2008 Rear Suspension Part II: Toe Change/Bumpsteer - Page 5 - Subaru Impreza WRX STI Forums: IWSTI.com
STI Forum
Join
Start New Thread Create Account Advertise FAQ Members List Contact
2008 Rear Suspension Part II: Toe Change/Bumpsteer
2008 Rear Suspension Part II: Toe Change/Bumpsteer
Go Back   STI Forums > > > > 2008 Rear Suspension Part II: Toe Change/Bumpsteer
Closed Thread
 
Thread Tools
Old 09-12-08, 07:00 AM   #41
Turninconcepts.com
Account Inactive
Feedback Score: 67 reviews
 
My Subaru Parts for sale

Join Date: Jul 2003
Turninconcepts.com is an unknown quantity at this point
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Quote:
Originally Posted by stretch View Post
TiC, I'm not sure your analysis is completely accurate. You mention toe-in occuring above or below ride height because the toe link is horizontal to the ground. I know we talked about this, and I couldn't understand why Subaru would do this. I found the error.

The false assumption is that the wheel travels perpendicular to the ground, which would be a tangent to the toe link at ride height. This does not happen since the wheel travels in an arc determined by the upper and lower control arms.

At droop, it appears the lower control arm and toe link are- for the most part- parallel. (Confirm this, I have only looked at the pictures featured here.) This means that- despite neither arm being parallel to the ground- there will be no initial toe change (at least for the moment the links are parallel). Only camber and the track width will change.

However, since the two links are different lengths, the links become further from being parallel under compression, creating the toe-in. The toe curve will accelerate as the angle between the two links grows. Since the links appear to be parallel at full droop, you'll get toe-in starting very gradually (but accelerating) at anything beyond full-droop. This also means toe-out going from ride height back to full droop, albeit a minimal amount since that's towards the beginning of the toe curve.
that's not what we've seen, but we can remeasure.

I just remembered we have the 08 up in the air for a pad swap before Tony goes to instruct at Putnam this weekend. I placed a straight edge between the lower arm pivots and took a look at how it compares to the toe arm. they are close to parallel, but not quite. The toe arm droops a little more. Also keep in mind that the toe arm is shorter than the control arm so changes in arc travel between the two will not be consistant through travel. (edit: whoops - just noticed that you pointed out the different lengths).

I keep reading through this and either way, yours or mine, it's not good. toe in on the loaded tire (in both our statements) = not good. Toe in on the unloaded tire (in my statement) = ok, but not the best. Toe out on the unloaded tire (in your stament) = not so good, and much like the behavior I outlined on the results of lowering the car.

Last edited by Turninconcepts.com; 09-12-08 at 07:10 AM.
Turninconcepts.com is offline  
Likes 0 Dislikes 0
 
Sponsored Links
Advertisement
 
Old 09-12-08, 07:34 AM   #42
stretch
Advanced Mastery Level II
Feedback Score: 4 reviews
 
Fav Mod: Seat Time
My Subaru Parts for sale

Join Date: Mar 2006
Location: Columbus, GA
stretch is on a distinguished road
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Quote:
Originally Posted by Turninconcepts.com View Post
that's not what we've seen, but we can remeasure.
I wasn't sure if you had actually measured, but if you did, then completely ignore what I said. I've done no measurements, and the pictures I've seen could be misleading. I thought your spreadsheet was predicted toe, not measured toe.

However, I wouldn't draw the conclusion that rear toe-steer in this direction is bad. I'm not saying the opposite is true either. IIRC, it'll hurt trade some turn-in for straight line stability, but it'll cause the thrust of the rear tires to point to where the car is steering. This should help corner exit (and is also why rear-drive cars don't usually run toe-out). Worth it? Dunno.
stretch is offline  
Likes 0 Dislikes 0
 
Old 09-12-08, 08:08 AM   #43
Turninconcepts.com
Account Inactive
Feedback Score: 67 reviews
 
My Subaru Parts for sale

Join Date: Jul 2003
Turninconcepts.com is an unknown quantity at this point
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Quote:
Originally Posted by stretch View Post
I wasn't sure if you had actually measured, but if you did, then completely ignore what I said. I've done no measurements, and the pictures I've seen could be misleading. I thought your spreadsheet was predicted toe, not measured toe.

However, I wouldn't draw the conclusion that rear toe-steer in this direction is bad. I'm not saying the opposite is true either. IIRC, it'll hurt trade some turn-in for straight line stability, but it'll cause the thrust of the rear tires to point to where the car is steering. This should help corner exit (and is also why rear-drive cars don't usually run toe-out). Worth it? Dunno.
THAT spreadsheet is predicted and just happens to use the toe arm length. it was just used for demonstration purposes to illustrate what I was talking about.

I'm still not sure on the toe steer either. We have an idea to do passive 4 wheel steering using the toe arm, and it would be really trick in turns, but at the same time would make the rear a little less planted feeling in bumps in a straight line.

that one I think we're going to have to build and real world test as we just can't predict the feeling of what it'll end up doing on something like a daily driver.
Turninconcepts.com is offline  
Likes 0 Dislikes 0
 
Sponsored Links
Advertisement
 
Old 10-09-08, 07:08 PM   #44
caveguru
Developing Advanced Mastery
Feedback Score: 0 reviews
 
Fav Mod: hammer down!
My Threads
Posts: 971
My Subaru Parts for sale

Join Date: Jun 2008
Location: Gaithersburg, MD
caveguru is an unknown quantity at this point
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Had the car on the alignment rack today, so we tried to measure rear toe change on compression and extension. Although the rack may not have been able to measure this acurately, we tried anyway. After the normal alignment was finished, we lifted the rear of the car up, re-leveled the machine's heads, and measured the toe. Got toe-in on extension this way, but not very much- maybe 0.04 degrees if the machine was reading accurately. Then we lowered it back down with three 2x4's stacked under one wheel and then re-leveled the heads. This results in toe-in on the opposite (extended) side and toe-out on the compressed side. Toe-out was around 0.14 degree, which is fairly significant to my understanding of toe behavior. Tire was not completely up in the well yet, so there should be more toe-out under greater compression. Again, all this depends on the machine being able to read the toe accurately when one corner is up off the alignment rack. Also, my rear's ride height is around 13.9" with the 275/35/18's mounted. The toe was set to zero initially at this ride height.

Frank
caveguru is offline  
Likes 0 Dislikes 0
 
Old 10-10-08, 10:16 AM   #45
Chayse
Becoming A Master Contributor
Feedback Score: 6 reviews
 
Fav Mod: 18x9.5 w/ 275/35-18
My Threads
Posts: 634
My Subaru Parts for sale

Join Date: Apr 2005
Location: SE PA
Chayse is on a distinguished road
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Quote:
Originally Posted by caveguru View Post
Had the car on the alignment rack today, so we tried to measure rear toe change on compression and extension. Although the rack may not have been able to measure this acurately, we tried anyway. After the normal alignment was finished, we lifted the rear of the car up, re-leveled the machine's heads, and measured the toe. Got toe-in on extension this way, but not very much- maybe 0.04 degrees if the machine was reading accurately. Then we lowered it back down with three 2x4's stacked under one wheel and then re-leveled the heads. This results in toe-in on the opposite (extended) side and toe-out on the compressed side. Toe-out was around 0.14 degree, which is fairly significant to my understanding of toe behavior. Tire was not completely up in the well yet, so there should be more toe-out under greater compression. Again, all this depends on the machine being able to read the toe accurately when one corner is up off the alignment rack. Also, my rear's ride height is around 13.9" with the 275/35/18's mounted. The toe was set to zero initially at this ride height.

Frank
Hey Frank,
So, basically, you are seeing the exact opposite behavior as what TiC predicted? The compressed wheel, i.e. the outside wheel in the turn, actually toes out, and the unloaded wheel toes in? That should be what we want, right? Although the ratio of toe change for the loaded wheel to the unloaded wheel is somewhat out of balance based on your measurements.
Did I read that correctly?
Chayse is offline  
Likes 0 Dislikes 0
 
Old 10-10-08, 10:36 AM   #46
caveguru
Developing Advanced Mastery
Feedback Score: 0 reviews
 
Fav Mod: hammer down!
My Threads
Posts: 971
My Subaru Parts for sale

Join Date: Jun 2008
Location: Gaithersburg, MD
caveguru is an unknown quantity at this point
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Quote:
Originally Posted by Chayse View Post
Hey Frank,
So, basically, you are seeing the exact opposite behavior as what TiC predicted? The compressed wheel, i.e. the outside wheel in the turn, actually toes out, and the unloaded wheel toes in? That should be what we want, right? Although the ratio of toe change for the loaded wheel to the unloaded wheel is somewhat out of balance based on your measurements.
Did I read that correctly?
You read correctly, but there are two very big caveats:

1) I did not speak to the manufacturer of the alignment rack to determine what effect lifting one end of the car has on the accuracy of the toe measurement.

2) The difference in the change in rear toe could have been due to several things, including a non-linear toe-change curve due to the complexity of the stock suspension or the likely event that I did not have the left side extended by the same amount as I had the right side compressed (because the sway bar was still connected and the car is a square, not a triangle, so the 4 corners are not obligated to put equal loads on each wheel).

I think the bottom line here is that a better method is needed to measure the toe change in the rear, but it is by no means certain that the rear toes-in on compression.

Frank
caveguru is offline  
Likes 0 Dislikes 0
 
Old 10-10-08, 05:20 PM   #47
Turninconcepts.com
Account Inactive
Feedback Score: 67 reviews
 
My Subaru Parts for sale

Join Date: Jul 2003
Turninconcepts.com is an unknown quantity at this point
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Alignment racks have to be specifically designed to allow that kind of measurement. THe one we are looking at will do it with a $5000 add on module. Otherwise, even at $60000 it won't.
Turninconcepts.com is offline  
Likes 0 Dislikes 0
 
Old 10-21-08, 05:02 PM   #48
caveguru
Developing Advanced Mastery
Feedback Score: 0 reviews
 
Fav Mod: hammer down!
My Threads
Posts: 971
My Subaru Parts for sale

Join Date: Jun 2008
Location: Gaithersburg, MD
caveguru is an unknown quantity at this point
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

I thought about this some more. Because the WL rear camber bushings are effectively changing the length of the upper control arm, they are also changing the camber curve and most likely the toe curve. This could be making a significant difference.

The good news (for maximum performance) is that my earlier measurements above were done with the rear camber set at -2.1, which had the bushings most of the way in. In other words, as the upper control arm got shorter, it may have changed the toe curve to toe out on compression, which we would expect. With my preliminary feel that ~2 degrees of rear camber will be a good choice for the track, and based on the rough measurements above, I see the WL rear camber bushings as a good temporary solution to solving the toe curve concern.

Since the bushings also change the camber curve, I'll try to measure that next time I'm in as well. I think I'll be able to produce accurate measurements if I compress the suspension by using straps to pull the car down while keeping both wheels planted on the rack. This will prevent the error from having the alignment heads at different heights. Of course, with 450 lb/in springs, I'd better use some really strong straps and choose my attachment points carefully! Likewise, I could have gotten a more accurate reading at max extension if I'd stopped raising the car just before the wheels left contact with the rack.

Frank
caveguru is offline  
Likes 0 Dislikes 0
 
Old 10-21-08, 07:03 PM   #49
altaic
Advanced Contributor Level 2
Feedback Score: 0 reviews
 
Car: DGM '08 STI
Fav Mod: Coming Soon
My Threads
Posts: 238
My Subaru Parts for sale

Join Date: Sep 2007
altaic is an unknown quantity at this point
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Why not just toss ten or twenty bags of sand in the back? Seems safer than straps.
altaic is offline  
Likes 0 Dislikes 0
 
Old 10-21-08, 07:20 PM   #50
caveguru
Developing Advanced Mastery
Feedback Score: 0 reviews
 
Fav Mod: hammer down!
My Threads
Posts: 971
My Subaru Parts for sale

Join Date: Jun 2008
Location: Gaithersburg, MD
caveguru is an unknown quantity at this point
Default Re: 2008 Rear Suspension Part II: Toe Change/Bumpsteer

Quote:
Originally Posted by altaic View Post
Why not just toss ten or twenty bags of sand in the back? Seems safer than straps.
hmmm, assuming the KW max compression is close to stock (it's probably a bit more) and my ride height of ~13.8", I should have around 1.8" of bump travel (based on part one of this thread). At 450 lb/in, that would mean I need 810 lbs. With the kw's bumpstops, an even thousand should ensure success. Since this would be at andrewtech, that's how many 6 speed trannies? four?

but seriously, I doubt I'll actually use the strap idea. Probably just back the perches out and have someone sit in there. good point.

Frank
caveguru is offline  
Likes 0 Dislikes 0
 
Sponsored Links
Advertisement
 
Closed Thread

Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


New To Site? Need Help? More

All times are GMT -8. The time now is 04:43 AM.


Powered by vBulletin
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Designed & Powered by Domain Architect
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.