary working/loading of the CV joints during rotation.

The suspension needs to be adjusted so that the lower wishbones are parallel to the ground with someone sitting in the driver's seat. (front suspension also). Adjust the spring heights to achieve this condition. As Rohan has said, the workshop manual clearly shows what it should look like.
Rear Suspension.jpg and

If when adjusting the ride height with parallel wshbones you still have an acute driveshaft angle, then there is something seriously wrong with the whole assembly.

BTW, assuming suspension bushes are rubber/metal types, they should all be tightened when the suspension is as shown in the manual.
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PostPost by: summerinmaine » Tue Jul 20, 2010 7:47 pm

Thanks for all the replies. I never thought to look in the manual for a figure. :roll: I've been away from Elan work for quite a while, so I've got some mindset adjustment work to do.

My suspicion at this point is that the ride height was never set properly, and the suspension bolts were probably tightened before the body was laid on. The car is mostly stripped at this point, so I'll probably need to wait until re-assembly to make these adjustments. It seems that there were a number of "oddities" in the work done by the PO. I just pulled the seats out, and one set of clamps was installed upside down. This makes about 10-12 assembly errors I've found over the course of time.

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PostPost by: summerinmaine » Tue Jul 20, 2010 7:55 pm

Galwaylotus wrote:What's your overall wheel (tyre) diameter? Is it close to factory spec? If you're running low profile tyres then the ride height will be affected and needs to be taken into consideration when setting up.



The tyres are Pirelli P3000 175/70x13s. That would probably affect the ride height a bit, but I'll need to pull out a pencil to figure out how much.
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PostPost by: Galwaylotus » Tue Jul 20, 2010 8:04 pm

summerinmaine wrote:
Galwaylotus wrote:What's your overall wheel (tyre) diameter? Is it close to factory spec? If you're running low profile tyres then the ride height will be affected and needs to be taken into consideration when setting up.



The tyres are Pirelli P3000 175/70x13s. That would probably affect the ride height a bit, but I'll need to pull out a pencil to figure out how much.


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PostPost by: summerinmaine » Tue Jul 20, 2010 8:13 pm

bcmc33 wrote:The suspension needs to be adjusted so that the lower wishbones are parallel to the ground with someone sitting in the driver's seat. (front suspension also). Adjust the spring heights to achieve this condition. As Rohan has said, the workshop manual clearly shows what it should look like.
Rear Suspension.jpg

If when adjusting the ride height with parallel wshbones you still have an acute driveshaft angle, then there is something seriously wrong with the whole assembly.

BTW, assuming suspension bushes are rubber/metal types, they should all be tightened when the suspension is as shown in the manual.


From my manual, dimension "F" in the diagram should be 10.75 inches, so that gives me a benchmark to start with for ride height. If dimension F is correct, then dimension "H" should be adjusted, and factory specs achieved, right? Again, according to the manual, Dimension H should be 22.6 inches under "normal" conditions, and allow +/- 3 inches for bump and droop. With the rear end off the ground, and the suspension allowed to droop fully, if I measure H as ~25.8 inches, is this a good way to tell if my ride height is approximately correct under normal conditions (allowing for a bit of difference in my tire profiles)?
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PostPost by: summerinmaine » Tue Jul 20, 2010 8:16 pm

Galwaylotus wrote:
summerinmaine wrote:
Galwaylotus wrote:What's your overall wheel (tyre) diameter? Is it close to factory spec? If you're running low profile tyres then the ride height will be affected and needs to be taken into consideration when setting up.



The tyres are Pirelli P3000 175/70x13s. That would probably affect the ride height a bit, but I'll need to pull out a pencil to figure out how much.


See below:



Thanks Amigo! It appears that my tire diameter is 1/2 inch greater than stock. So I'll take that into consideration when I'l doing my measurements. I guess I should expect dimension F to measure out at ~11.00 inches.
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PostPost by: summerinmaine » Wed Jul 21, 2010 8:39 pm

Okay, I've got her back on the rear wheels and checked Dimension F to find that it's less that 0.1 inches longer than the factory spec (with tires at 24psi). But it's clear that the lower A-arms are not parallel to the garage floor at rest. Does anyone know what the dimension should be from the centerline of the lower A-arm inner or outer fixing bolt to the garage floor? If I measure mine, they come in at ~6.25 inches for the outer, and ~9.00 inches for the inner. :shock: Now the motor and trans are out and the car isn't weighted down at all, but that seems excessive to me.

My plan is to loosen all the suspension fixing bolts, then load the trunk of the car until the arms are parallel to the floor, then re-torque. Does this sound reasonable? I think that the Spax dampers are adjustable for ride height, but if not, is there any other way of adjusting this? Perhaps I won't need to if the A-arm set-up is correct.

Remember, mine is a Spyder dual wishbone set-up.
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PostPost by: paddy » Wed Jul 21, 2010 9:26 pm

summerinmaine wrote:My plan is to loosen all the suspension fixing bolts, then load the trunk of the car until the arms are parallel to the floor, then re-torque. Does this sound reasonable? I think that the Spax dampers are adjustable for ride height, but if not, is there any other way of adjusting this? Perhaps I won't need to if the A-arm set-up is correct.

Remember, mine is a Spyder dual wishbone set-up.


I'm pretty sure the Spyder arrangement makes no difference to what I'm about to say.

Tightening the fixing bolts when the car is at its normal static ride height is primarily for the benefit of the bushes - so they are minimally stressed most of the time. If they are tightened at some other position, then they will be under stress but it will make bugger all difference to the ride height. (OK, maybe a tiny bit but it's not the root cause of your problem.)

You need to set the ride height via the spring platform adjustment.

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PostPost by: summerinmaine » Wed Jul 21, 2010 10:05 pm

paddy wrote:
summerinmaine wrote:My plan is to loosen all the suspension fixing bolts, then load the trunk of the car until the arms are parallel to the floor, then re-torque. Does this sound reasonable? I think that the Spax dampers are adjustable for ride height, but if not, is there any other way of adjusting this? Perhaps I won't need to if the A-arm set-up is correct.

Remember, mine is a Spyder dual wishbone set-up.


I'm pretty sure the Spyder arrangement makes no difference to what I'm about to say.

Tightening the fixing bolts when the car is at its normal static ride height is primarily for the benefit of the bushes - so they are minimally stressed most of the time. If they are tightened at some other position, then they will be under stress but it will make bugger all difference to the ride height. (OK, maybe a tiny bit but it's not the root cause of your problem.)

You need to set the ride height via the spring platform adjustment.

Paddy


That make sense. I'll check the other dimensions, but I'm a bit mystified as to why there should currently be such a dramatic difference between the measurements at the ends of the A-arms.
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PostPost by: elansprint71 » Thu Jul 22, 2010 11:33 am

Might also be worth checking the height of your diff in the chassis; there is a small amount of adjustment available using washers on the top mounts. However, it sounds like your angle of dangle is being mostly affected by the tyre/suspension relationship.
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PostPost by: stugilmour » Fri Jul 23, 2010 4:10 am

I am just going through the same issue on my Plus 2. Not sure if your Elan manual will have the same "Section X: Overhaul and Rebuild" at the very back at the book. Anyway, for the Plus 2 this section provides the weight to add to the trunk and the seats prior to tightening. They call up the following; Elan would be a bit less I guess, but this may provide an idea, or see if you have similar section in manual:
Front seats............168 lb
Back Seats..............80 lb
Boot.......................72 lb
Fuel Tank................50 lb, equiv to 5 imp gallons, 23 l, 6 US gallons

Comparing the total weight the GVW of the car, I decided these weights provided were for both seats, not each.

I opted for as you describe; added enough weight to get the wishbones close to level and tighten away. My rear is still sitting high with the weight removed (other than a full tank and the spare in place). I am trying to look into options to adjust the ride height now, but without adjustable perches in the rear I think I am screwed? Any suggestions welcome. Spyder frame, new springs & shocks both ends from Spyder, conventional setup geometry. Replacement springs were longer than the old ones by about the same ride height discrepancy, so thinking a shorter spring or adjustable perch is required.

BTW, I don't see how changing the tire diameter will effect wishbone angle and am focused as mentioned above on getting the rear wishbones flat rather than matching ride height floor to bottom of frame.

HTH Let us know how you make out.
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PostPost by: Galwaylotus » Fri Jul 23, 2010 7:25 am

stugilmour wrote:BTW, I don't see how changing the tire diameter will effect wishbone angle.

It won't but an earlier post mentioned ride height and this is affected by tyre diameter.
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