Roll Center Explained - Suspension Design

[Treefrog]

Similar setup, but with the swaybar arm angled. M[SUB]sb[/SUB] is different because of the angle between the swaybar link and the lower link.

In this setup, changing y and leaving x the same changes the install ratio (effective ratio). When y increases, the swaybar link length increases, meaning that the sway bar arm is shorter, increasing the rate.

I left off the upper link in the diagram for simplicity.

 

[Stephen Wilson]

You also have to consider the motion ratio of the trailing arm. The effective swaybar rate is measured at the wheel, so the effective arm length X is much longer, extending to the hub centerline. The same way wheel rate is affected by the spring motion ratio on A-arm suspensions. With the swaybar mounted basically at the trailing arm pivot, it becomes a torsion bar.

 

[Treefrog]

You also have to consider the motion ratio of the trailing arm. The effective swaybar rate is measured at the wheel, so the effective arm length X is much longer, extending to the hub centerline. The same way wheel rate is affected by the spring motion ratio on A-arm suspensions. With the swaybar mounted basically at the trailing arm pivot, it becomes a torsion bar.

There are some other factors as well.

1. The angle at which it leans in throughout flex affects it as well, seen as a change in swaybar link length in the side view.
2. The direction of the force at the wheel and having to correct it to vertical.
3. The axle end of the link not being at the centerline of the axle.

1 can be large and has more change as the sway bar link pivot gets closer to the axle. 2 is generally small when the links are close to horizontal. 3 only occurs if the swaybar is connected to the suspension link and is almost always very small.

 

[Stephen Wilson]

Yup, #1 is why a high-mounted swaybar is a good idea. The 2 ways to minimize rate change from link angularity is obviously to use a longer arm, less obvious is a longer the end link reduces the angle created by the arc of the arm and trailing link .

 

[bdkw1]

The 2 ways to minimize rate change from link angularity is obviously to use a longer arm, less obvious is a longer the end link reduces the angle created by the arc of the arm and trailing link .

You say that like rate change is a bad thing. I count on it and plan for it.

 

[Treefrog]

You say that like rate change is a bad thing. I count on it and plan for it.

It can be good if you plan for it, but it can be bad if you don't.

 

[Stephen Wilson]

You say that like rate change is a bad thing. I count on it and plan for it.

True, coming from road-racing it's considered a bad thing, for articulation I suppose it would be a good thing. Either way, something to be aware of and accounted for.

 

[bdkw1]

True, coming from road-racing it's considered a bad thing, for articulation I suppose it would be a good thing. Either way, something to be aware of and accounted for.

There are a lot of things that you can tune more easily when you have 30" of travel as opposed to getting it all done in under 6". But some things are applicable. Variable grip in the rear is 1, good traction at ride height and increased rate/less grip as it heads towards bump makes for a much easier to drive car. It also lets you run a lighter bar. Sway bars are evil, they are an undamped spring that you are just throwing into the mix. The lighter bar you can get away with the better off you'll be. Running bars front and rear lets you really cut down on the rates and gain a lot of traction in the rear exiting corners.

 

[HQA]

The Busted Knuckle videos are brilliant and in the Roll Center Video when talking about the roll axis front to rear (after minute 11 in video) he said the roll axis needs to "slope from rear to front downward" and that made me realize what might be my problem and my roll axis goes the wrong way.

I am guessing my roll axis is why my rig has such poor handling at high speed on and off road. The rig leans over side to side rocking back and forth unlike any off road rig I have. I have eliminated a lot of that rocking side to side with stiffer shock valving and swaybars but its still there.

On highway if I swerve fast to avoid something it is like a tail heavy trailer going out of control and needs calm minimal steering input to bring back under control.

The rig is a 1997 Land Cruiser with coil overs and solid hum9 portal axles. The handling got worse when I added 600-800 lbs of weight to the rear axle by adding a cargo box and a 40 inch spare tire to the rear of the box.

Originally 20 yrs ago, I added the portal axles and cut 500 plus lbs off by removing most of the top, interior and bobbing the rear. I also raised the front axle panhard 5 inches from stock and left the rear stock location.

3 years ago I put the roof back on, rear seats back in and added the Australian style cargo box behind a chopped cab at the C pillar. I have put over 6k miles of highway and off road driving on it, 600 mi round trips to Rubicon, Shaver Lake area and Death Valley. I don't want to trailer the rig.

Could my roll axis slope from rear to front upward and the added weight on the rear be compounding the handling issue?

I am in the middle of raising (edit I meant lowering) the front panhard 5 inches however I want to know what you guys think.

 

[gt1guy]

HQA

How did it handle 20 years ago after you removed all the weight?

To your question, yes, the roll couple axis and the addition of weight on the rear will work together to make your handling suck. But you really only need to look at the rear by itself. Adding a bunch of weight up high and not adjusting the height of the rear RC will make the rear handle different.......spring rates being the same.

Your last sentence is confusing. Your going to raise the front RC even higher without any change to the rear? That will make things worse. If anything, I'd raise the rear RC and leave the front alone. Get the roll couple axis going down hill to the front and then start looking at spring rates.

Are you linked and coilovers front and rear? Got any corner weights? Total weight?

 

[Bebop]

HQA
Do you still have the factory radius arm style suspension? Or was it switched to a 3/4 link style?
Radius arms are known to act like a bid sway bar, and while they are killing travel, they are great for road manners.

BTW That truck looks dope, post more pics please.

 

[Firstram]

This might clear things up














You have to hold the dildo by the heavy end

 

[Ghetto Fab.]

Yo dude!

Isn't it the Ultimate 80 from way back when? Honestly I think most of your problem is too much weight up high and too far behind the rear axle. What does the rear suspension roll center look like on its own? It sounds like you have roll oversteer issues and there are a bunch of factors that can go into that.

 

[HQA]

HQA

How did it handle 20 years ago after you removed all the weight?

To your question, yes, the roll couple axis and the addition of weight on the rear will work together to make your handling suck. But you really only need to look at the rear by itself. Adding a bunch of weight up high and not adjusting the height of the rear RC will make the rear handle different.......spring rates being the same.

Your last sentence is confusing. Your going to raise the front RC even higher without any change to the rear? That will make things worse. If anything, I'd raise the rear RC and leave the front alone. Get the roll couple axis going down hill to the front and then start looking at spring rates.

Are you linked and coilovers front and rear? Got any corner weights? Total weight?

My mistake, I meant lowering the front panhard not raising.

20 year ago handling was touchy on highway it felt like roll steer and I just got used to it.

I thought with my roll axis higher in the front vs rear (roll couple thank you) would make the weight transfer to the rear of vehicle in a turn vs a more desirable lower front roll axis and higher rear would make the weight transfer to the front. The stock land cruiser had the front roll axis 2” lower vs rear.

3 link with panhard front and rear is stock 4 link plus panhard

Total weight 7k lbs wet
3100 front
3900 rear

I can raise the rear panhard 4” before running axle into frame.

I think I can raise the roll center higher by ditching the panhard and going to triangulated upper links. However that is gonna be a to of work.

I have a big trip coming next week so I can leave the same or do a easy adjustment.

 

[HQA]

Yo dude!

Isn't it the Ultimate 80 from way back when? Honestly I think most of your problem is too much weight up high and too far behind the rear axle. What does the rear suspension roll center look like on its own? It sounds like you have roll oversteer issues and there are a bunch of factors that can go into that.

Hey Kevin, I’ve been meaning to have a conversation with you about this rig and the 4 wheel independent rig I starting next.

Yes it’s the U80 2.0 crazy it’s been 20 years since I first built it.

Over this winter I plan to move all the links into better positions lessening the roll steer and raising the roll center.

 

[HQA]

rear using the 3 link calculator

 

[HQA]

front

 

[HQA]

here is what I think I can fit for the rear without loosing the gas tank

what you guys think?

 

[Bebop]

here is what I think I can fit for the rear without loosing the gas tank

what you guys think?

The rear steer on this is going to be terrible.

 

[HQA]

The rear steer on this is going to be terrible.

move frame end of lower links inwards?

 

[bdkw1]

here is what I think I can fit for the rear without loosing the gas tank

what you guys think?

That's a swing arm, not a 4 link. Get the pivot axis about the front bumper. Your uppers are way too short, see roll steer comment above.

 

[Bebop]

move frame end of lower links inwards?

Longer upper links

 

[Ghetto Fab.]

He's trying to work around the gas tank. Its right in front of the rear axle. Moving the lowers in and lower on the frame would help, but he'd have to loose the gas tank in that spot. What you could do is move the lower links up on the rear axle. That should flatten the links a bit and move the IC further forward and get the AS down. It will increase the forces on the upper links, but thats a tradeoff you might need to make.

The front looks good.

 

[SHARPMACHINE]

Damn good thread for a dumb guy like me wanting to get smarterer!!!

 

[HQA]

Here is what I’m working with.

4” up travel at ride height

The lower links at axle are 2” higher than the wheel center

What about doing a reverse 4 link like the Stellar guys 4 runner the Moscow Mule?

 

[Bebop]

Shocks are in the wrong location
Panhard bar needs to go way up

I'd make the lower links into radius arms. Easy to test the theory.

 

[gt1guy]

Damn, you got no room under there.

I notice the coilovers are pretty inboard too.

I thought with my roll axis higher in the front vs rear (roll couple thank you) would make the weight transfer to the rear of vehicle in a turn vs a more desirable lower front roll axis and higher rear would make the weight transfer to the front. The stock land cruiser had the front roll axis 2” lower vs rear.

Yes, the higher RC end will make the opposite end do more work. It isn't necessarily transferring weight so to speak, but it is carrying more of the roll load.

The rig has the F/R weight distribution of a rear engine car and the roll couple axis (sloping to the rear) is feeding the problem, making it worse.

As Bebop just said, raise the rear panhard bar. And the shock location isn't helping fighting roll. With the F&R weight distribution, I'd bet lowering the front panhard would help get the front to do a little more work.

 

[bdkw1]

Since you're limited on making the upper link longer, maybe do kinda like what Bebop is saying about radius arms. Giant motorsports makes the DD 4 link kit which is basically radius arms that can articulate and a panhard bar. Move arm and frame radius arm mounts to the outside of the frame, panhard should be level at ride hieght.

Ford Ranger Daily Driver Link Kit – Giant Motor Sports

www.giantmotorsports.com

 

[Dougal]

Granted it's been awhile but I don't agree (at this time) with the idea of a high roll center and your rear end is going to "pop" on a impact. RC are the resistance to lateral forces, no?

Roll centre height determines whether your vehicle will jack or squat when subjected to lateral force.

A low roll centre (imagine one below the road surface) means the car body will be pulled lower by lateral forces. A high roll centre means the car body will be jacked higher and faster. Can be desirable or undesirable depending on what's happening.

Think of the swing axles on the old VW and Corsair cars. That's why the back end would bite and lift. The swing axles on those would rapidly gain roll centre height as it lifte so it got ugly quickly.

A beam axle with horizontal panhard will have the roll centre at panhard height. Independent suspension can be designed to put them anywhere from under the ground to above the CG.

Standard setup is to have the rear roll centre higher than the front.

The RangeRover Classic, D1 and Defender1 all ran panhard front and a ball joint rear with a higher roll centre. Beam axles with better handling than you get with a panhard front/rear.
Triangulated 4 link gives similar effect. It's always worth considering roll centre heights when you're laying stuff out.

 

[Dougal]

The Busted Knuckle videos are brilliant and in the Roll Center Video when talking about the roll axis front to rear (after minute 11 in video) he said the roll axis needs to "slope from rear to front downward" and that made me realize what might be my problem and my roll axis goes the wrong way.

I am guessing my roll axis is why my rig has such poor handling at high speed on and off road. The rig leans over side to side rocking back and forth unlike any off road rig I have. I have eliminated a lot of that rocking side to side with stiffer shock valving and swaybars but its still there.

On highway if I swerve fast to avoid something it is like a tail heavy trailer going out of control and needs calm minimal steering input to bring back under control.

The rig is a 1997 Land Cruiser with coil overs and solid hum9 portal axles. The handling got worse when I added 600-800 lbs of weight to the rear axle by adding a cargo box and a 40 inch spare tire to the rear of the box.

Originally 20 yrs ago, I added the portal axles and cut 500 plus lbs off by removing most of the top, interior and bobbing the rear. I also raised the front axle panhard 5 inches from stock and left the rear stock location.

3 years ago I put the roof back on, rear seats back in and added the Australian style cargo box behind a chopped cab at the C pillar. I have put over 6k miles of highway and off road driving on it, 600 mi round trips to Rubicon, Shaver Lake area and Death Valley. I don't want to trailer the rig.

Could my roll axis slope from rear to front upward and the added weight on the rear be compounding the handling issue?

I am in the middle of raising (edit I meant lowering) the front panhard 5 inches however I want to know what you guys think.

Yes that'll do it. The other thing you need to calculate is natural frequency. Which is simply springrate/mass at each axle. Rear needs to be equal or higher than the front when loaded or it'll be a pig with a steering wheel.

Panhards get pretty sucky at longer travel as the angle causes them to jack in one direction and squat in the other. That's why triangulated 4 links and ball joints are better.