Body roll under power questions

[Waternut]

So I have some questions regarding the wheeling characteristics of my YJ. I'm running semi triangulated 4 link setups with 14" coilovers front and rear. The Jeep does absolutely amazing but I have some concerns that maybe you guys can help with.

When I'm in 2wd, there is some minor torque related body roll but when I'm 4wd it's considerable. For example, in 4lo and in gear with my foot on the brake, the front drivers corner will move several inches in either direction depending on how much gas I give it or how bound up the tires are. Generally it's not a problem but this torque roll (not sure if that's a real term) can be very alarming if I'm on a side slope. I did the rear 4 link first and ran leaf springs in the front, this wasn't even an issue. However, when I did the front 4 link, it was very noticeable immediately. Here's a video I just made showing what I mean. I think the last roll at around 55 seconds is probably the most violent and I revved the engine to about 1300rpms with my foot on the brake.
https://www.youtube.com/watch?v=RnPZvR18vvs

Is this just part of having soft slinky suspension or is there something I can do to minimize it?

I'm not currently running any kind of anti-sway bar and have considered adding one but this would be my only reason for wanting it.

In case it helps or my suspension setup is grossly to blame.... My rear anti-squat is 90% and my front anti-squat is 120%. My Jeep only weighs 3800lbs so my spring rates are 150/110 in the front and 130/95 in the rear and I sit 9-10" in the shock travel at ride height.

 

[Treefrog]

Hopefully someone with more experience will comment, but it looks to be torque roll.

It,s part of having a soft slinky suspension, and as far as I know there are only four ways to reduce it.

1. Move the diff from the axle to the body (independent suspension).
2. Outboard the springs as much as possible, their reaction to the roll increase as the square of the distance to the roll axis.
3. Have lower axle gearing. This results in less torque trying to roll the vehicle for the same amount of torque at the wheel.
4. Run a sway bar.

 

[Waternut]

I guess if a sway bar does limit this, that is probably the best solution.

 

[Rockbuggy84]

I had the same issue with my last rig. My Coilovers were straight up and down like yours.

Front- Moved upper mount to inside of tower and lower as close to ball joint on the “c” as possible.

Rear-moved upper to inside of tower and lower out as far as possible.

Huge difference.

 

[Waternut]

I had the same issue with my last rig. My Coilovers were straight up and down like yours.

Front- Moved upper mount to inside of tower and lower as close to ball joint on the “c” as possible.

Rear-moved upper to inside of tower and lower out as far as possible.

Huge difference.

Thanks. Funny that I was considering that last night and ordered up some new shock mounts. I recently moved my fenders up and that's opened up the option of moving the shocks around a bit without messing with EVERYTHING else. My shocks aren't even vertical and they actually lean the wrong way by a few degrees so I'll work on that. Looks like I should be able to go from negative 3-4 degrees to 5-10 positive by moving the shocks outboard off the axle tube and onto the C. Maybe more if I can clear the frame and move the shocks more inboard.

 

[mobil1syn]

post a side shot at ride height.

 

[Waternut]

post a side shot at ride height.

Sorry...Jeep has been sitting in the garage half stripped. This is the closest thing I've got after the fender hi-line mod until I can get it back together. Not sure what you're after but the front shock angles forward slightly and the rear shock angles forward a bit more.

 

[rattle_snake]

Dang that is a lot of roll for just switching gears.

 

[arse_sidewards]

Where are your front and rear roll centers? The closer the overall roll center line is to your crank center-line the more efficiently torque will translate to body roll.

 

[Waternut]

Where are your front and rear roll centers? The closer the overall roll center line is to your crank center-line the more efficiently torque will translate to body roll.

Suspension roll center is 8-9" below the vehicle roll center but it is almost right on top of the crank center line. Hadn't heard that before but it makes sense.

 

[Ghetto Fab.]

I'm going with too much anti-dive in the front. The rear being 90% also doesn't help the situation.

Just my opinion.

Kevin

 

[Waternut]

I'm going with too much anti-dive in the front. The rear being 90% also doesn't help the situation.

Just my opinion.

Kevin

You'll have to explain that one to me. Not sure I understand how anti-squat/anti-dive translates into torque roll.

 

[Ghetto Fab.]

You'll have to explain that one to me. Not sure I understand how anti-squat/anti-dive translates into torque roll.

Basically the jeep chassis is trying to drive over the front axle when your on the brakes. The torque roll sets this in motion and the high anti dive magnifies it. Its acting like a short arm TJ with a ton of lift.

I really don't know what is common these days, but 20yrs ago I was taught to keep the rear anti-squat to 60-70% and the front at 40-50%. I've been hearing lately that a higher rear anti-squat percentage(90-100%) works out better for shock tuning, but haven't heard why yet so I still lean to lesser numbers.

I'm guessing if you raised your frame side upper link mounts in the front up 2-3" on the frame things would be better. Obviously that is probably one of the hardest solutions.

Kevin

 

[Waternut]

Basically the jeep chassis is trying to drive over the front axle when your on the brakes. The torque roll sets this in motion and the high anti dive magnifies it. Its acting like a short arm TJ with a ton of lift.

I really don't know what is common these days, but 20yrs ago I was taught to keep the rear anti-squat to 60-70% and the front at 40-50%. I've been hearing lately that a higher rear anti-squat percentage(90-100%) works out better for shock tuning, but haven't heard why yet so I still lean to lesser numbers.

I'm guessing if you raised your frame side upper link mounts in the front up 2-3" on the frame things would be better. Obviously that is probably one of the hardest solutions.

Kevin

Thanks. That makes a lot of sense and explains a lot. I was shooting for the higher AS numbers to limit the changes in suspension/tire loading under acceleration. My plan for the high anti dive was to get a slight squat in the front under fwd acceleration. I believe it does do that on flat ground but realistically, that condition never exists in rock crawling. What does happen is I hammer the throttle when I'm stuck or essentially holding the brakes (with a rock) which causes the high anti dive to raise the front of the vehicle.

I'll look into lowering the anti dive but like you said, it probably won't be easy. Then again neither is fixing the Jeep if I roll it bc the torque roll put me on my side. So may be worth it.

 

[Treefrog]

Basically the jeep chassis is trying to drive over the front axle when your on the brakes. The torque roll sets this in motion and the high anti dive magnifies it. Its acting like a short arm TJ with a ton of lift.

I really don't know what is common these days, but 20yrs ago I was taught to keep the rear anti-squat to 60-70% and the front at 40-50%. I've been hearing lately that a higher rear anti-squat percentage(90-100%) works out better for shock tuning, but haven't heard why yet so I still lean to lesser numbers.

I'm guessing if you raised your frame side upper link mounts in the front up 2-3" on the frame things would be better. Obviously that is probably one of the hardest solutions.

Kevin

I'm not convinced that its the anti-dive that is the biggest issue. If anything its trying to dive the front less because its high, which would keep it from compressing the passenger corner more.

As for anti-squat and shock tuning, 100% means that the tuner doesn't have to deal with tuning for the car reacting when accelerating, just for the suspension reacting to bumps. That said there are downsides to high AS or we wouldn't see go fast cars tending towards flatter upper and lower links.

Upon watching that video again, particularly starting from around 55s at 25% speed, the passenger side shock really doesn't change length until after the body has rolled. This is because once it has entered steady state the shock forces have to split the weight, and the driver side has less on it. But the important part here is that because the passenger side doesn't compress during the roll it means that it isn't reacting to that torque. Instead its tucking under the body, towards where it is pivoting. So based on that, RockBuggy's advice of outboard the lower end of the shock and inboard the upper end is probably the best solution since it means that the body will try to roll into the spring instead of around the spring.

 

[Waternut]

I'm not convinced that its the anti-dive that is the biggest issue. If anything its trying to dive the front less because its high, which would keep it from compressing the passenger corner more.

As for anti-squat and shock tuning, 100% means that the tuner doesn't have to deal with tuning for the car reacting when accelerating, just for the suspension reacting to bumps. That said there are downsides to high AS or we wouldn't see go fast cars tending towards flatter upper and lower links.

Upon watching that video again, particularly starting from around 55s at 25% speed, the passenger side shock really doesn't change length until after the body has rolled. This is because once it has entered steady state the shock forces have to split the weight, and the driver side has less on it. But the important part here is that because the passenger side doesn't compress during the roll it means that it isn't reacting to that torque. Instead its tucking under the body, towards where it is pivoting. So based on that, RockBuggy's advice of outboard the lower end of the shock and inboard the upper end is probably the best solution since it means that the body will try to roll into the spring instead of around the spring.

Thanks. I ordered up some steel to move the shock mounts a week ago but for some reason it's being extra slow and still hasn't even shipped. I plan to tackle that first and see what it does.

 

[Gordon-B]

Just spitballing here but drag racers have been putting a bit more antisquat on the passenger side for years now.

I'm not good enough with geometry to tell you how to translate that to the front axle but if your links / brackets are adjustable you could reduce the AS on one side and see how the Jeep reacts. If it's better or whatever then just keep adjusting accordingly.

I have a homebuilt 4 link with triangulated uppers and in 2wd I can launch full throttle and the driver side lifts maybe 2" over the passenger side so I'm considering dropping the AS on the driver side or raising it on the passenger side to see what happens.

My AS was designed to be around 50-60% but my build ended up a bit higher than anticipated so it's probably more like 60-70%

 

[Gordon-B]

Just an update, I tried out uneven AS, more on passenger side rear. I guestimated the numbers into the 4-link calc and it put me around 50% driver side, 65-ish passenger side.

It is definitely more level but you can feel it pushing from the passenger side now.

If you've ever ridden in a powerful inboard boat, you will understand the feeling of launching and feeling like you are going a bit sideways even though the boat is level (Steering trim).

It's kind of like that.

The push feels like it comes more from the passenger side than directly behind you.

Also instead of being unsettled under max power, now it shimmys a bit when you let off.

I may try putting the passenger upper link back at the center hole and raise the driver side to decrease the AS to around 40% on the driver side. That may give a similar decrease in torque roll but without the shimmy.

It is interesting to experiment at least.

 

[Waternut]

Well before I start messing with uneven anti-squat, I decided to change the shock angle and see how that does. I went from about 4 degrees negative angle to 9 degrees positive. Hopefully this helps because it was a lot of work. Nearly free to do short of consumables but WAY more work than I anticipated due to various things I have under the hood like alternator welder, full hydro hoses, brake lines, and the coolant overflow bottle. I still need to find a home for all of that stuff but hopefully I'll be able to at least do a dry run in the garage this weekend.

 

[Gordon-B]

It looks like you not only moved the body side in but also moved the axle side out.

I think, or would guess that the axle side moving out will have a bigger impact than moving the body side in.

In fact, you may get slightly lower roll stiffness (narrower mounts at body side) when the body is perfectly centered, however the stiffness will increase as the body rolls whereas before it was stiffest at center and got worse and worse as the body rolled over.

Hopefully this makes a difference for you.

I am going to try the same thing mounting the shocks to the top of the inner C. Should help a lot with roll. From what I understand, you want the shock to be as far out as possible on the axle and also as far out on the body, but it should have a slight angle (as yours does) to provide increasing roll stiffness as the body rolls.

Once the "axle - to - shock" angle passes 90 degrees you are back to decreasing but that is some serious body roll on the street. On the trail it is of less concern since your rear is often pushing you the other direction.

Exception to this would be side hill stability and this is where even light weight sway bars really do help.

 

[Waternut]

Well I finally got it all back together...or at least most of it and enough to test it. The old shock mounts were as far outboard on the axle tube as they would go and the frame mounts were as far inboard as they would go while allowing the shocks to clear the frame. Changing that was no small act since moving the shock mount outboard means moving it up about 3" onto the top of the inner C and moving the axle mount up means moving the frame side up and moving the frame side up means everything has to move and the shocks have to be mounted more inboard or they'd go through the fender.

So far it seems to have helped a lot. There is still a big squat when I put it in reverse but the forward roll is greatly reduced. I do still need to mess with spring preload and get it right. Changing the shock angle forced me to take out about an inch of preload on the springs to make them sit down and the front still looks a little high so I need to tune my shocks again.

 

[Treefrog]

How much are the rear shocks leaned in? It will have some effect on the feel

 

[Waternut]

How much are the rear shocks leaned in? It will have some effect on the feel

It's leaned in the correct way by about 8 degrees. I'm less worried about what happens when I throw it in reverse but it was just something I noticed. Ultimately, I'm calling it a success for now considering I'm still not running a sway bar.

 

[Treefrog]

It's leaned in the correct way by about 8 degrees. I'm less worried about what happens when I throw it in reverse but it was just something I noticed. Ultimately, I'm calling it a success for now considering I'm still not running a sway bar.

Its not so much about reverse as it is about the front and rear being coupled and effecting each other.

 

[Gordon-B]

To piggyback off this, if the front end roll stiffness is higher than the rear the car will tend to push thru a turn.

If the rear is higher than the front, well it'll tend towards oversteer.

From the sound of it you have greatly stiffened up the front which will keep you more stable at speed, but it may be worth testing to see how the rear end behaves when turning hard. Try it on dirt or when it's rainy to get a feel for what will happen at low speed / low grip. The same thing will likely happen at high speed / high grip, but if you aren't prepared, it can end poorly.


as I'm sure you are well aware, a lighter spring with more preload will be better off road but also give more body roll than a stiffer spring pack with less preload.

If you are running high speed in the desert you need preload to force the shocks to extend quickly.

If you are mainly trail riding preload is much less important as long as your springs are not so stiff that you cannot bottom them out when twisted up.

Also, congrats on fixing it