Whitey
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Hey all,
Recently I’ve been going over some of my old measurements on the 120 IFS, and I’ve noticed a significant variation of the leverage ratio for coil-in measurements. This is particularly prominent when using Dobinsons coils which tend to relax considerably at droop and then start compressing back to working height as the lower arm is jacked up for measurements. This coil relaxation pushes the strut to irreproducible open lengths, meaning coil-in measurements will give different answers for the wheel travel:strut shaft travel leverage ratio. The strut will get pushed to a different open length each time you measure it, depending on how much the coil has relaxed from its normal working length at ride height.
You can observe the relaxation of Dobinsons coils by putting your Prado up on jack stands, letting the IFS droop, and then dropping the Prado back to the ground. You will notice the ride height has increased considerably, and this can take several days of driving for the coil to relax again to the previous ride height. This affects both your ride height and strut length.
I have performed coil-out measurements before on the IFS, eg., for the lower control arm bumpstop separation, but I haven’t done it for the wheel:strut leverage ratio. I assumed my coil-in measurements were sufficient, but they are not.
One other important point is that some of these measurements are performed with OEM bush/washer in the strut cap, while others use after market bushes/washers. The difference can be 10mm in bush thickness. The photo below shows an after-market bush on the left, and the OEM bush on the right, which you can see sits higher inside the cap.
I have checked the OEM bush/washer dimensions, and the bottom surface of the washer is at the same position as the top surface of the strut cap, or directly underneath the top of the strut boss.
As such, I have now completed some strut-out/coil-out measurements from the strut boss to bottom strut bolt center line. These measurements are full stroke for the OEM arms, from lower arm bumpstop contact down to ball joint bind.
There has also been some confusion about CV bind on the 120/150/FJ/Hilux series, and what open length it occurs at. I have now double checked some old threads on PP, and compared those measurements with my recent coil-out IFS cycling measurements, shown below.
The wheel travel:strut shaft travel leverage ratio for coil-out measurements is 1.89:1, and is significantly lower than my previous coil-in measurements, which could be up to 2.20:1.
There is a commonly used figure of maximum open length of 570mm for the 120 series struts, which was originally believed to be estimated on the basis of CV bind. This is a reliable figure, but as you can see from the plot, the figure of 570mm is useful more from the perspective of ball joint bind on the OEM upper arms. I see the ball joint on the OEM arms bind at an open length of 575mm (with respect to OEM bush/washer).
For my original CV bind investigations, I used my bottle jack to force the arms down (with ball joint binding), and have felt the rotors getting resistance to turning past 580mm with a strut cap in/strut and coil out measurement using after market strut cap bushes. Adding the extra after-market bush thickness (about 10mm uncompressed) to 580mm open length is consistent with the earlier findings from MickL, who used after-market SPC light racing upper control arms. These arms have a much larger range of motion in the ball joint, and allow the IFS to droop to 915mm bottom rim to guard. At 915mm, MickL found the rotors getting difficult to turn, so CV binding begins on the 120 series at around 589mm open length based on both his and my measurements. You may feel the rotors binding up at lengths shorter than 589mm, and I would say from 585mm onward you can begin to feel some resistance. In OEM geometry, the droop is limited to around 890mm due to the ball joint binding, so there is potentially 25mm of wheel travel to be gained with after-market arms. However, I would not consider this extra travel useable.
The next plot shows the variation of CV angle with strut length. I had a diff drop in for these measurements, but I know from my previous measurements on CV angle that there is little difference at full droop, around 1 degree.
I also re-measured the lower arm-bumpstop gap, and obtained a ratio of 1.32:1, measurements are shown below;
From bumpstop contact to ball joint bind, there is 110mm of stroke in the OEM geometry, giving 209mm or 8.23” of wheel travel. You won’t get that much into the bump travel though, as the coils will prevent full articulation, they will end up lifting the Prado rather than the IFS lifting relative to the chassis
What you can’t see from the plots is what the CV looks like. At 890mm bottom rim to guard, my CV boots are already binding up, the individual folds of the boot are pushing into each other, and at full lock the boots rub on the back of the strut ring eye. For this reason I’ve rounded off the edge on my ring eyes so it reduces the possibility of the boot tearing. MickL also saw this problem. At 890mm, my rotors still rotate freely. You can see in the photo below where I’ve marked the ring eye for rounding off.
My after-market struts were put together by Driveline Services, and I asked for an open length of 570mm. What I didn’t pay enough attention to was the after-market bushes, which sit 10mm lower than the OEM bush in an uncompressed state. With everything torqued up, my struts extend to 577mm, a couple of mm past the ball joint bind. There is enough unsprung weight and force from the coil to push the arms down a couple of mm with the ball joint binding.
While the struts are different lengths and use different bushes between 120/150/FJ and Hilux, I believe the upper and lower arms are the same dimensions. The strut ring eye cradle on the FJ/150 arms is slightly wider to accommodate the wider FJ/150 crush tube, but that is the only geometrical difference.
We can also see a similar ball joint bind length on the FJ (573mm), and measurements on the 150 show CV bind starts in the 585-590mm range.
This re-analysis suggests the CV bind is up around 590mm on all models. Diff drops may affect this length slightly, with perhaps 1 degree of variation in CV angle at full droop.
Even with this long length for CV bind, in OEM geometry the ball joint binds at a much shorter length of ca. 575mm, so it is advisable if you run OEM upper arms to limit your open length to 570mm so you don’t top out on the binding ball joint. Further, the CV boot is looking ugly at 575mm, and it will rub on the back of the strut ring eye at full lock.
After market arms will fix the OEM ball joint binding problem, and they will also fix the problem of the OEM ball joint boot/upper arm rubbing/binding on the coils at full droop. Even so, you would still want to limit your strut maximum open length to 570-575mm due to the CV boot rubbing on the ring eye at full lock, and the potential for the CV boot to tear. You also need to keep an eye on the thickness of after-market strut cap bushes, which can drop the strut down a long way.
I hope this clears up some of the numbers around ball joint bind, CV bind and maximum strut open length.
As always, for those running long open length struts, don’t give the Prado a bootful of accelerator with a wheel off the ground, as you will pop out a tripod or potentially snap a CV shaft.
Best
Mark
Recently I’ve been going over some of my old measurements on the 120 IFS, and I’ve noticed a significant variation of the leverage ratio for coil-in measurements. This is particularly prominent when using Dobinsons coils which tend to relax considerably at droop and then start compressing back to working height as the lower arm is jacked up for measurements. This coil relaxation pushes the strut to irreproducible open lengths, meaning coil-in measurements will give different answers for the wheel travel:strut shaft travel leverage ratio. The strut will get pushed to a different open length each time you measure it, depending on how much the coil has relaxed from its normal working length at ride height.
You can observe the relaxation of Dobinsons coils by putting your Prado up on jack stands, letting the IFS droop, and then dropping the Prado back to the ground. You will notice the ride height has increased considerably, and this can take several days of driving for the coil to relax again to the previous ride height. This affects both your ride height and strut length.
I have performed coil-out measurements before on the IFS, eg., for the lower control arm bumpstop separation, but I haven’t done it for the wheel:strut leverage ratio. I assumed my coil-in measurements were sufficient, but they are not.
One other important point is that some of these measurements are performed with OEM bush/washer in the strut cap, while others use after market bushes/washers. The difference can be 10mm in bush thickness. The photo below shows an after-market bush on the left, and the OEM bush on the right, which you can see sits higher inside the cap.
I have checked the OEM bush/washer dimensions, and the bottom surface of the washer is at the same position as the top surface of the strut cap, or directly underneath the top of the strut boss.
As such, I have now completed some strut-out/coil-out measurements from the strut boss to bottom strut bolt center line. These measurements are full stroke for the OEM arms, from lower arm bumpstop contact down to ball joint bind.
There has also been some confusion about CV bind on the 120/150/FJ/Hilux series, and what open length it occurs at. I have now double checked some old threads on PP, and compared those measurements with my recent coil-out IFS cycling measurements, shown below.
The wheel travel:strut shaft travel leverage ratio for coil-out measurements is 1.89:1, and is significantly lower than my previous coil-in measurements, which could be up to 2.20:1.
There is a commonly used figure of maximum open length of 570mm for the 120 series struts, which was originally believed to be estimated on the basis of CV bind. This is a reliable figure, but as you can see from the plot, the figure of 570mm is useful more from the perspective of ball joint bind on the OEM upper arms. I see the ball joint on the OEM arms bind at an open length of 575mm (with respect to OEM bush/washer).
For my original CV bind investigations, I used my bottle jack to force the arms down (with ball joint binding), and have felt the rotors getting resistance to turning past 580mm with a strut cap in/strut and coil out measurement using after market strut cap bushes. Adding the extra after-market bush thickness (about 10mm uncompressed) to 580mm open length is consistent with the earlier findings from MickL, who used after-market SPC light racing upper control arms. These arms have a much larger range of motion in the ball joint, and allow the IFS to droop to 915mm bottom rim to guard. At 915mm, MickL found the rotors getting difficult to turn, so CV binding begins on the 120 series at around 589mm open length based on both his and my measurements. You may feel the rotors binding up at lengths shorter than 589mm, and I would say from 585mm onward you can begin to feel some resistance. In OEM geometry, the droop is limited to around 890mm due to the ball joint binding, so there is potentially 25mm of wheel travel to be gained with after-market arms. However, I would not consider this extra travel useable.
The next plot shows the variation of CV angle with strut length. I had a diff drop in for these measurements, but I know from my previous measurements on CV angle that there is little difference at full droop, around 1 degree.
I also re-measured the lower arm-bumpstop gap, and obtained a ratio of 1.32:1, measurements are shown below;
From bumpstop contact to ball joint bind, there is 110mm of stroke in the OEM geometry, giving 209mm or 8.23” of wheel travel. You won’t get that much into the bump travel though, as the coils will prevent full articulation, they will end up lifting the Prado rather than the IFS lifting relative to the chassis
What you can’t see from the plots is what the CV looks like. At 890mm bottom rim to guard, my CV boots are already binding up, the individual folds of the boot are pushing into each other, and at full lock the boots rub on the back of the strut ring eye. For this reason I’ve rounded off the edge on my ring eyes so it reduces the possibility of the boot tearing. MickL also saw this problem. At 890mm, my rotors still rotate freely. You can see in the photo below where I’ve marked the ring eye for rounding off.
My after-market struts were put together by Driveline Services, and I asked for an open length of 570mm. What I didn’t pay enough attention to was the after-market bushes, which sit 10mm lower than the OEM bush in an uncompressed state. With everything torqued up, my struts extend to 577mm, a couple of mm past the ball joint bind. There is enough unsprung weight and force from the coil to push the arms down a couple of mm with the ball joint binding.
While the struts are different lengths and use different bushes between 120/150/FJ and Hilux, I believe the upper and lower arms are the same dimensions. The strut ring eye cradle on the FJ/150 arms is slightly wider to accommodate the wider FJ/150 crush tube, but that is the only geometrical difference.
We can also see a similar ball joint bind length on the FJ (573mm), and measurements on the 150 show CV bind starts in the 585-590mm range.
This re-analysis suggests the CV bind is up around 590mm on all models. Diff drops may affect this length slightly, with perhaps 1 degree of variation in CV angle at full droop.
Even with this long length for CV bind, in OEM geometry the ball joint binds at a much shorter length of ca. 575mm, so it is advisable if you run OEM upper arms to limit your open length to 570mm so you don’t top out on the binding ball joint. Further, the CV boot is looking ugly at 575mm, and it will rub on the back of the strut ring eye at full lock.
After market arms will fix the OEM ball joint binding problem, and they will also fix the problem of the OEM ball joint boot/upper arm rubbing/binding on the coils at full droop. Even so, you would still want to limit your strut maximum open length to 570-575mm due to the CV boot rubbing on the ring eye at full lock, and the potential for the CV boot to tear. You also need to keep an eye on the thickness of after-market strut cap bushes, which can drop the strut down a long way.
I hope this clears up some of the numbers around ball joint bind, CV bind and maximum strut open length.
As always, for those running long open length struts, don’t give the Prado a bootful of accelerator with a wheel off the ground, as you will pop out a tripod or potentially snap a CV shaft.
Best
Mark
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