Steering Rack Extensions - Tie Rods and Bump Steer

[4129R]

I have long been wondering about the steering racks and bump steer, since I have converted 7 steering racks to right hand drive.

I understand for there to be no bump steer, the ball joint pivot point on the steering arm at each end of the rack, needs to line up with the pivot points of the wishbones top and bottom, where they meet the chassis at the long 1/2" rod.

The steering racks have been lengthened by adding a hexagonal metal rod on to each end of the rack, which has moved the tie rod pivot point out a lot, beyond the line connecting the top and bottom wishbone 1/2" pivot rod. This will lead to a lot of bump steer I believe.

See the attached photos.

Where the blue and red tape meet is where the ball joint pivot point should be to line up with the two wishbone pivot points.

I think the steering rod on the end should be lengthened, not the rack fixed rigid to the chassis. The top new rack has the right pivot points but the tie rod is not long enough.

The bottom rack is the old rack with the rack lengthened, which I think gives huge bump steer, as the ball joint pivot is considerably misaligned with the pivots for the wishbones.

I made a dummy rack out of copper tube to mark where the pivot points should be and put red and blue tape on to show the centres.

For those who understand steering geometry and bump steer please tell me if I am wrong. If I am right, all steering racks seem to have been lengthened at the wrong part.

[TurboFource]

The inner tie rod end should line up with the line going through through the inner control arm pivot points …. Why would the length of things change going from left hand to right hand drive?

[4129R]

The inner tie rod end should line up with the line going through through the inner control arm pivot points …. Why would the length of things change going from left hand to right hand drive?

When I took the rack out of a LHD casing and put it in a RHD casing, I saw the hexagonal extension pieces which got me thinking.

I think all Europa racks potentially have been made wrong, by extending the rack, not the arms. By lengthening the rack, they don't line up. The rack is the right length, the arms are too short.

[Clifton]

The amount of bump steer is minimal when the tie rod is horizontal. Mines pretty lowered and angle at 9 degrees,  I assume a stockish height Europa is 0-5*.  You aren't going to have too much bump steer when near horizontal and the small angle on the LCA. A longer tie rod would reduce the bumpsteer though. IMO, the only negative is that it only adds a little toe in when turning but on a mid engine car, the front inner tire is so unloaded it doesn't cause as much understeer as on a front engine RWD car or a very heavily loaded inner tire of a FWD car.

Lotus likely didn't care to make it perfect or many other things (caster and camber) because it is a street car and more toe in = more understeer = safer. This would be at the limit too, not at fun street driving speeds.

[richyb66]

My S2 rack hasn't got the hexagon extenders on the end of the rack. The tie rod knuckle ends are screwed on directly and held with a locknut.
The threaded end of the tie rod has a hex adaptor on it that the track rod end screws on to.

[314159td]

Early cars use extensions (likely off the shelf male-female threaded standoffs) on the tie rods. That stopped somewhere around S2 production, presumably in the earlier half based on various murky sources. My S1 has extensions on the tie rod.

Later cars used the larger, also hexagonal extension on the steering rack itself as you've shown. Most people seem to believe this improves bump steer so your data here opposes that somewhat. Not sure if Lotus has any notes in their service bulletins or manuals regarding the reasoning for the change.

Regardless, always good to have more data points!
If you could post a picture that shows the entirety of both racks and your copper tube simulator, that might make it easier for folks to understand. 

[RonPNW]

I worked on this recently. Check out:   https://www.lotuseuropa.org/LotusForum/index.php?topic=6125.msg66740#msg66740

In the end I left things stock. It was easy to make dramatic bump steer changes with small geometry changes. After playing with several "tweaks", I retested the stock configuration and found it to be the best per my goals. While getting the geometry correct is a good start, you only know what you have after you measure it. Even then the actual, on road, performance could change dramatically as bushings compress.

It is also worth noting that it seems each S2 is somewhat unique. What works for one chassis may not be correct for another.

Ron

[4129R]

I worked on this recently. Check out:   https://www.lotuseuropa.org/LotusForum/index.php?topic=6125.msg66740#msg66740

Thanks for that. The photo of the wishbones and steering rack end shows my point about alignment very well.

[4129R]

If you could post a picture that shows the entirety of both racks and your copper tube simulator, that might make it easier for folks to understand.

As you asked.

Where the blue tape meets the red tape is where the tie rods should pivot.

The black tape is where the rack should mount in the aluminium clamps.

The white tape with C marks the centre of the racks in the straight ahead position.

The top old rack has the hexagonal extensions which extend the tie rod pivots too far.

The lower new rack needs a 70mm extension on the left passenger end, and a 40mm extension on the drivers end.

[Nockenwelle]

I don't think that the chassis engineers at Lotus back at the time were not knowing how to properly design a sportscar suspension. Bump- or roll steer is designed into the suspension for a good reason. Have a look at the following article:
https://www.vehicledynamicsinternational.com/features/john-miles-toe-the-line.html

[S2Zetec54]

Interesting article….I assume this is John Miles who drove the 47 with the twin periscopes?

[4129R]

I don't think that the chassis engineers at Lotus back at the time were not knowing how to properly design a sportscar suspension.

As I live in Norfolk UK, I know a lot of people who have worked for Lotus, and in the pub tonight, an ex-Lotus employee told me exactly that.

In the early 70s, they did not understand bump steer.

They just wanted the cheapest way of adapting a mass production steering rack to work in a Lotus, and if that meant putting 50mm extensions on each side, so be it, and let us find the cheapest way of doing this.

I intend to remove the hexagonal 50 mm extensions on the rack, and lengthening the tie rods beyond the ball joint to see if the car goes down a straight road better, without wandering. As I have 4 road going Europas at the moment, I can compare standard with adjusted, to see if it is worth the effort of removing the extensions from the rack, and making new extensions to the 1/2" tie rods from the rack to the track rod ends.

[Nockenwelle]

In the early 70s, they did not understand bump steer.
They just wanted the cheapest way of adapting a mass production steering rack to work in a Lotus, and if that meant putting 50mm extensions on each side, so be it, and let us find the cheapest way of doing this.

I get your point that cheap was probably the most important thing for Chapman - maybe even before the 'lightness' aspect. It could well be the reason behind that steering extension on a third party rack at the expense of the suspension geometry.
But I can't imagine Chapman did not fully understand bump steer back in the days. Of course he may just ignored it for cost reasons.

There are other geometry aspects of the Europa suspension that are also surprising at the first look. For example the very little amount of castor of only 3°. More castor would greatly improve straight line stability and add more camber gain when cornering. Both are highly welcome. And a bigger castor angle would have been for free so no cost aspect here.

It took me some time to figure out the reason. Reducing the mechanical trail due to a small castor angle gives the driver a lot more feedback on the limit when the dynamic/pneumatic trail of the tire disappears and the steering gets light.

Have a look at that Gordon Murray interview praising the Elan S3 suspension designed at the same time when the Europa was developed:
https://youtu.be/6JP9Swop_Fg?si=miZ1Gg0U3nVKOZmH&t=2699

[4129R]

I think we have established that the S2 rack had the steering rack extensions on the tie rod ends, not on the rack, and the TC and TCS vice versa. So the S2 should not have bump steer.

I wonder if the addition of the chin spoiler was anything to do with keeping the nose straight. I believe the S2 had no spoiler, and the TC and TCS had them added. Is this related, or just co-incidence?

[BDA]

I always understood that the chin spoiler on the TCs and TCSs was for highway stability