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IRS 3 Link Trailing Arm Toe Change Over Travel

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troberts9893 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote troberts9893 Quote  Post ReplyReply Direct Link To This Post Topic: IRS 3 Link Trailing Arm Toe Change Over Travel
    Posted: Oct/20/2014 at 11:13pm
Currently in the middle of designing a 3 link trailing arm rear suspension. I've been running into some issues with toe change over travel which I did not initially foresee. As long as there is 0 degrees of toe at ride height, is it acceptable to have some toe change over? 


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Richie_Dagger View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Richie_Dagger Quote  Post ReplyReply Direct Link To This Post Posted: Oct/21/2014 at 12:47am
Some guys might want some toe change in travel to change handling, like roll steer. Some guys might want to keep it constant. It depends on what you want your suspension to do.
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joey.m.gonzalez View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote joey.m.gonzalez Quote  Post ReplyReply Direct Link To This Post Posted: Oct/21/2014 at 1:32am
There is no way to avoid getting some toe change with a 3-link trailing arm suspension.  When I was designing the one for our car, I was able to get it down to an acceptable amount of change.

Good luck.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ks_sid Quote  Post ReplyReply Direct Link To This Post Posted: Oct/22/2014 at 4:46am
keeping constant toe over the wheel travel in three link suspension is difficult. look for the changes in the stability during the turns and bumps and make changes accordingly. Toe out at the rear outer tire can help during the turn by reducing the turning radius.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Soccerdan7 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/22/2014 at 11:54am
People tend to talk about roll steer as being a good thing and bump steer as being bad, but they are really just the same thing. 

You can design a passive roll steer system that toes out the outer wheel when in compression and maybe toes in your inner rear a little too in droop (although the inner is a little less important depending on how much weight transfer and roll stiffness you have). What is bad about this though is you will scrub off a lot of speed on every bump, jump, log and obstacle as when both wheels are compressed and rebound together, they both toe in or out at the same time. This effect is not too bad if you run a stiff suspension with relatively small travel, but if you run 12-14 inches of travel at a very low rate like we did it is a huge problem. This is the main reason why we ran a 3 + 1 link IRS in 2012 as it functioned like a pure 3 link but had independent toe control as well, so we completely eliminated any bump steer and help us carry tons more speed through bumps and jumps.
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paasch View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote paasch Quote  Post ReplyReply Direct Link To This Post Posted: Oct/22/2014 at 1:19pm
I agree with Dan. Toe change in the rear suspension might be useful in maneuverability, but in the higher speeds of endurance it's a bad thing. I've watched BajaSAE cars with so much toe change in the rear that they became unstable coming out of a high speed 90, oscillating back and forth as unwanted roll steer threw the back end from one side to the other as the driver fought for control.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Oct/22/2014 at 2:48pm
Our 2005 car with rear steer had a tie rod mount point which had close to zero toe change for endurance, accel, etc; and a different mounting location for land manuv. which had significant toe change to help with getting the car to turn.  Easy to do, and nice to incorporate early in design so that if you find that it's not what you expected, you can "turn it off" without having to burn metal.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pj_rohlf12 Quote  Post ReplyReply Direct Link To This Post Posted: Oct/23/2014 at 5:02pm
Through experience there will be toe change with any trail link system, no matter if 3 link, 5 link, 5 link modified. For there to be no toe change the pivot point would have to be located directly in line with center of wheel.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andy.R Quote  Post ReplyReply Direct Link To This Post Posted: Feb/22/2015 at 10:01pm
As said earlier, a large part of the 3-link is having toe in on your outer tire while cornering. One way to reduce the amount of tow during travel is to change the angle of the swing arm. The more in line the arm is with the car, the less toe you will have. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pj_rohlf12 Quote  Post ReplyReply Direct Link To This Post Posted: Feb/26/2015 at 10:40pm
make the swing arm rotate parallel to the CL of the car and boom no toe.
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