Whitey
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Hey all,
With a lot of talk about the Prado IFS and travel limits, it’s easy to forget how critical it is to get decent droop in the rear of the Prado, an often neglected and little discussed geometry. There’s been a bit of discussion around lately about short open length rear shocks giving limited droop for the solid axle setup in the rear of the Prado.
I had addressed these questions a few pages into another thread, but thought it would be easier to have a devoted thread which has the necessary plot for understanding the problem.
In the plot below you can see how the rear lift, droop and shock open length are related;
This graph is constructed on the basis of the rear 1:1.1 motion ratio, measured in horizontal geometry.
One of the most commonly used limited open length rear shocks is the Bilstein BE5-A713, which is 368-580mm stroke. This compares to 359-574mm for the OEM 120 series shock. At a typical OEM 770mm ride height in the rear of the 120 Prado, the OEM rear shock will give ca. 105mm droop. As such the 713 is a good replacement for the OEM shock, and the valving of the 713 is matched well to a softer OEM coil.
When the 713 is matched with a typical C59-327 Dobinson rear coil, it then ends up in a lifted geometry anywhere from around 2-3” up (depending on rear load) from the OEM ride height. This means a ride height in the range 820-845mm. If you follow the “580mm open length” line in the plot, you will see that this means you are limited to around 40-65mm of droop in the rear by using a BE5-A713/C59-327 combination. This range matches well commonly reported droop figures of around 50mm for the 713.
It should also be mentioned that the 713 is valved most appropriately for a soft coil with low rebound and high compression of 1820N:1520N at 0.52m/s. This suggests the best use of the 713 is in a non-lifted geometry as a direct OEM replacement that matches the soft OEM rear coil rate of 200lb/in.
A more appropriate Bilstein for the rear to obtain excellent droop and which are valved for much stiffer coils (to suit a typically loaded Prado) are the 24-217897 (3700N:515N at 610mm open) or the B46-1478LT (4000N:900N at 625mm open). Both of these rear shocks will be well matched with rear coil rates up to 300lb/in, and will give ca. 100mm droop.
I am currently running a rear 618mm open length (spaced a little with thicker bushes) shock with 280lb/in 430mm coils which gives me a ride height of 835mm with a measured 96mm of droop.
Using stiff rear coils that are well matched to the front coils is also critical for controlling vehicle pitch, a notorious handling problem in the Prado. 80 series shocks are typically valved nicely for stiff rear coils and have an open length typically around 620mm (in their shortest versions), allowing for an excellent touring (stiff coils for big loads) and handling (minimal pitch) setup in the rear of the Prado that maintains excellent droop.
As always, the Pradopoint Suspension Database is a good source of information for shock choices. The rear geometry is also close to identical for the 120, 150 and FJ, so this discussion applies equally well to all models.
Lastly, keep in mind the binding limits of the OEM swaybar which is at the limit of its arc at 615mm open length. If you choose shocks longer than 615mm open you will need longer swaybar links and longer brake lines in the 120, see here;
http://www.pradopoint.com/showthread...imum-extension
You should also check that the coil free height you choose is matched to your shock open length, coil-bucket separation as a function of shock open length can be seen here;
http://www.pradopoint.com/showthread...ado-120-series
Good luck with setting up the rear of your Prado!
Best
Mark
With a lot of talk about the Prado IFS and travel limits, it’s easy to forget how critical it is to get decent droop in the rear of the Prado, an often neglected and little discussed geometry. There’s been a bit of discussion around lately about short open length rear shocks giving limited droop for the solid axle setup in the rear of the Prado.
I had addressed these questions a few pages into another thread, but thought it would be easier to have a devoted thread which has the necessary plot for understanding the problem.
In the plot below you can see how the rear lift, droop and shock open length are related;
This graph is constructed on the basis of the rear 1:1.1 motion ratio, measured in horizontal geometry.
One of the most commonly used limited open length rear shocks is the Bilstein BE5-A713, which is 368-580mm stroke. This compares to 359-574mm for the OEM 120 series shock. At a typical OEM 770mm ride height in the rear of the 120 Prado, the OEM rear shock will give ca. 105mm droop. As such the 713 is a good replacement for the OEM shock, and the valving of the 713 is matched well to a softer OEM coil.
When the 713 is matched with a typical C59-327 Dobinson rear coil, it then ends up in a lifted geometry anywhere from around 2-3” up (depending on rear load) from the OEM ride height. This means a ride height in the range 820-845mm. If you follow the “580mm open length” line in the plot, you will see that this means you are limited to around 40-65mm of droop in the rear by using a BE5-A713/C59-327 combination. This range matches well commonly reported droop figures of around 50mm for the 713.
It should also be mentioned that the 713 is valved most appropriately for a soft coil with low rebound and high compression of 1820N:1520N at 0.52m/s. This suggests the best use of the 713 is in a non-lifted geometry as a direct OEM replacement that matches the soft OEM rear coil rate of 200lb/in.
A more appropriate Bilstein for the rear to obtain excellent droop and which are valved for much stiffer coils (to suit a typically loaded Prado) are the 24-217897 (3700N:515N at 610mm open) or the B46-1478LT (4000N:900N at 625mm open). Both of these rear shocks will be well matched with rear coil rates up to 300lb/in, and will give ca. 100mm droop.
I am currently running a rear 618mm open length (spaced a little with thicker bushes) shock with 280lb/in 430mm coils which gives me a ride height of 835mm with a measured 96mm of droop.
Using stiff rear coils that are well matched to the front coils is also critical for controlling vehicle pitch, a notorious handling problem in the Prado. 80 series shocks are typically valved nicely for stiff rear coils and have an open length typically around 620mm (in their shortest versions), allowing for an excellent touring (stiff coils for big loads) and handling (minimal pitch) setup in the rear of the Prado that maintains excellent droop.
As always, the Pradopoint Suspension Database is a good source of information for shock choices. The rear geometry is also close to identical for the 120, 150 and FJ, so this discussion applies equally well to all models.
Lastly, keep in mind the binding limits of the OEM swaybar which is at the limit of its arc at 615mm open length. If you choose shocks longer than 615mm open you will need longer swaybar links and longer brake lines in the 120, see here;
http://www.pradopoint.com/showthread...imum-extension
You should also check that the coil free height you choose is matched to your shock open length, coil-bucket separation as a function of shock open length can be seen here;
http://www.pradopoint.com/showthread...ado-120-series
Good luck with setting up the rear of your Prado!
Best
Mark