Bump Steer

[Momus]

I did some work on an RX3 Mazda and was running it around at Calder. The suspension was all someone elses. A Nissan Clayton factory suspension and later Gibson Race team engineer observed during testing and gave the diagnosis. From the driver's seat the car lurched into corners and seemed to lock down on the bump stops and would stump jump plow understeer until the throttle was lifted. It was difficult to finesse.
This cars settings which included grippy R radials a low and mobile roll center, moved out LCA with high king pin inclination (around 16 or 17 degrees) lots of scrub radius, and inadequate roll stiffness and springs and probably std wet legs was a prime candidate.

[jsa]

Thanks Marcus

[Gary P.]

[Mik Kay]

Over the last few days i have completed the bump steer check with the new bumpsteer gauge and it is virtually identical to the corrected version of the laser graph. Out by a lot!!. I made up a cad drawing ages ago of the front suspension and had copied all the dimensions over from this drawing as a base line, obviously i have made a mistake and the drawing is wrong. Re measured setup and transferred to 50% scale graph paper. Problem solved. In the process of drawing this graph out a couple of questions have arisen. The first is a general question concerning the position of the 'instant centers' when finding the roll centre. Is there a preferred general position for this point to intersect or does it not matter??. At the moment the graph is showing a roll centre of 130mm at centre and the roll couple of 224mm off centre L from the ground. 2) Please bare with me on this one. If the same RX3 as described in Momus's post about downward jacking is transferred over from understeer to a gravel corner where the car is deliberately placed into oversteer to complete the corner. What would be the major changes if any, in the front suspension loads as the front wheels are turned to correct the slide. Would this not decrease compression on outside front wheel and with good weight transfer and throttle application decrease this amount further??.

Thanks

Mik

[Momus]

On a strut the IC is the projection of the line drawn through the LCA pivots and the top pivot at the angle of the leg. (The roll couple is the distance between the RC and the height of the center of gravity)
Where the two imaginary lines intersect, probably within a few metres is the IC. The RC is the point where a 3rd imaginary line from the IC to the tyre contact patch passes the car centerline.
The position of the IC and RC is going to vary continuously during movement and seeing you have a bilaterally symetrical suspension, as do all known cars, you also have two roll centers much of the time. Mostly you are worried about the one on the loaded side but the other one is there. It may be better to assume there is a force based apportioning of the roll center.

To answer the down jacking on gravel question I'd say that the ground clearance issues would mean a sensible LCA angle (well up inboard) and low grip and a down jacking effect would be very unlikely.

With the mods you have and the removal of the stabiliser bar things could be quite different and some relatively large and severe adverse geometry changes are likely.

[Mik Kay]

Momus could you please elaborate a little more on "some relatively large and severe adverse geometry changes are likely". Due to 2" rain over the weekend i thought it might be a good opportunity to go and have a play. The front seemed to turn fine considering how wet it was so i think i will keep this as my base line setup. The car did have some push but with a locked diff i expect this. Will now add the plate type LSD and start playing with the 4 link bars and panhard rod positions to see if i can find a little bit more improvement.

Mik

[Momus]

You will get te geometry changes from bodyroll.
And almost certainly if your front springs and dampers are able to prop up the loaded side plenty of understeer.

[Mik Kay]

Anyway thought i would post results after fixing the front end woes and testing it out for 45 min. After some miner adjustment to most of the steering i got the bump steer down to 020 or 1/2 mm toe out. Tried with and without 18 mm swaybay and really didn't feel the difference. i didnt really notice any 'major' understeer even with the bias slightly to the front and braking heavy. Steering is now much lighter and am rapped with the results.

Mik

[gen mk 1rs2000]

the better you get the geometry
the lighter & better it will feel
wich also means more confidence & easier to drive , easier on tyres etc
usualy makes for going quicker too !!