Tech Tips: Bump-steer effects from ball joint and caster changes
Sep 14, 2017, 02:38 PM
#1
Tech Tips: Bump-steer effects from ball joint and caster changes
I'm working on the V2 uprights and making a few refinements and thought I would share some of my info on bump-steer effects from Whiteline ball joint, Whiteline Tie Rod, SSB Control Arms, and SSB Uprights.
This list became relevant for me as I'm working on the range of bump-steer adjustment and for OEM, WL, and SSB ball joints making sure I have whats needed to hit what I think is a magic caster number or down to close to stock if that's your jam.
A couple bits of info measuring parts, I'll give you the change from OEM with a +/- 1 mm tolerance. Plenty good for getting in the ball park before actually measuring your real bump steer.
Ball Joints
Whiteline - 7 mm longer
SSB Ball Joint - 10.8 mm longer (part of the control arms)
Tie Rods
WL - 8 mm longer
I could post pictures of the CAD model to show how I got to all these scenarios but the info is the important part. Thinking about the clearest way to present the info, a simple table seemed reasonable.
So pick your combo, see if it matches with how much caster you've added.
Another couple bits of info on what you get from caster.
Perrin PSRS: +1.25 deg (IIRC)
Cusco Offset Bushing: ?
10 mm Top Hat Shift: +0.85 deg (Roughly, depends on ride height)
Rake Adjustment: 1/2" = 0.27 deg
And for what I'm calling magic, feeling pretty good now about +7.5 deg caster. Running 9+ seemed to cause more understeer at higher steering angles. It worked really well at lower angles but really punished you if you over drove as grip really fell off at higher angles. This was felt by a couple others I threw in the car as well.
+7.5 is easy enough for me to reach with 1.5 on top and 2.5 at the control arm (3.5(Stock) +1.5 + 2.5 = 7.5) and gets my wheels forward to clear the back of the fender. Its also how my 295s nearly don't rub.
This list became relevant for me as I'm working on the range of bump-steer adjustment and for OEM, WL, and SSB ball joints making sure I have whats needed to hit what I think is a magic caster number or down to close to stock if that's your jam.
A couple bits of info measuring parts, I'll give you the change from OEM with a +/- 1 mm tolerance. Plenty good for getting in the ball park before actually measuring your real bump steer.
Ball Joints
Whiteline - 7 mm longer
SSB Ball Joint - 10.8 mm longer (part of the control arms)
Tie Rods
WL - 8 mm longer
I could post pictures of the CAD model to show how I got to all these scenarios but the info is the important part. Thinking about the clearest way to present the info, a simple table seemed reasonable.
So pick your combo, see if it matches with how much caster you've added.
Another couple bits of info on what you get from caster.
Perrin PSRS: +1.25 deg (IIRC)
Cusco Offset Bushing: ?
10 mm Top Hat Shift: +0.85 deg (Roughly, depends on ride height)
Rake Adjustment: 1/2" = 0.27 deg
And for what I'm calling magic, feeling pretty good now about +7.5 deg caster. Running 9+ seemed to cause more understeer at higher steering angles. It worked really well at lower angles but really punished you if you over drove as grip really fell off at higher angles. This was felt by a couple others I threw in the car as well.
+7.5 is easy enough for me to reach with 1.5 on top and 2.5 at the control arm (3.5(Stock) +1.5 + 2.5 = 7.5) and gets my wheels forward to clear the back of the fender. Its also how my 295s nearly don't rub.
Sep 14, 2017, 02:53 PM
#2
Dallas
Is higher-speed understeer perceived at high cafe setting in AX or track situation?
And how much steering angle?
Wondering if at some point a lot of caster at higher steering angle actually decreases contact patch?
What refinements are you considering?
Is higher-speed understeer perceived at high cafe setting in AX or track situation?
And how much steering angle?
Wondering if at some point a lot of caster at higher steering angle actually decreases contact patch?
What refinements are you considering?
Sep 14, 2017, 03:08 PM
#3
Right now with -3deg camber and somewhere north of 9deg caster it seems that somewhere around 15deg steering angle it starts to fall apart and push. So when I hit those hairpin turns or overshoot something and you crank the wheel to near lock my butt says grip is falling off.
I really only ran this much just to see if that was even usable and now feeling like thats just too far. 7.5 should get right back to a magic spot where 3degs is appropriate.
As for refinements, there's some changes in the shape of the steering mount, adding a little stiffness, and I think cleaning up some of the look. Nothing wrong with the old parts, and I'll still run them but the next ones are just a move in a more refined direction. New parts will also get etched and hard anodized.
Im also making two version with a 1.25" and 2" correction. I may also make an OEM and 10mm ball joint option on the control arms. The 1.25" is specifically requested and I can see the need for something with more street ride height adjustment. 2" + the 10mm ball joint is a lot of correction of mostly for us trailer queen cars. Shorter setup will also hopefully work for those that want to keep 17" wheels, maybe even certain 16" wheels.
I really only ran this much just to see if that was even usable and now feeling like thats just too far. 7.5 should get right back to a magic spot where 3degs is appropriate.
As for refinements, there's some changes in the shape of the steering mount, adding a little stiffness, and I think cleaning up some of the look. Nothing wrong with the old parts, and I'll still run them but the next ones are just a move in a more refined direction. New parts will also get etched and hard anodized.
Im also making two version with a 1.25" and 2" correction. I may also make an OEM and 10mm ball joint option on the control arms. The 1.25" is specifically requested and I can see the need for something with more street ride height adjustment. 2" + the 10mm ball joint is a lot of correction of mostly for us trailer queen cars. Shorter setup will also hopefully work for those that want to keep 17" wheels, maybe even certain 16" wheels.
Sep 14, 2017, 03:24 PM
#4
EvoM Guru
iTrader: (1)
So, with the whiteline RCK (ball joint and tie rod), and 6.8* of caster, I have a lot of bumpsteer...LOL
Sep 14, 2017, 03:27 PM
#5
Yep, and its toe out in compression so it will make the front feel very sharp cause as the car rolls the steering is basically increasing. But the easy improvement is to just swap back to OEM tie rods. It will also massively improve tramlining, at least fixing bump-steer on my car sure made a world of difference on our ****ty highways.
Sep 14, 2017, 03:44 PM
#6
EvoM Guru
iTrader: (1)
Yep, and its toe out in compression so it will make the front feel very sharp cause as the car rolls the steering is basically increasing. But the easy improvement is to just swap back to OEM tie rods. It will also massively improve tramlining, at least fixing bump-steer on my car sure made a world of difference on our ****ty highways.
I think (hope) I kept my stock tierod ends.
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Sep 14, 2017, 04:05 PM
#8
EvoM Guru
iTrader: (1)
Hmm. I like the way the car turns in. And the tram lining isn't terrible...
What is the inside corner doing (rebound)?
What is the inside corner doing (rebound)?
Sep 14, 2017, 04:09 PM
#9
Your car will turn in just as well, but it will do it with youre hands controlling the wheel instead of the leaning car causing it. The bad side of bump steer is of course hitting a bump and having toe changes. Its just creating a nervous car for no benefit.
Sep 14, 2017, 04:12 PM
#10
EvoM Guru
iTrader: (1)
So, I'll just have to get used to a bit more steering input. Gotcha.
Sep 14, 2017, 11:56 PM
#11
Evolved Member
Right now with -3deg camber and somewhere north of 9deg caster it seems that somewhere around 15deg steering angle it starts to fall apart and push. So when I hit those hairpin turns or overshoot something and you crank the wheel to near lock my butt says grip is falling off.
is this with stock wheel position or is the wheel moved forward?... as when you move the wheel forward (or back) your ackerman will change.. And that might influence the understeer because your relative toe left to right will change differently to stock..
Sep 15, 2017, 07:14 AM
#12
Im not sure how your seeing any significant ackerman effect. Any of the geometric changes is only minor in overall effect of ackerman.
One thing I plan to try next year is to make a steering arm version to try out less ackerman, maybe 50% less? I don't know what will be right and that will probably depend on what others use the car for but all way fun to experiment.
One thing I plan to try next year is to make a steering arm version to try out less ackerman, maybe 50% less? I don't know what will be right and that will probably depend on what others use the car for but all way fun to experiment.
Sep 15, 2017, 07:42 AM
#13
Evolved Member
well according to my model moving wheel line 15 mm forward is worth about 2 deg more total track difference at full lock. Moving wheel forward, or rack back, will increase ackerman. This is probably not that signifficant at lower steering lock but might want to keep it in mind if you push the wheel forward a lot.
I have seen what negative ackerman does to a lotus 7 clone (those thend to run negative ackerman on high speed tracks) when you run them on slow circuits so am always keen to check that too.. In fact my single seater that I am building will have adjustable ackerman just to see the effect..
anyway, I am not saying that is the problem, just an option...
I have seen what negative ackerman does to a lotus 7 clone (those thend to run negative ackerman on high speed tracks) when you run them on slow circuits so am always keen to check that too.. In fact my single seater that I am building will have adjustable ackerman just to see the effect..
anyway, I am not saying that is the problem, just an option...
Last edited by kikiturbo; Sep 15, 2017 at 08:34 AM.