Lotus Elan

Bump Steer

PostPost by: Dieschelan » Sun Jul 21, 2019 7:14 pm

Hi

What’s the correct way to correct the bump steer in an Elan? I know if you add shims to the steering rack clamp you can correct it. But how many shims are necessary?
Is possible to correct it without dissemble all the front suspension?

Regards

Diego
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PostPost by: Concrete-crusher » Sun Jul 21, 2019 9:21 pm

Hi first you need to have a way of measuring the bump steer. I used a laser gun sight mounted on the stub axil.

My first attempt was to mount it in line with the axil, then when you raise the suspension you can trace the line it makes. It should be verticle.

My second attempt was with it mounted at 90' on the wheel and reflected back from a mirror this also works.

I needed to lower my rack on one side to get the bump steer to zero. I did this by machining a solid. Rack mount down. The reason I needed to lower the rack was simply because the chassis had been pushed up at the front on one side, possibly past jacking or it had hit something

anyway best of luck Steve
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PostPost by: ericbushby » Sun Jul 21, 2019 9:47 pm

Hi Diego,
I do not know the correct way, but this is what I did, somewhat similar to Steve`s above.
Raise the car off the floor and ensure it is straight and level.
Assemble the front suspension without the springs. It is only nuts and bolts, no spring compressor needed.
Clamp a piece of wood to the bottom wishbone to act as a handle. The suspension can now be raised and lowered throughout its travel.
Fix a laser pen to the hub; a kitchen foam scrubber held it quite well in the hub recess. A clothes peg holds the switch in when needed.
A piece of cardboard was set up on the garage wall with a vertical line drawn on it. The garage must be wide enough to do this at both sides.
AS the suspension is raised and lowered The laser will show an angled line and you can mark on the cardboard what the current setting is.
Adding or removing shims under the steering rack mounts will change the angle of the line. Adjustments at one side also affects the other side.
Mine had been assembled with no shims and felt dangerous to drive. It now drives smoothly with no tendency to swerve sideways on bumps.
The chassis was engraved with 0.050" on one side and 0.060" at the other.
I finished up with 0.130" and 0.180" at the other side.
Adjustments of 0.010" intervals were necessary as I got closer.
I hope this is helpful
Eric in Burnley
1967 S3SE Type 45
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PostPost by: vincereynard » Sun Jul 21, 2019 10:08 pm

I tried a laser pen as described above to measure rear toe in.

The problem I found is that the laser light is not ( necessarily) in line with the pen body. Hold the "spot" on a point. Rotate the body and the spot moves.

I did not find an accurate way to fix this problem.
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PostPost by: ericbushby » Sun Jul 21, 2019 10:41 pm

I know what you mean Vince but in this case it does not matter as we are only interested in the angle at which it moves up and down not it`s actual position. I could remove and replace the pen at any time and realign the spot on the cardboard without affecting the angle at which it moves. it worked OK for me.
Cheers
Eric
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PostPost by: Slowtus » Sun Jul 21, 2019 10:43 pm

Ralph Broad once corrected a bad case of alleged bump steer on a Formula 1 car testing at Brands (can't remember which team even though I was there at the time) back in the '60s...twas the rear wheels which were apparently doing the damage as one/both were out of whack.

He used a piece of string to show the Formula 1 people where the problem possibly existed (tracking) and then promptly jumped into his Cooper (not an S) and blasted off into the distance. Okay, maybe not blasted but...

Bump steer can be caused by anomalies at both ends of the car, especially with those which are independent all round.

So they tell me.

PS, this is not a technical response, I am no engineer but I do (kinda) remember the article from Motorsport or similar from back in the day which explained all the hither and thither and bits of string being wielded by Ralph as the F1 people looked on.

Sorry, i went away from the issue here to a gentle reminiscence :D
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PostPost by: vxah » Sun Jul 21, 2019 11:17 pm

As I have the rack setting up to do on my new chassis I was thinking about a way of measuring the bump steer. I had read about the laser pen method but I can’t quite work out if it’s correct? I say this because as we know the front suspension uses quite short arms for track control there must be an “arc” drawn as the suspension moves through its travel, this will cause the actual vehicle track to widen and narrow over undulations so, if your laser draws a vertical line is it correct? I would have thought an arc of some sort would be correct?
Then again I thought as we have unequal length wish bones this will change the arc drawn by the stub axle or maybe remove the arc all together? I convinced myself that the only accurate way to measure the bump steer is with tracking gauges checking at various points in the travel or, with the bare chassis, fit a pair of wheels on and measure the distance between the front and rear of them while the suspension is moved, the distance may change due to the arc but, as long as the change is the same front and back of the rims then it’s good?

Perhaps I’m overthinking this?
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PostPost by: nthSteve » Mon Jul 22, 2019 12:29 am

Bump Steer - my two cents.

I highly recommend Prepare To Win and Tune To Win by Carroll Smith. The former describes it and how to adjust it. The latter continues with more attention to dynamics.

As I understand it, bump steer relates to change in the Front (or Rear) Toe as the suspension moves / the body rolls. We esp. don't want the front to toe out under bump, because that wheel is loaded and thus steering us.

Adjusted at the front by changing the relative heights of the outboard and inboard ends of the track rods - including by raising or lowering the ends of the rack; also if castor is changed. At the rear by altering the hub carriers' inclination angles.
-Steve, SoCal, '72 Sprint DHC
formerly Lotus 47, Lotus Cortina
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PostPost by: baileyman » Mon Jul 22, 2019 1:29 am

Smith is great on it.

I had to set mine up, with 26R bits, before bolting it all up as an initial setting. My approach was to model the angles of the control arms and adjust the angle of the steering arms appropriately.

I explained to my machinist that I set the suspension at neutral with horizontal lower links, then found the intersection of the lower and upper link pivots in space (way over near the other suspension), then raised the rack until the steering arms pointed to the same spot. Then I massaged the whole thing so that the roughly linear part of the curve was all in bump instead of half in droop (added another spacer). My machinist said, "Sounds perfect to me!"

Then I recounted the process to a local Type IX racer and he said, "Who cares what happens to the inside wheel?"

And I guess that about sums it up. The trouble is toe changes on the outside wheel when it is carrying most of the car.
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PostPost by: john.p.clegg » Mon Jul 22, 2019 5:20 am

All the above,but the hub scribes an arc as it moves up/down , what you are looking for is that it increases toe-in at both droop and compression ( both sides ) , so it needs setting so that at normal ride height it has least toe-in.

As you alter one side , the other will alter as the steering rack acts as a see-saw..so it's set one side,set the other side , check and repeat.

John :wink:
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PostPost by: rgh0 » Mon Jul 22, 2019 10:01 am

vxah wrote:As I have the rack setting up to do on my new chassis I was thinking about a way of measuring the bump steer. I had read about the laser pen method but I can’t quite work out if it’s correct? I say this because as we know the front suspension uses quite short arms for track control there must be an “arc” drawn as the suspension moves through its travel, this will cause the actual vehicle track to widen and narrow over undulations so, if your laser draws a vertical line is it correct? I would have thought an arc of some sort would be correct?
Then again I thought as we have unequal length wish bones this will change the arc drawn by the stub axle or maybe remove the arc all together? I convinced myself that the only accurate way to measure the bump steer is with tracking gauges checking at various points in the travel or, with the bare chassis, fit a pair of wheels on and measure the distance between the front and rear of them while the suspension is moved, the distance may change due to the arc but, as long as the change is the same front and back of the rims then it’s good?

Perhaps I’m overthinking this?

That is why measuring it with a laser at right angles rather than straight ahead is better as you don't get the influence of the track change versus toe change on suspension travel

cheers
Rohan
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PostPost by: vxah » Mon Jul 22, 2019 10:56 am

Ah yes Rohan, I see this now and why one needs a wide enough workshop!
It make perfect sense to me.. If I affix the laser in the stub axle hole and carefully raise the suspension up and down without a steering arm attached it should scribe a vertical line or, actually a line the same as the castor angle?
As was also said about toe change when cornering hard being undesirable, how does the bump steer get affected by the steering being turned as in, the inner rack joint is not where it was when we set things up?
Should one check what happens to the toe at say, 10 degrees turn in?
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PostPost by: rgh0 » Mon Jul 22, 2019 11:36 am

In general bump steer setting is only critical on straight ahead driving and under braking. Once you turn into a corner there is a lot of other things happening depending on body roll. turn angle of the steering and castor angle. Outside a formula 1 team its all to much to try to compute :lol: So set it up for straight ahead and hope the rest works OK. If racing an Elan and playing with caster, camber , toe in and roll stiffness its trial and error unless you happen to be mates with a formula 1 designer and their simulation software. There is other commercial suspension design software but not sure if its much use in these circumstances

cheers
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PostPost by: nthSteve » Mon Jul 22, 2019 11:27 pm

vxah - Not to be pedantic, but you have to have the steering arm(s) attached, since the Toe (wheel steering angle) depends upon the steering arm arc as well as that of the suspension itself. Basically we want to mimic going over bumps or under braking, as Rohan said.
-Steve, SoCal, '72 Sprint DHC
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PostPost by: Andy8421 » Tue Jul 23, 2019 6:09 am

nthSteve wrote:vxah - Not to be pedantic, but you have to have the steering arm(s) attached, since the Toe (wheel steering angle) depends upon the steering arm arc as well as that of the suspension itself. Basically we want to mimic going over bumps or under braking, as Rohan said.


Steve,

I think vxah has a good point, one which I had not considered before. Setting bump steer can only be correct at the straight ahead position. when the steering is turned, the pivot points of the track arms move relative to the suspension mounting points and any bump steer setup is lost.

I guess a smarter man than me could design the suspension to minimise this effect, (perhaps it is already taken into account with the position of the rack), but at first sight, Rohan's point is well taken, bump steer settings are only for the straight ahead position.
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