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Hopping on corner exit

 
Old May 6, 2019, 10:22 AM
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Hopping on corner exit

No changes to my car other than a TRE rear diff and going from a 255 NT01 to 275 NT01.

Now, when exiting under full throttle in right turns (think autox), the rear of the car is bouncing and I'm worried about breaking an axle.
It used to just get a bit sideways and I could control it with the throttle. Now it's gripping and bouncing.

The rear diff may has about 300-500 miles on it, I think. It's on the original assembly fluid; I'm finally due to change it.

I upped the rear rebound dampening once, don't think it helped, upped it again but didn't get to try it out.

Ideas? Thanks, in advance!
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Old May 6, 2019, 10:36 AM
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Hopping in the evo is a ***** to fully solve. Every time you fix it then add more grip it seems to come back. There are two version of the hop as well, corner entry and corner exit. Entry is fixed with the front uprights fixing RC and soft front swaybar. I have yet to experience it on entry since I made those changes.

On throttle is a bit harder. Too much rear bar and too much rear compression seem to make it worse for me. Dropping compression a bit and adding rebound does seem to help, as well as less rear bar for more rear spring.

I also imagine too much sidewall flex in the tire will make things worse as well. So if you dont run enough tire pressure or are pushing tire width vs wheel width, things will be worse.

Unfortunately theres no single smoking gun fix. its circular and with more grip comes more hop. 315s at crows brought out just a little in my setup but it was very little with the low power (under 400) I was running.
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Old May 6, 2019, 10:43 AM
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I'll check my notes on tire pressure. I know a couple tires were rolling over a bit, so I was going to add more psi. I can't remember which.
The tire has an upright sidewall and a slightly narrower tread width. It almost looks like an ever-so-slight stretch.
I'm on a 25'm RSB on the next to softest setting and a 10k spring.
I can't adjust compression, so that's out.

I'll start with the notes on tire pressure and go from there.
Sounds like I just to try new things until it goes away.

Just when I had the car set the way I liked it. : lol:

Should this be a problem on a road course? If it's autox only, then I can totally deal with that.
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Old May 6, 2019, 10:48 AM
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I am glad someone made a thread about this! I used to think the hopping was normal with cars that had crazy grip. If there is a systematic way to fix it, even better!
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Old May 6, 2019, 10:51 AM
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Originally Posted by kaj View Post
Should this be a problem on a road course? If it's autox only, then I can totally deal with that.
High steering angles and low gears exacerbate the problem for me. Road courses should be fine in most cases.
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Old May 6, 2019, 11:00 AM
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I have a feeling I'll be also dealing with this issue on corner exit. I just got my diff back from TRE (Cusco 141FG) and I'm trying to put some gentle miles on it to break it in before my first autocross this coming weekend. I've noticed on super tight turns, she likes to start bouncing. I can only imaging that effect is going to be multiplied once pushed! Granted, I've also been seeing how the Reinharte's are handling some street miles. I was running full soft on rebound, high/low speed compression(s) so that probably may have attributed to some of it.
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Old May 6, 2019, 11:09 AM
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Originally Posted by kaj View Post
I'll check my notes on tire pressure. I know a couple tires were rolling over a bit, so I was going to add more psi. I can't remember which.
The tire has an upright sidewall and a slightly narrower tread width. It almost looks like an ever-so-slight stretch.
I'm on a 25'm RSB on the next to softest setting and a 10k spring.
I can't adjust compression, so that's out.

I'll start with the notes on tire pressure and go from there.
Sounds like I just to try new things until it goes away.

Just when I had the car set the way I liked it. : lol:

Should this be a problem on a road course? If it's autox only, then I can totally deal with that.
The corner entry version is more of an issue on track with diving into corners. Usually power out of corners in 3rd or 4th doesnt put enough torque to the ground to drive the issue. It really seems to be when you are in that territory of spinning the tires on exit. 2nd is of course much easier to get to that point.
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Old May 6, 2019, 11:21 AM
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Originally Posted by Construct View Post
High steering angles and low gears exacerbate the problem for me. Road courses should be fine in most cases.
Originally Posted by Dallas J View Post
The corner entry version is more of an issue on track with diving into corners. Usually power out of corners in 3rd or 4th doesnt put enough torque to the ground to drive the issue. It really seems to be when you are in that territory of spinning the tires on exit. 2nd is of course much easier to get to that point.
This is what I'm hoping. The car did great otherwise, so I'd like to not mess with it.
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Old May 6, 2019, 12:08 PM
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My car does that hopping autocrossing on corner exit but never on track. TRE diff, big rear sway, fairly stiff coilovers.
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Old May 7, 2019, 11:19 AM
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yea normally too much grip in the rear will cause hopping. or if things are just too tight in general.

Dallas will be able to answer but im not sure if its got anything to do with rake/corner weight also? maybe less corner weight on that outside rear corner and it wont dive and hop so much on power?
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Old May 7, 2019, 11:51 AM
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I've fought with corner exit hopping on track for the past year now. Usually happens in 2nd but I've managed it in 3rd before.

Check your rear bushings. I swapped out for all poly in the rear suspension this winter as well as in the diff mounts. Hoping that well help a tad.

Shock tuning and tire pressure changes help as well.
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Old May 7, 2019, 11:54 AM
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Originally Posted by ayoustin View Post
I've fought with corner exit hopping on track for the past year now. Usually happens in 2nd but I've managed it in 3rd before.

Check your rear bushings. I swapped out for all poly in the rear suspension this winter as well as in the diff mounts. Hoping that well help a tad.

Shock tuning and tire pressure changes help as well.
If you changed the lower rear control arm to all poly, change the outer to ends back to spherical. Because of the trailing arm and its steep angle for anti-squat there is quite a bit of arc the lower control arm moves through. With poly on either of those two outer point you are asking the suspension to bind and twist on the poly instead of freely move an OEM-like spherical. There is definitely a reason the OEM arm has sphericals in those two locations.
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Old May 7, 2019, 01:29 PM
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Originally Posted by ayoustin View Post
I've fought with corner exit hopping on track for the past year now. Usually happens in 2nd but I've managed it in 3rd before.

Check your rear bushings. I swapped out for all poly in the rear suspension this winter as well as in the diff mounts. Hoping that well help a tad.

Shock tuning and tire pressure changes help as well.
All my bushings are poly except the sphericals Dallas mentioned. I ALMOST pressed those out but, luckily, they caught my eye before. My mustache bar has stock bushings, though.

You can hear it at 3:35 and 4:13, so does it both directions. I was down a couple psi in two tires, so there's that.
My TOB gave out before I could work on pressures, rebound, and My driving line.


Last edited by kaj; May 7, 2019 at 01:35 PM.
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Old May 7, 2019, 02:08 PM
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Originally Posted by Dallas J View Post
If you changed the lower rear control arm to all poly, change the outer to ends back to spherical. Because of the trailing arm and its steep angle for anti-squat there is quite a bit of arc the lower control arm moves through. With poly on either of those two outer point you are asking the suspension to bind and twist on the poly instead of freely move an OEM-like spherical. There is definitely a reason the OEM arm has sphericals in those two locations.
All OE sphericals stayed. Just the rubber stuff got replaced.
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Old May 7, 2019, 02:23 PM
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Originally Posted by ayoustin View Post
All OE sphericals stayed. Just the rubber stuff got replaced.
Werd, just had to have this convo with another about aftermarket lower arms and not running them. With my rear uprights, I think the binding angle will actually break those control arms. Should probably just make an appropriate replacement for people but I'm vehemently opposed to having a threaded end for adjustment with all the load of the shocks going through it in bending.
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