Twin Cam front suspension geometry different than S2?

[TurboFource]

It appears the front suspension geometry on a Twin Cam chassis is different than an S2 based on the drawing for an S2.

I measure 31.0" vs 30.360" for upper distance between points
25.125" vs 24.660" between lower points
8-7/8" vs 9.0" for the lower point from top of the frame

TC measurements listed first....done with a tape measure but within less than 1/64" (.015) error

Nothing is bent and it diagonal dimensions are identical.

Was this done when they raised the ride height to meet Fed regs

[MRN I J]

I once plotted the camber change on my S2 just to see how my tyres were going to behave on the car in roll, figures long gone now

[gideon]

Turbo - this article gives some explanation of the things they changed for the Twin Cam, straight from the horses mouth too.

https://drive-my.com/under-the-skin-of-the-lotus-europa-twin-cam/

[TurboFource]

Thanks Gideon! They did change the geometry.

[jbcollier]

They changed the ride height at the front and then lowered the floor for more headroom.  Geometry-wise, suspension arms fit across the range with the exception of the S1 and early S2 upper rear arms.

[TurboFource]

I now read that Lotus also changed steering rack spacers on the Twin Cam and Special to improve bump steer in addition to suspension geometry changes. So does a Twin Cam handle better than the S1/S2? Any skidpad numbers available?

[jbcollier]

The S1/2 extend the rack by adding spacers at the outer tie rod end.  The TC/S use spacers to extend the inner tie rod end out to where it is roughly inline with the a-arm pivots.  In the straight ahead position, this reduces bump steer.  This is important in racing but less so on the road.

With identical tires, the S1/2 would give higher skid pad numbers due to being lighter than the TC/S.

[TurboFource]

Thanks JB

[MRN I J]

The S1/2 extend the rack by adding spacers at the outer tie rod end.  The TC/S use spacers to extend the inner tie rod end out to where it is roughly inline with the a-arm pivots.  In the straight ahead position, this reduces bump steer.  This is important in racing but less so on the road.

With identical tires, the S1/2 would give higher skid pad numbers due to being lighter than the TC/S.

So the spacers screw on to the rack bar & extend the ball joint position, by how much JB

[jbcollier]

Good question, don't know for sure.  Would assume they extend the same amount as the S1/s ones.

[dakazman]

   Thanks guys for bringing up a subject I really never heard of. Reading through a list of Google hits I found myself being drawn into being a wheel man and not a builder. Having driven literarymadness’s car I loved the handling in a turn.  My entire suspension is in place but not setup or serviced.  I will not race mine or test the limits so what should one , as a builder shoot for? 
  My only suspension / steering problem was on my corvette.  having the power steering shims set to high  and  when driving went lock to lock in a slight motion. Fun at 40mph in 2nd gear 🤪

Dakazman
 
 
 

[GavinT]

Years ago I cut 'n' shut an S2 rack to remedy the bump steer. Only later did we discover the TC/S used longer rack sleeve nuts to achieve the same thing.
The added rack length provided on the TC/S is about 3.25 inches in total, so reasonably substantial. I don't have precise measurements to hand.

It's also possible to raise the stock rack to effect a similar result but that requires the rack mounts be positioned horizontally as distinct from their stock vertical mounting on the front of the T.
The internal metal work kinda gets in the way, otherwise. Shims are used to attain the final position and equalise both sides.

[4129R]

The S1/2 extend the rack by adding spacers at the outer tie rod end.  The TC/S use spacers to extend the inner tie rod end out to where it is roughly inline with the a-arm pivots.

I was advised that the tie rod ball joint should be in line with the top and bottom arms suspension pivot points.

I have measured mine on a TCS, and the steering rack is about 20mm longer each side than the in line with suspension pivot length should be.

Is this normal, or are my spacers too long? Will this longer tie rod length give bump steer and straight line instability?

[4129R]

I have just found all the dimensions for the steering rack in the workshop manual section H page 7. 

The whole length centre tie rod ball joint to centre tie rod ball joint should be 1,224.50mm and centre rack ball joint to rack ball joint should be 698.50 mm with the two tie rods being each 263mm long including the tie rod end.

Time to take some measurements.

[4129R]

Well I have just measured the steering rack.

As you would expect, the total length is correct at 1224.5mm, but my tie rods are much shorter than the workshop manual dimension 1 on page 7, Section H. The manual says they should be 263mm, but mine are only 225mm long. That in effect means that they do not line up with the centres of the top and bottom suspension arms, so I have the potential of around 40mm of out of line bump steer.

Is the workshop manual correct? Are these dimensions for a TC or an S2 which may be different?

My manual has these dimensions:-

1    263.0mm
2    30.04mm
3    85.85mm
4    26.16mm
5  432.56mm

Therefore the dimension from the centre of the pinion/steering column to the shorter track rod end = 1 + 2 + 3 + 4 = 409.05mm.

I am confused.