Optimal bump steer?

[baileyman]

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

Just measuring my 26R setup. I have a stack of 1/8" shims five deep each side for 0.625". I know this sounds high, but one must consider that the geometry is about the same as the standard Elan already 1" into bump. So raise the wheels on a standard Elan an inch and then set the rack.

Further, after modeling the situation, I aimed at setting the rack at the bottom of the roughly zero bump steer zone, as I saw no reason to be concerned about droop steer. And so far, no reason has shown up.

John

[snowyelan]

Hi John,
If steering links were bent inwards with the 26R setup as is typical then its not really apples to apples. The rack tie rod lengths will be shorter. I can easily change the model to suit but would need to know what the new dimensions are.

Got a bit more done on the modelling. Its a bit of a tough go due to different dimensions from different sources.

Steering rack ball to ball:
WSM 698.5mm 27.5"
Spitfire drawing 694.69mm 27.35"
BB book 692.40mm 27.26" (used this in the model)

Steering rack tie rod end to tie rod end:
As modelled based on available drawings 1065.92mm 41.96" Used this for the model)
As per BB book 1071.88mm 42.2"

Steering arm offset #4 in the above drawing:
Spitfire/herald drawing 9.32mm 0.367"
BB book 7.925mm 0.312" (used this in the model)

By the tape measurements:
The lower trunnion is from the dimensions supplied above
The seal space between the trunnion and upright is modelled at 6.35mm. This could in theory vary quite a bit.

The 55.63mm (2.19") from the center of the lower pivots and the 158mm from the top of the upper pivots is within 1/3mm of each other so both methods are pretty much interchangeable.

Preliminary results in degrees per side are:
0.22 toe out at 1" droop
0.27 bump toe in at 1" compression
0.61 bump toe in at 2" compression

I have toe set at 0 for the above which I want to change to toe in. This shortens the tie rods a bit which will change things...a tiny bit..I am also going to use the spitfire herald dimensions to see what the difference ends up being. Standby.....

edited to add steering rack plus tie rods dimensions.

[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

Just measuring my 26R setup. I have a stack of 1/8" shims five deep each side for 0.625". I know this sounds high, but one must consider that the geometry is about the same as the standard Elan already 1" into bump. So raise the wheels on a standard Elan an inch and then set the rack.

Further, after modeling the situation, I aimed at setting the rack at the bottom of the roughly zero bump steer zone, as I saw no reason to be concerned about droop steer. And so far, no reason has shown up.

John

Droop steer is a term I have not heard of before. Maybe its just a different name for bump steer. My take on bump steer is that it is 'unwanted change of wheel direction resulting from suspension movement'. Whether its suspension movement above or below the ride height is a bit irrelevant, its still undesirable.

Maybe on a race car with limited suspension travel and on a flat track, steering geometry can be tweaked because there will be predictable for each corner.

Ian

[Foxie]

Just measuring my 26R setup. I have a stack of 1/8" shims five deep each side for 0.625". I know this sounds high, but one must consider that the geometry is about the same as the standard Elan already 1" into bump. So raise the wheels on a standard Elan an inch and then set the rack.

Further, after modeling the situation, I aimed at setting the rack at the bottom of the roughly zero bump steer zone, as I saw no reason to be concerned about droop steer. And so far, no reason has shown up.

John

Droop steer is a term I have not heard of before. Maybe its just a different name for bump steer. My take on bump steer is that it is 'unwanted change of wheel direction resulting from suspension movement'. Whether its suspension movement above or below the ride height is a bit irrelevant, its still undesirable.

Ian

I think Elanman has a good point here. It would be straightforward to calculate the tyre loading at say 2" compression and the decreased loading at 1" droop. I would imagine that the increased grip coming from the extra tyre loading would be much more likely to increase the effect of any steering deflection on bump, so the "droop steer" would be much less significant than the "bump steer".

[Andy8421]

I think Elanman has a good point here. It would be straightforward to calculate the tyre loading at say 2" compression and the decreased loading at 1" droop. I would imagine that the increased grip coming from the extra tyre loading would be much more likely to increase the effect of any steering deflection on bump, so the "droop steer" would be much less significant than the "bump steer".

I have also given this some thought. As pointed out by Elanman, in a racing situation, bump is more likely than droop to be an issue. The front suspension will go into bump under braking because of weight transfer, or in a corner when the outer wheel gets loaded. It is the 'squirrely under braking' issue that bump steer is supposed to cause, and general corner instability. Although the inner front wheel will be in droop in a corner, it will be lightly loaded and won't contribute much to the party. A case in point is Jim Clark's Lotus Cortina drives with the inner wheel in mid air in the corners.

Just to add more complexity to the issue, in a corner, the rack isn't centered, and the inboard steering balljoint will be in a different place to straight ahead - which will have an effect on bump steer. It might be interesting to see what happens to bump steer for a series of different steering angles. Possibly there is a compromise to be had between minimising bump steer with the steering straight ahead, and minimising bump steer at (say) 10 or 20 degrees of lock. Perhaps this is the root of the suggestions that Lotus didn't aim for zero bump steer in the straight ahead position.

It all gets more complicated the more you look into it.

Andy.

[snowyelan]

Hi All,

Did a bit more polishing on the modelling. Below are the assumptions for unknown/unverified dimensions. There’s quite a list so if anyone can verify them or, even better, provide a drawing with some pedigree/traceability it would be appreciated.

-After thinking about it for a bit I decided the Triumph rack numbers were probably the best to use. The Brian Buckland numbers are very close.

-I used the Brian Buckland rack tie rod center to center dimension of 42.2” to set the tie rod pivot ball position cross car. The upright drawings I previously posted didn’t dimension the steering link arm mounting boss relative to the spindle axis cross car, so I had to settle for the 42.2” dimension for positioning. I did a reality check against my car, and it was pretty close.

-The steering link arm drawing was used for height and fore/aft position of the ball joint. Cross car was ignored due to the issue above, and conflicting information between triumph and Brian Buckland drawings

-I couldn’t find a dimension for the tie rod ball joint ball center so I used 26mm from the steering link top surface to the center of the ball based on what I measured on my car.

-The triumph upright has a dimension to what I believe is the theoretical position of the upper ball joint. I used this dimension.

-I used 25.4mm for both of the trunnion dimensions I was looking for previously.

-Arms level with the chassis vacuum tank bottom, not ground due to arm sweep/3 deg caster.

-3deg caster (Later angle).

-Steering rack bottom face set to 2.19” above the lower pivot C/L.

-Steering rack bottom to center of rack 24mm (measured from my car).

Arms level with 3mm toe total

1 inch bump

2 inch bump

1 inch droop (pretty much zero toe)

Links projected arms level

Links projected 1in bump

Links projected 2in bump

Links projected 1in droop

If I can get some steering arm/upright documented dimensions I will play around with the ackerman/steering angle for those interested.

[Bigbaldybloke]

Nice work, does your model allow you to calculate the roll centre position?

[snowyelan]

I didn't design it specifically for roll center but all the bits are there to calculate it.

[snowyelan]

Roll center below. The lines appear not to line up with the pivots due to the castor. The lines intersect the mid point of the pivot pins fore/aft. Tire size and contact patch compression impact position.

[baileyman]

That's beautiful work. I'm pondering why I did not expect as much toe change. Hmmm.

I think I remember the standard Elan was designed with a bit of positive camber. Maybe that changed.

John

[snowyelan]

Decided to see how things looked in roll with differing steering angles at stock ride height.

5 deg roll, 10 deg steer

5 deg roll, 5 deg steer

5 deg roll, 0 steer

10 deg roll, 10 deg steer

10 deg roll 5 deg steer

10 deg roll, 0 deg steer......Fun!

[snowyelan]

And now for some opposite lock fun!!! Quite impressive that the tire doing the majority of the work stays pretty much vertical.