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-   -   for those who've adjusted front bumpsteer... (http://986forum.com/forums/performance-technical-chat/77430-those-whove-adjusted-front-bumpsteer.html)

jmitro 03-23-2020 07:14 AM

for those who've adjusted front bumpsteer...
 
...on a lowered car with standard (non-GT3) wheel carriers, and adjustable bump steer kit.

did you space the tie rod end CLOSER or FARTHER away from the steering knuckle?

I have the Rennline adjustable bump steer kit with H&R sport springs (GT3 lower control arms also) on my Boxster, and trying to achieve essentially zero bumpsteer. But no matter if I'm using NO spacers or ALL spacers, I'm getting what appears to be toe out on full suspensions compression. What am I doing wrong?

maytag 03-23-2020 08:41 AM

Quote:

Originally Posted by jmitro (Post 613860)
...on a lowered car with standard (non-GT3) wheel carriers, and adjustable bump steer kit.

did you space the tie rod end CLOSER or FARTHER away from the steering knuckle?

I have the Rennline adjustable bump steer kit with H&R sport springs (GT3 lower control arms also) on my Boxster, and trying to achieve essentially zero bumpsteer. But no matter if I'm using NO spacers or ALL spacers, I'm getting what appears to be toe out on full suspensions compression. What am I doing wrong?

Following this.
I haven't yet... but I probably need to.
I think I understand the premise of how it works.... but I'm in sponge-mode right now. :dance:

Deadeye 03-24-2020 03:49 AM

shimming the outer end of the tie rod will correct the geometry for a limited travel of the suspension but you will still have some toe change at full compression.

truegearhead 03-24-2020 09:25 AM

This is something I need to do as well. Reading up on it they talk about grip and bump steer separately, are they truly separate or does bump steer directly effect grip? I can't seem to find a clear statement. My car is lowered as much as it can without comprising my control arm angles but I haven't felt anything of the negative effects I've read up on.

jmitro 03-24-2020 03:18 PM

thanks guys; I posted this on the Rennlist 996 forum also since it gets a lot of traffic.

what I'm trying to do is dial out (what I perceive to be) understeer on corner entry and midcorner. It occurred to me that, after installing springs/shocks along with adjustable tie rod ends, I hadn't set my bump steer.

It's PROBABLY not necessary, as the bump steer is probably minor, if anything, and the understeer is likely driver-induced rather than setup. ( I need more practice :D)

Additionally, I think my alignment was off (per my home measurement of toe, I had 1/8-3/16" of total front toe out. So I've got some other changes to make as well.

Running a square setup with 255/40/17 RE71R on 8.5" rims all around.

Gilles 03-24-2020 03:23 PM

Jmitro, do you have adjustable bars? These will allow you to adjust the turn in and rotation to your like :-)

maytag 03-24-2020 03:52 PM

Good read here.
Technical Articles

truegearhead 03-25-2020 03:37 AM

Quote:

Originally Posted by maytag (Post 613975)
Good read here.
Technical Articles

That’s a great summary. Still I don’t see any actual grip impact and I’ve never noticed my car “steering itself” I only drive the car on smooth tracks so maybe this is why. I’m curious if 986/996 really need bump steer adjustment.

steved0x 03-25-2020 06:31 AM

Quote:

Originally Posted by jmitro (Post 613969)
what I'm trying to do is dial out (what I perceive to be) understeer on corner entry and midcorner.
...

Running a square setup with 255/40/17 RE71R on 8.5" rims all around.

With a square 255 setup on all 4 corners, you should have tremendous amounts of front grip. The time I drove a spec boxster with a square setup like this the front grip was incredible. The challenge with a square setup is to tame the oversteer at the limit.

Even with my 225 front 255 Boxster I didn't have any understeer trouble on corner entry, especially if I had any amount of trailbraking, even like a big toe worth.

I did have mid-corner understeer on long sweepers like at Roebling.

And then I would have a little understeer on full throttle track out.

What sway bars are you running, and if adjustable, what settings do you have them on?

What brand of springs/shocks and if adjustable, what settings do you have them on? Since you talk about ride height, I am guessing coilovers?

I have seen 2 cars now that had terrible oversteer due to too stiff springs in the rear and also set too low and running out of strut travel. I've never seen one that had that condition in the front but maybe it is possible?

Racer Boy 03-25-2020 07:20 AM

Quote:

Originally Posted by jmitro (Post 613969)
what I'm trying to do is dial out (what I perceive to be) understeer on corner entry and midcorner. It occurred to me that, after installing springs/shocks along with adjustable tie rod ends, I hadn't set my bump steer.

It's PROBABLY not necessary, as the bump steer is probably minor, if anything, and the understeer is likely driver-induced rather than setup. ( I need more practice :D)

Additionally, I think my alignment was off (per my home measurement of toe, I had 1/8-3/16" of total front toe out. So I've got some other changes to make as well.

You say that the perceived understeer may be driver-induced. What do you mean by that? Are you giving the steering wheel a large input all at once while braking hard, or something else?

How much track driving experience do you have? Sorry for the questions, I just want to understand what is happening.

Straying too far from zero-toe will cause understeer, whether it's too much toe-in or too much toe-out. 3/16" of toe-out is a LOT.

Bump steer refers to toe change as the suspension moves through it's travel, either up or down. All suspensions have bump steer. You will get toe change when the suspension reacts to a bump, but you will get the same amount of change if the suspension reacts to body lean caused by hard cornering (lateral loads).

jmitro 03-25-2020 09:23 AM

thanks all for the excellent responses. the longacre article is good information; I looked at that a few days ago.
here's a link to the rennlist thread which was also helpful:
https://rennlist.com/forums/996-forum/1186896-for-those-who-ve-adjusted-front-bumpsteer.html#post16503630

my suspension is really a street setup- H&R sport springs with Bilstein B8s; it's not coilovers, so ride height is not adjustable. Similarly, the sway bars are stock non adjustable. The only adjustments are front toe and bump steer, rear toe, and GT3 front control arms for additional camber.

I've raced with BMWCCA for about 5 years so I have a lot of track experience, but I think my problem is a tendency to turn in too early and hit the apex early or midpoint which means sometimes breathing the throttle on exit to get the nose to turn. It depends on the turn, but generally higher speed turns. So my sensation of "understeer" might be a combination of poor turn in and suboptimal car setup as well

Gilles 03-25-2020 09:57 AM

Quote:

Originally Posted by jmitro (Post 614012)
my suspension is really a street setup- H&R sport springs with Bilstein B8s; it's not coilovers, so ride height is not adjustable.
Similarly, the sway bars are stock non adjustable.

Jmitro, H&R has a great set of sway bars that are 'Very' reasonable priced :-)

jmitro 03-25-2020 03:47 PM

So I inserted three 1/8" spacers between the tie rod end and steering knuckle, then took the car to get a professional alignment today. F/R toe was pretty far out.

Final settings were:
Front: 1 degree total toe out, -3.0 camber
Rear: 0.1 degree total toe in (was trying to get zero toe), -2.0 camber
Pretty excited to try these new settings at my next track day.

Thanks all for the suggestions. I'll look into more adjustability including sway bars as I progress in the future :cheers:

bcrdukes 03-25-2020 05:31 PM

Quote:

Originally Posted by Gilles (Post 614014)
Jmitro, H&R has a great set of sway bars that are 'Very' reasonable priced :-)

I don't mean to steal the thunder from the original poster, but aren't there reports that the H&R sway bars are loud and make crashing noises? I've been tempted to get a set, but there are varying experiences.

Gilles 03-26-2020 08:11 AM

Quote:

Originally Posted by bcrdukes (Post 614038)
I don't mean to steal the thunder from the original poster, but aren't there reports that the H&R sway bars are loud and make crashing noises? I've been tempted to get a set, but there are varying experiences.

Bcrdukes, I had the H&R sway bars on my '07 CS for more than 10 years and never heard a noise from them, the connecting links yes as they wear down and had to be replaced, but ended getting a set of adjustable links from Tarett and never had an issue again.

The H&R is bigger than the OEM bar on the CS and does not have adjustment, you balance the bars (and the car) with the rear bar.

bcrdukes 03-26-2020 11:16 AM

Thanks, Gilles. I think that the other complaint is that they are bigger (and thus heavier) so a lot of owners did not like them.

(Sincere apologies to jmitro for stealing your thread!)

steved0x 03-26-2020 12:37 PM

Quote:

Originally Posted by jmitro (Post 614012)
my suspension is really a street setup- H&R sport springs with Bilstein B8s; it's not coilovers, so ride height is not adjustable. Similarly, the sway bars are stock non adjustable. The only adjustments are front toe and bump steer, rear toe, and GT3 front control arms for additional camber.

I had a student that had that setup, and the car felt fantastic! He was on 225 front 255 back though, Hankook R-S4.

Quote:

Originally Posted by jmitro (Post 614012)
I've raced with BMWCCA for about 5 years so I have a lot of track experience, but I think my problem is a tendency to turn in too early and hit the apex early or midpoint which means sometimes breathing the throttle on exit to get the nose to turn. It depends on the turn, but generally higher speed turns. So my sensation of "understeer" might be a combination of poor turn in and suboptimal car setup as well

With 255 square and stock swaybars, you should have no understeer except maybe some power on understeer at low speeds on full throttle if you have the wheel turned a lot. I like to turn in a little early with slower hands, and unless it is a really fast corner I have some brake on all or most of the way down to the apex, and I try and make it so that I can go full throttle at the apex, and this takes me out to the track edge. (unless it is a double apex or a chicane which are different) When I have a student with a higher powered car (most recently a 996TT) we had to take later apexes due to the difference in power.

If that BMW you mentioned has less power than the Boxster, it may be like you said, (I wouldn't call it poor turn in, just a different style based on a different car :cheers: )

Do you have any track video of your current setup?

spendy 03-28-2020 03:10 PM

jmitro - As Steve mentioned, most people have oversteer when going square. The camber difference front/rear should reduce understeer as you are increasing front grip vs the rear.

MaxD 03-28-2020 03:51 PM

Quote:

Originally Posted by spendy (Post 614405)
jmitro - As Steve mentioned, most people have oversteer when going square. The camber difference front/rear should reduce understeer as you are increasing front grip vs the rear.

Exactly. -3 to -3.5 rear camber is common running square. Also it is common to disconnect or run a smaller (stock) rear bar when using a GT3 front bar.

Play around and see what you like.

Similar to running an E36 with no rear bar and a square setup.

jmitro 03-29-2020 06:52 AM

I'm anxious to get back on the track and see how it feels. unfortunately most of the events around here have been canceled or postponed for a month or two so it will be some time before I can get a feel for the car. I think I screwed up the alignment doing it myself, so hopefully with the changes above the car will feel different.

my BMW was a euro-powered E36 M3 with 320hp and about 2500lb so this Boxster is a different experience!


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