Land Rover Discovery Suspensions: Caster

[michaelgroves]

R_, some adjustable rate springs (i.e. air springs) would push the tail up and the nose down thereby shifting some weight back onto the front.

I'm not sure changing springs actually shifts a significant amount of weight forward, does it? - mostly it just looks that way!

 

[ntsqd]

I'm not sure changing springs actually shifts a significant amount of weight forward, does it? - mostly it just looks that way!

You sure won't move all that was added to the rear up to the front, but some will shift.
Gravity always acts on the CG towards the center of the Earth. With the vehicle sitting nose-high that Normal vector is biased towards the rear. In other words the point where the vector meets the ground is not in the same place within the wheel base as when the vehicle is sitting level. No amount of spring rate change is going move the CG forward, but changing the attitude of the vehicle can change the weight distribution.
That clear as mud? It's not an easy concept for me to describe.

 

[michaelgroves]

You sure won't move all that was added to the rear up to the front, but some will shift.
Gravity always acts on the CG towards the center of the Earth. With the vehicle sitting nose-high that Normal vector is biased towards the rear. In other words the point where the vector meets the ground is not in the same place within the wheel base as when the vehicle is sitting level. No amount of spring rate change is going move the CG forward, but changing the attitude of the vehicle can change the weight distribution.
That clear as mud? It's not an easy concept for me to describe.

No, I think I understand. Just as lifting the rear of the vehicle would mean that a plumb-bob hanging under the middle of the car would point to a slightly different point on the gound (further forwards)?

The weight transfer will be some trig function of the change in angle (of the car, or the plumb-bob).

 

[R_Lefebvre]

Think of it this way. You know that if you're driving down a very steep incline, if the truck nose were pointed down far enough, at some point the full weight of the truck would be balanced on the front axle, and the truck would be ready to flip over forwards. Ditto when driving up a hill, it all goes onto the back axle until you're ready to flip over.

Changing the pitch angle of the truck has a similar effect, but to a smaller degree, which is related by trigonometry. The effect is small.

 

[Lars70]

That's pretty much the crux of the problem in general! If the pinion angle weren't a problem, then the simplest solution for any lift would be to fit radius arms that retain the optimum caster. But the lift offsets the pinion further (vertically) from the transfer case output shaft, and makes the propshaft angle steeper.

It seems to me that there is some (low) level of lift where the steeper angle doesn't matter, some greater amount of lift where it would be adequate to use a modified propshaft that can cope with the greater angle, and a yet greater amount of lift where it's not possible to operate a reliable shaft at that angle. In this last case, it would be necessary to point the pinion upwards to make the angle acceptable, and then solve the resulting caster problem by redrilling (or cutting and welding) the swivel pin housings.

Once again, I'm ignorant of the nuances of the Discovery radius arms, but in the case of the Bronco, the radius arms pivots are roughly at the same fore/aft location on the frame as the front output on the transfer case. Stock, the front axle is located on the radius arms such that the pinion shaft on the differential points at the t-case output. Which, since the Bronco comes stock with a CV style driveshaft, is what you want (thus very little angle between pinion and driveshaft). As the radius arms droop, either because you just got airborne or because the vehicle was lifted, the radius arm geometry keeps the pinion shaft pointed at the t-case output. The angle between t-case output and pinion shaft changes as the radius arms go up/down, but the angle between the pinion shaft and the driveshaft remains constant.

With a CV style driveshaft, there is only one correct angle for the pinion shaft relative to the driveshaft: zero (or 180 if you want to be fussy). Regardless of the lift.

Time to go educate myself on the details of the Discovery front end so I'm not just spouting theoreticals.

 

[ntsqd]

It is a small difference, but I have experienced it make enough of a difference to make it worth the effort. Usually, though, the weight delta involved is much higher.

 

[michaelgroves]

With a CV style driveshaft, there is only one correct angle for the pinion shaft relative to the driveshaft: zero (or 180 if you want to be fussy). Regardless of the lift.

Hi Lars,

I guess with that geometry, a lift does create a specific problem - you have to change the angle of the pinion as you add longer springs, so as to keep it pointing perfectly at the transfer case putput. You don't have the option of altering the radius arms to optimise your caster. So the axle rotation puts your caster out, even on a mild lift. On the plus side, am I right in thinking that your stock radius arms are good, no matter what lift you go for? (In terms of the bushes binding, I mean - there may be other issues with them when lifting).

 

[Lars70]

Hi Lars,

I guess with that geometry, a lift does create a specific problem - you have to change the angle of the pinion as you add longer springs, so as to keep it pointing perfectly at the transfer case putput. You don't have the option of altering the radius arms to optimise your caster. So the axle rotation puts your caster out, even on a mild lift. On the plus side, am I right in thinking that your stock radius arms are good, no matter what lift you go for? (In terms of the bushes binding, I mean - there may be other issues with them when lifting).

The stock Bronco radius arm bushings (the ones at the frame) will eventually bind on down travel. I fabricated custom extended arms with urethane-lined spherical joints that eliminated that issue. Not much of the front suspension under my ancient, obsolete (but reasonably durable) Bronco is stock.

Could you or anyone else point me to a technical description of the Disco's front suspension? I'm enjoying this discussion because it doesn't seem like the Pirate board, but I feel like an ignoramus since I don't know the details.

 

[muskyman]

gone

 

[Lars70]

the Bronco and the Disco/RRC have very similar bushings at the frame stock. as you lift these trucks you preload the stock bushings and limit down travel or droop. We were doing heim jointed front radius arms on the bronco's 30 years ago to prevent this binding. Add in a cut and rotate of the inner 'C' to correct caster and you were good to go. There were lots of bronco's running 38-44" tires in the group I started wheeling with 30 years ago.

the bronco's front suspension is far from obsolete, infact it was well ahead of its time . The hot set up used to be to graft on the radius arms from the front onto the rear and get rid of the leaf springs alltogether. this would yeild a very compliant suspension that would flex like no tomorow.

With the Disco/RRC instead of going to a heim jointed front radius arm the most common change is to offset the frame end of the radius arm so the radius arm goes through the bushing center line and does not preload the bushing. This is a good fix in that you still have the rubber bushing to isolate the noise from the passenger compartment. allthough the heim joints or urathane core johnny joints are not at all limited in flex as the stock bushings are, the noise transmission from them may be more then desired by a overland traveller might want to deal with.

there is one other important modification needed once you reach 3" or more of lift with a Disco/RRC. once you rotate the swivel balls the front drag link location changes as well. this change moves the panhard bar and drag link out of a parralel position. this causes the steering to see-saw over bumps. to do away with this you will need to raise the location of the Panhard bar on the axle housing so that the two are once again in the same plane. this then makes the truck drive like it should.

As I stated before you need to look at the front suspension and steering as a whole and address all the issues. once you do that driving quality returns to that of a stock Land Rover and you will feel the extra steps are worth it.

From your description it sounds like the Discovery's front suspension is very similar to the early Bronco's- what I suspected, but since I've never seen one, I wasn't sure. The issues you describe are the same ones I've been through with my Bronco as I've evolved the suspension over the 13 years I've owned it.

Longer springs lifted it (currently 4 inches). I cut the inner knuckles off the Dana 44 axle housing and rotated them back 10 degrees to get the caster back while keeping the correct pinion angle.

The addition of a longer drivetrain (NV4500 & Atlas II) moved the front yoke on the transfer case 6 inches aft. This gave me some front driveshaft vibration since the pinion no longer stayed aligned (pointed at the t-case yoke) as the suspension cycled. 6 inch longer radius arms with Currie Johnie joints took care of that- also gave me another degree of caster.

Meanwhile, rotating the knuckles on the axles rotated the tie rod up & aft, along with the drag link. Since I had already moved the tie rod & drag link above the knuckles, it really threw the drag link/ panhard rod relationship out of whack. I fabricated a new panhard mount on the axle that is outboard of the radius arm rather than inboard. The drag link and panhard rod are once again parallel, and the outboard upper mount means that the two rods are now the same length. So they remain parallel as the suspension moves.

There are probably more details, but I don't remember them offhand. In the end, I've got about 6 degrees of positive caster and very stable steering that readily returns to center, with no bumpsteer.

Whew! Now I need to find a Discovery to crawl under so I can see it for myself.

 

[muskyman]

gone

 

[michaelgroves]

Meanwhile, rotating the knuckles on the axles rotated the tie rod up & aft, along with the drag link.

I can't quite visualize the Bronco setup, but wouldn't the rotating of the knuckles (the cutting and welding?) have simply returned the tie-rod and drag-link mounting points back to their original positions relative to the chassis?

I'm seeing the at-rest position of the chassis being raised relative to the front axle.
The axle's pitched (rotated) forward, to keep the pinion pointing in the right direction. (This would shift the steering rod mounting points out of alignment).
Then you cut and weld the knuckles (including the mounting points?) 10 deg back to restore caster.

So aren't the tie-rod and drag-link mounting points back where they were?

 

[muskyman]

gone

 

[michaelgroves]

see you dont know this stuff and you question people that speak from a knowledge base like they are wrong...very laughable.

I bet you have never actually built your own rig...I just bet.

Hmm? I was aking a question. I freely admitted that I couldn't visualize the setup, and I was interested in knowing more about it.

Why's that laughable?

 

[roverdoc]

it is a ref to your recent posts on open vs locked as well as here....