Optimal bump steer?

[snowyelan]

So I've finally decided to model this up in CAD to try and figure out where the rack should be in theory.

I've hit 2 stumbling blocks. The first is a discrepancy in the manual between dimensions for the front lower pivots. I think I can simply measure the chassis for figuring this out.

The second is I can't find the distance between the upper and lower pivot on the upright. Anyone know where I can find this? I roughly measure it at 235mm but the balljoint center is buried...

LEN snip.JPG (43.18 KiB) Viewed 1224 times

Thanks.

[Elanman99]

I have always assumed the ball in the top mount is centered between the two mounting bolts.

ian

[Andy8421]

Snowyelan,

This is great stuff. You may have seen on another thread I was thinking of having a go at this myself, but didn't have the skills to complete it.

I am afraid I can't help with an accurate measurement of the centre of rotation of the top balljoint - although I think the suggestion of 'between the bolts' is a good start.

I do have a question though, are you assuming that the centre of rotation of the outboard steering balljoint is in line with the track rod? Eyeballing the setup, it wasn't clear to me that this is the case.

Andy.

[snowyelan]

Morning Andy,
I decided to start off with a relatively simple 2D sketch and go from there. For this model I am only looking at the change in position (grey dimension) between the the upright and the tie rod outer ball joint.
I do intend to do a full 3D mock up of the suspension but thought I'd start with the basics. Any tie rod fore aft sweep isn't considered yet.

[snowyelan]

Making a bit of progress with the 3D model. I found the dimension for the upright. Sort of.

What I still need is the 2 dimensions below. I don't have a spare to measure. Anyone got one they could check?

I also need the tie rod balljoint height but can measure that on my car.

[Craven]

Interesting project but there is some uncertainty as whether zero bump is how Lotus set things up.
Both trunnion dimensions are 25 – 26mm.

[snowyelan]

Thanks for the dimensions.
I agree that zero was probably not what was used. There is an interesting dimension on the chassis drawing that could possibly be the designed rack C/L height. I'm curious to see what the bump looks like at that height.

[baileyman]

This is so cool. I made mine out of wood scraps rather than pixels.

When you get your model set, try to articulate it. Then you can project the upper and lower link ends to find where they intersect. And then you can point the steering links to the same point. That solves for rack height at an instantaneous zero bump zero droop steer. It changes once you get off that point.

John

[snowyelan]

Hi John,

It does articulate currently. Its a little wonky because I need to clean up some of the pivot locations. Some are just a rough guess at this stage.

Couple more pdf's for reference
Elan steering rack - Socket to socket is 698.5mm

Triumph rack dims - Spitfire is noted as 660mm but i think this is the rack only, not to the socket centers. I played with the math but couldn't make it work out to a reasonable answer

What has me really curious is the dimension circled below.. Is it just the wheel axis or is it referencing the "Lotus rack height".
The Maths for axis is tire dia (XASFF155) (582/2=291)-127mm=164mm (6.45") ride height to the cross member. Pretty close, tire diameter was probably smaller when the drawing was done (145 series)
Dimension 9 (2.25") in the upright diagram above plus the 1" dimension supplied by Craven plus 1/4" for the seal seems to match 3.5" (88.9mm) from the chassis drawing as well...

The semi official 2.190 from the lower pivot C/L to the rack bottom works out like this compared to the 88.9mm:
88.9mm-24mm(rack bottom to axis)=64.9mm(2.56") Significantly higher than the 2.19".

If we use the top of the 1/2" lower pivots we get closer... 58.55mm (2.305") My thought process behind this is how the jig would have looked when the chassis was measured for rack shims. It was probably a simple bar with a 2.19" step in the middle.

I haven't shared my rack shim #'s till now but I ended up with approx 9mm of shim to get an almost perfect bump line with the laser method I described previously. I suspect this is quite a bit thicker than anyone has with their original shims. I seem to recall 0.140" (3.5mm) being mentioned before.

Does anyone have their original shim sizes they would like to share?

chassis detail.JPG (46.72 KiB) Viewed 1056 times

[661]

About 10.5mm each side.
I have the 26R set up.
Steering arms bent in and a bit more caster.
The dual gauge bump steer equipment measured virtually no deflection from 20mm droop to 35mm bump

[Donels]

Brian Buckland recommends measuring the required shim thickness from a straight edge across of the top suspension spindles. The dimension to the top of the shim should be 158 mm. Perhaps you could model this.

Using his method my shims came out to 1mm one side and 2 mm the other. He says this method applies to the Elan and +2.

[Donels]

Also see rack dimensions from BB's book. They’re slightly different to yours.

[Bigbaldybloke]

You might all want to take up RC model car racing! The top spec ones of these such as made by X-ray, Schumacher, Serpent, and others have fully adjustable front suspension, in fact more adjustable than our Lotuses.
To get the best handling we all have setup jigs to put the cars on to get the optimum handling for any particular car, tyre and track combination. With the jigs fitted in place of the wheels you can accurately measure caster, camber, toe-in, bump steer and Ackerman angle. You can adjust the inner and outer ends of the track rods up and down and see the effects on bump steer in the straightahead position and on any angle up to full lock. You can see how moving the suspension pivot points up and down on both inner and outer ends affects the roll centre position and the suspension geometry as the suspension travels up and down, and how moving the track rod inner ends forward and backward affects Ackerman angles.
Basically we find that if the track rods are the same length as the bottom wishbones and parallel with the bottom wishbones with the car loaded so it is at normal ride height and steering straight ahead then bump steer is so small as to be too small to mention. At the front we tend to run 2 degrees negative camber, 1 degree toe out, 4 degrees caster and depending on track we may add in some anti-dive. At the rear we run anything up to 3 degrees toe in, maybe some anti-squat and some cars have active rear suspension so we can dial in toe in or toe out as the suspension compresses. So much more adjustable than a road going Lotus but really good to help you understand how small changes to suspension geometry affect the movement of the wheels.

As for me, I’ve stuck to the shim sizes provided with my replacement chassis, I doubt I’d ever push the car hard enough on public roads to notice small changes to rack height unless it really was a long way out. Much more important for me is getting the static toe-in setting correct and ensuring everything is in as good a condition as possible with no wear in suspension joints, dampers, wheel bearings etc. And the right tyres at the correct pressure

[snowyelan]

Hi BBB,

I used to race R/C. First foam tire touring car then switched to 12th scale. The rapid evolution of touring was to expensive for me. I dabbled in the 2wd stadium truck and 2wd buggy too. A good friend of mine is still pretty heavy into it so I do keep track of what's going on.

Donels,

Thanks for the shim thicknesses. I can model in the 158mm and see where that lands compared to the measurement from the lower pivots

[Elanman99]

About 10.5mm each side.
I have the 26R set up.
Steering arms bent in and a bit more caster.
The dual gauge bump steer equipment measured virtually no deflection from 20mm droop to 35mm bump

I have no knowledge of the 26R setup but 10.5mm doesn't sound like a shim, more a spacer or packer.

The rack shims measurements I have come across have all been less than 3mm.

Ian