PDA

View Full Version : Bump Steer



BitHed
11-15-2005, 06:31 AM
...this came up somewhere else and wasnt really addressed...I tremembered it and did some BASIC reseacrh but I am actually MORE confused then when i didnt know AYNTHING!!

...All this info I got from Longacre Racing Online (http://www.longacreracing.com/articles/art.asp?ARTID=13)

...this is what bump steer is...

"Bump Steer is when your wheels steer themselves without input from the steering wheel. The undesirable steering is caused by bumps in the track interacting with improper length or angle of your suspension and steering linkages."

...and this seems to be the fix...

"In order to accomplish zero bump the tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper a-arm pivot and the lower control arm pivot. In addition, the centerline of the tie rod must intersect with the instant center created by the upper a-arm and the lower control arm."

...There is more info at Longacre but I still dont understand it...

QUESTION: Do i need to CHANGE the ANGLE (forward or back) of something or raise something up or down? They mention shims but where do i put them? I know the KYOSHO boys addressed this but buggered if I can figure it out...help :confused:

acsubie
11-15-2005, 10:23 AM
Bit i was able to fix my "bump" with a little experimenting....what i ended up doing was switching the ball cups with captured ends, i also used some of the pivot balls that have the "lip" on both sides... that seemed to help the angle...it might not be 100%, but 98% is good enough for me, i'll take some pics of the before and after once i get ahold of a camera.

BitHed
11-16-2005, 02:39 AM
...well well well...first time up at bat and i think i fixed it...pictures first...

...FRONT LEFT...


http://halfeight.com/gallery/data/500/30011_20687_025.jpg

...FRONT RIGHT...

http://halfeight.com/gallery/data/500/30011_20687_023.jpg

BitHed
11-16-2005, 03:53 AM
...I replaced the 5mm balls with KYOSHO 5.8 hard balls (#W1036) and added the big fat wide KYOSHO 5.8 ballend (#1295)...Its all held together with a 3x12 (?) screw and the all important RED nut...Pretty quick and extremely simple...

..So as i understand it, compression of the shocks makes either or both wheels TOE-IN, move inwards at the front...Well that doesnt happen anymore...Its pretty cool...

...BUT...The problem has moved...

...Now when i steer i only get the very OUTSIDE edge of the inside wheel and the very INSIDE edge of the outside wheel; ie turning left means i get the leftside EDGES of each wheel...I am figuring this is a side effect of changing these parts...I am sure it ios going to cause some HORRIBLE understeer :eek: But fixing that is the NEXT challenge...

pointcrossed
11-19-2005, 01:49 PM
Cant believe no one else has responded to this, anyways it is not much of a science as the site makes it. The diagram is for a real vehicle that has smaller tires in scale to an RC car the larger tires do hide bump steer a bit as well as the caster angle. From my past racing kits I eliminated the bump steer by placing washers (shims) under the inner or outer ball studs if they are long enough i know that after I upgraded to the 3 racing steering components that I had more flexibility with adjusting for bump steer...

I hope this helps...

Hawk
11-19-2005, 04:06 PM
...I replaced the 5mm balls with KYOSHO 5.8 hard balls (#W1036) and added the big fat wide KYOSHO 5.8 ballend (#1295)...Its all held together with a 3x12 (?) screw and the all important RED nut...Pretty quick and extremely simple...

..So as i understand it, compression of the shocks makes either or both wheels TOE-IN, move inwards at the front...Well that doesnt happen anymore...Its pretty cool...

...BUT...The problem has moved...

...Now when i steer i only get the very OUTSIDE edge of the inside wheel and the very INSIDE edge of the outside wheel; ie turning left means i get the leftside EDGES of each wheel...I am figuring this is a side effect of changing these parts...I am sure it ios going to cause some HORRIBLE understeer :eek: But fixing that is the NEXT challenge...
Bit, remember that what you see sitting on the bench looks entirely different when the car is in a dynamic state like driving down the track and turning left.
Usually, the chasis rotates around the roll center. The camber change you are seeing on the bench is an adjustment to the chassis lean. This is usually more extreme with an off road vechicle as they are tossed about by bumps and such. It is 4 wheel drive and the tires tend to need to be more round across the top to be more forgiving of a rough surface.
If you are goning to drive it on a prepared (asphalt or concrete) roadway, you can do away with lots of suspension travel, tighten the sway bars to limit chassis rotation. You would also take out a lot of the camber change too. You do have the ability to change all these things with the car you have but it is purpose built to off road environments.
The other thing to remember is the changes in camber, and ackerman offset are different at ride height than at full compression or full extention. Only use static settings to acheive the handling charateristics on the track you desire.
And remember, it is a very dynamic environment.
d :cool:
P.S. Pointcross, it is not science, it is witchcraft at its finest.

pointcrossed
11-19-2005, 04:39 PM
That was very well put Hawk...

I did not have witchcraft in mind but it is some form of magic.