[BryceSmith]

Hi guys, was playing around after fitting new radius arm bushes today and i found a whole lot of extra travel in the rear if i took out the rearmost bolt, but also noticed a lot of wind up when accelerating. So i want to know who has done this and what pros and cons youve encountered, i would only run this setup when out on the tracks occasionally anyway. Any advice appreciated.
‘96 coily.

[Gwagensteve]

::squee! - rubs hand together:: - one of my favourite topics

Yes, I've done it.

It's a terrible idea, especially in the rear of a coil sierra.

You've slightly misrepresented the effect - it doesn't increase travel - in the rear of a coil sierra, travel is limited by the shock length. What happens is roll stiffness drops to (almost) nothing. Roll stiffness needs to be reasonably matched front to rear. Additionally, in order for the suspension to work when the car is climbing, it's advisable to have slightly higher rear roll stiffness than front.

What you'll find is with a bolt removed, on flat ground the car will follow the nose - the front will flex after the rear has reached the limits of its travel. When climbing, this results in the car flopping over and lifting front wheels. It'll do it on even very mild climbs, and as you've noticed, is accentuated by torque.

Here's a visual demonstration, on my car. It ran Range Rover front radius arms in the rear and leaf front at this point a gazillion years ago, but the general arrangement is pretty similar to an SJ80. It had much longer shocks though so the effects are exaggerated.



Bolts all in. Note how hard the front tyre is tucked into the guard and the angle the body is on.



Bolt pulled. Note the front axle is barely flexing and the body is now on a huge angle.

there's another effect that's not immediately evident - instead of the axle rolling on the median between the two radius arms, it now rolls with the arm that has both bolts in. That can lead to some dramatic driveshaft angles.


Both bolts in



one bolt pulled

Because of the radius arm design, with a bolt removed, all the torque will be directed through one arm only. This tends to make the side with both bolts in jack up, whilst the other side with the missing bolt stays at normal ride height. Again, on a climb this is accentuated as the weight transfers rearwards, and the car will take on some very strange side angles.

I'll add with some SJ80 specific tech - pulling a bolt puts big loads on the radius arm bracket on the side with both bolts in place, and they're already too weak.

If you want to experiment with this further, put the rear bolts back in but pull the passenger front bolt in the front. Freeing the front up will transform the feel of the car off road, especially when climbing - the body will stay much more level and the car will feel much more balanced. You will notice some increased brake dive and some weird characteristics at speed, but it'll feel nice in the bush. Pulling a front radius arm bolt is a fairly common Range Rover practice, as, due to their suspension design, the rear has very low roll stiffness and the front has relatively high roll stiffness. This is done to promote road handling by making the car understeer gently and reducing body roll, but off road it's unhelpful.

Every car will get to the point where the front wheels will lift and the front will "unflex" and "push off" if it's on enough of an angle - its just a result of the front getting progressively lighter as the angle gets bigger. The idea is to try and move the angle this happens high enough that you don't encounter it under normal driving. Removing a rear bolt moves this angle closer to flat, removing a front bolt moves this angle higher.

Here's some ancient photos of my Gwagen for reference of working within and outside this angle. as I discuss below, the Gwagen is radius arm front and rear like the SJ80 but has matched roll stiffness, not lower rear roll stiffness like the SJ80.



Front flexing hard even with the car on a big climb



Woah - front unloaded due to the huge compound angle. (This is about 40˚ climb and a huge side angle. This was a very, very scary hill, long closed)


Here's some context:

Suzuki fell into the same trap as all the japanese manufacturers - they copied the British (land rover) and Germans (Gwagen) when they designed their coil suspensions but missed the point on roll stiffness. Both the Range Rover and Gwagen have parallel chassis rails that allow the rear coils to be placed outside the chassis (like the front of your SJ80) and that which allows the springs to add rear roll stiffness and helps to balance front to rear roll stiffness. The Gwagen is radius arm front and rear like the SJ80, but the parallel chassis and outboard rear springs matches front and rear roll stiffness well. Mercedes then added a very stiff front swaybar to make the car understeer. with that removed the Gwagen handles excellently and is very balanced off road. The Range Rover has inherently very low inherent rear roll stiffness with a three link A-frame arrangement, but outboard springs in the rear that introduce some roll stiffness. Whilst the radius arm front has inherent roll stiffness, the Range Rover wasn't designed to run a front antisway bar and one was only added because of ignorant drivers.

The japanese saw these coil designs but didn't want to build a clean sheet design - mostly they wanted to add coils to their exiting leaf sprung chassis. to do this they generally added radius arms in the front and a 5 link or radius arms in the rear because they're easy to package, but they missed the memo to run parallel chassis rails and outboard rear springs, so the end result was very little added roll stiffness added via the springs, because they're closer to the centreline of the vehicle - so basically, the worst possible combination. and that's why every bogan reverses their GQ patrol up obstacles to take photos with the front flexing.

[greenzook89]

Anything is possible but bear in mind that the diff requires both bolts to be in to prevent axle wrap. Axle wrap can lead to issues like breaking rear shocks and binding unis, It will also feel pretty floppy in the rear. A coily (like any 4x4) needs the front to work more to climb up over obstacles

Edit. Beaten by Steve.

[Brenno]

Great tech. Thanks Steve

[alien]

This doesn't even apply to me and I read it.

[Marko_SJ]

So on a radius arm setup we add roll stiffness by bush type, spring spacing?

Is there a way to increase rear roll stiffness relative to the front without fab?

I like the way that Gwagen sits whilst ramping up that obstacle.

[sideways]

I typed this the other day but never hit send...

To further the discussion. Removing the bolt increases flex by reducing roll stiffness. The roll stiffness is reduced because you aren't compressing the bushes on that arm. So aside from pulling a bolt, what other ways can we look at reducing roll stiffness in a radius arm vehicle?

1. Softer bushes. Look up "holey bushes" on Aulro and pirate4x4. Landrover guys have been playing with this since forever and getting impressive results. GU patrol bushes come from the factory with slots to allow greater deflection. To me this is the cheap an easy way to reduce roll stifness in a radius arm car. It's also pretty unique in that you'd never get a yellow sticker for it. Bushing life may be short but hey, there's a downside to most easy ways of doing things. You could also look at getting Bruce (member on here) to make you some extra soft bushes. Personally, I'd look at running some super soft/holey bushes on one side and standard ones on the other.

2. Bigger bushes with more deflection. Suzukis and rovers have 50mm bushes, landcruisers and patrols have 60mm bushes. You'd have to make new radius arms and given the clearance between the radius arm and the diff housing I just don't think this is all that feasible without moving the bushes further apart which would defeat the purpose. If you were going to make new radius arms you'd go superflex style.

3. Move the bushes closer together. This is essentially what the Superior Engineering Superflex arms do. On one side you have an arm with standard bush spacing (in some cases the standard radius arm), on the passengers side (due to the action of the panhard) you have a plate that bolts to the standard diff end radius arm mounts and a radius arm with the mounts super close together which bolts to that plate. It's a clever design and requires no chassis/diff mods, to me this is the realistic step past holey bushes if you're sticking with radius arms. If you're making a radius arm for offroad use you'd be mad not to go for this design IMO.

4. Make the radius arms longer. A longer arm has less of an angle on it when the car flexes, or more to the point the angle difference between the 2 arms is less. Less angle means less bushing deflection. Again, you'd have to have new arms made and figure out how to package it all without compromising your break over angle/clearance. Coily/Jimny front radius arms are super short so this could be worth playing with but personally for all the mounting and manufacturing nightmares, I'd just convert to a 3/5 link.

Personally, I'd be chasing flex in the front before the rear.

FWIW whilst I agree the inboarded rear springs are a negative on the Japanese 4WDs for suspension balance, the wide chassis and 5 link rear end is fantastic for packaging. Where a Range Rover classic carries 76L of fuel a Landcruiser 80 carries 145L and the spare tyre! Furthermore, it's easy to vastly increase the 80s fuel capacity where as the Rangie requires small auxiliary fuel tanks to be added in the most venurable areas on a 4wd, the sills and rear 1/4s behind the wheels. Naturally, I don't think Toyota or Nissan cared at all about those of us who wanted to fit 37s and destroy the car offroad but they sure made a far better touring vehicle which was the market they aimed for.