Main Fuel Tank – rear trailing UCA separation on Prado 120 series

[Whitey Shockie Maker of the Month Award Find all posts View Profile Close Whitey]

Hey everyone,

With many reported failures of the rear upper trailing control arms (on 90’s and 120’s), and apparent binding of the passenger side arm on the fuel tank, I was interested to know the exact geometry.

With coils and shocks out, and swaybar in, I find the following relationship;



At full droop with the swaybar binding, I get about 610mm shock travel, very close to the commonly reported figure of 615mm. I’m using after market bushes that are a bit thicker than factory which accounts for the 5mm difference.

The rear axle is pushed into the bump stop at 405mm shock travel.

I’m running 80 series shocks, 365-618mm, so the swaybar links definitely need to be lengthened by around 10mm to make sure the shocks top out before the swaybar binds.

With the rear axle level at full droop with the swaybar in, the minimum separation between the main fuel tank and the rear UCA is about 26mm. Extrapolating the data suggests the arm will bind at a very long shock length of around 740mm.

The more realistic situation is at what shock travel the arm will bind at full flex. With my Prado flexed out on Stanfred ramps, I can get the shock length out to 615mm, and in the flexed state, the fuel tank – trailing UCA separation is about 35mm, an increase of 9mm compared to when the axle is level.

This suggests I’m never going to get anywhere near binding the rear trailing UCA with the shocks I’m using.

For the more extreme users who run the rear end with the swaybar out with shocks longer than 615mm open length, have any of you observed the rear trailing UCA bind, and if so, at what shock length?

Even though binding appears unlikely, I’m still very interested in these offset adjustable upper trailing arms purely from the perspective of strength;

https://www.superiorengineering.com....filter=239,355

I’ve read reports where the factory upper trailing arms have snapped in the middle of the Simpson on the Madigan line and required re-welding, not something I want to be doing in the middle of the desert! They will also be excellent for adjusting the diff for lifts over 2”.

Best

Mark

[Razorback81 Advanced Member Find all posts View Profile Close Razorback81]

what is the pinion angle before the lift & then after the lift?

[Whitey Shockie Maker of the Month Award Find all posts View Profile Close Whitey]

Hey Razorback,

I haven't checked the change in pinion angle, but I'm only running a 2" lift at the moment. I don't have my factory coils either, so I can't check what the pre-lift angle was. I'm not aware of anyone worrying about the pinion angle at 2" lift, but I'll soon be putting in 3" coils, so I might have to start worrying about it! The adjustable upper arms can hopefully roll the diff back and decrease any ugly looking angles.

Do you have an idea what the maximum pinion angle is before you should start worrying? My tailshaft is sitting at about 9.3 degrees above horizontal right now with a rear ride height around 820mm, or pretty much bang on 2" lift.

I still have some more measurements to do on the lower trailing arms, so I can also measure the change in pinion angle from bumpstop to swaybar bind, but this would be with the coils and shocks out.

How informative will this be without the coils in? i.e., is the pinion angle more of a concern at highway driving ride height in terms of wear and tear, or are you more concerned about how extreme the pinion angle can get when the rear is fully flexed out?

Is there a general guideline we can follow for 'healthy' pinion angle?

Best

Mark

[Razorback81 Advanced Member Find all posts View Profile Close Razorback81]

Hey Mark

it is something i think we need to look at
i know back in my days when helping guys with Drag racing pinion angle was critical for traction
being a multi link set up, they dont get the changes like the leaf spring set up, which you had to run tramp bars on

as with our Off Road racing same deal if the pinion angle is too high the rear diff starts to bunny hop a little , traction is lost, but more importantly that makes damaging the ring & pinion very easy with the on / off back lash that happens

we need to find someone with a Std car that can measure it as it sits on the ground, then raise it to see the change in pinion angle using a clinometer on the phone will do the job

my car has a set of springs in the rear that are King HD with + 50mm so my rear tyres are already lower than chassis ( i have had a few comments that it is a 3" lift)

James

[Razorback81 Advanced Member Find all posts View Profile Close Razorback81]

i spoke with MickL a while back after he fitted the new rear arms & adjustable pannard rod

his pannard rod was 7mm longer to correct the rear position back to centre
and i think from memory he said cost was a bit pricey & didnt seem to notice the difference

any comment MickL ?

James

i know he doesnt get on here much

[Whitey Shockie Maker of the Month Award Find all posts View Profile Close Whitey]

Hey James,

Although you're talking about drag cars and 4wd's, the common connection is the anti-squat geometry.

I'm no expert in anti-squat, but I know of a few basic things.

For drag cars, anti-squat is crucial. When they burst off the line with huge acceleration, all the load is passed down through the multi-link at the rear, and if it isn't set up correctly, the car will squat too much at the rear, and then there is potential for diff steering and tramping/bunny hopping, as you mentioned above.

For 4wd's, the situation is obviously a bit different, as the 4-link in our Prados doesn't experience the massive acceleration forces that a drag car does. Anti-squat geometry is highly sensitive to acceleration, and also to ride height. The higher you lift, the poorer the anti-squat geometry.

I think the most 'extreme' situation the 4-link in the Prado experiences is in desert/dune climbing situations, where you have a very heavily loaded vehicle (near GVM), that is lifted with a high centre of gravity. When fully flexing on sand moguls or climbing a steep rutted dune (accelerating hard), the rear end can get heavily loaded through the multi-link, and this is where failures have been reported for the upper trailing arms.

The OEM upper trailing arms are ridiculously tiny compared to the lower arms, so if you are going to the desert with a loaded vehicle, I'd suggest replacing them with the stronger adjustable arms I mentioned above.

In general, I think the diff is only off centre after lifting over 2", which is easily fixed with an adjustable panhard rod, as MickL and others running 3-4" lift can describe more accurately.

Regarding the pinion angle, I'm not sure what it is like with lifts over 2", and I'm also unsure if the diff starts to roll around the axle housing axis. If it does roll, then the after market adjustable upper trailing arms can easily fix that.

I think that's the limit of what can be done with the trailing arms and the panhard rod on the Prado. The adjustments are probably minor, in the order of 5mm, and it is more the vehicle lift changing centre of gravity that affects the anti-squat geometry in the largest manner.

I'd definitely like to hear some more about anti-squat for the Prado, and the effects it has on the diff and the trailing arms.

Best

Mark