Whitey
Tweet
Hey everyone,
After reading of the problems that some FJ cruisers were having with after-market bump stops and extended travel struts with longer closed lengths, I thought it would be a good idea to see what the relationship is between strut travel and lower control arm-bump stop separation for the Prado 120.
Problems with FJ Cruiser and after market bump stops;
http://www.fjcc.com.au/f36/superbump...-details-1413/
I spent an afternoon, and pulled the coil out of one of my 712 struts and put the strut by itself back in. This way there’s no coil bind holding anything up, and I can freely measure all the way to the bump stop.
This is the relationship I measured;
As you can see from the graph, the ratio between strut travel and control arm-bump stop separation is 1.5:1. This is a substantial difference to the 2:1 assumption in the FJ thread. Note that I did not use the shroud for these measurements, this is from washer seat to ring-eye centre.
As discussed in the FJ thread, the after market bump stop was far too soft, and compressed so far that the struts bumped out, and actually forced the shock body into the shroud. This suggests the internal strut bump stop has failed also.
The Prado 120 and FJ Cruiser IFS geometry are very similar (identical?) as far as I can see from various discussions.
If you were to use a typical 433mm closed length extended travel bilstein, and then you also put a shroud on it, then both the closed and open lengths are shifted. On the top of the shroud there is a small metal cylinder which slips over the strut shaft. This cylinder is 7.8mm high (call it 8mm).
So by using the shroud, the strut is physically lowered by 8mm from the bottom of the washer, giving a new ‘effective’ closed length of 433+8 = 441mm.
The after-market bump stops used in the FJ thread compressed much more easily than the OEM bump stop, up to 25mm (possibly more). If you follow the extrapolation in the plot, you will see -25mm compression is at a strut length of 440mm, which is when the long travel strut (with shroud) will have bumped out in the FJ.
You can see for my Prado 120, I could get the OEM bump stop to compress by around -6mm with the jack under the control arm supporting the weight of the car.
So from these figures, I initially thought it was safe to use long travel bilsteins with closed length 433mm, and as far as I know, nobody has reported any bump-out issues with 5100’s so far?
Something else of note is that the FJ must have had solid coil bind, or very close to it. I also left the coil seat on my strut when I did the measurements without the coil, so I could measure the approximate coil working height. This is what the plot looks like;
For my ZC7296, I calculate a solid coil length of around 207mm.
The FJ user didn’t specify what coil lengths or coil type he was using, but even so, the coils must have been very close to full solid bind.
It must have been quite a hard hit to do this. We’re talking 3 times the normal corner car weight in Force to bind a coil solid at -25mm bump stop compression.
So after pulling out my 712 strut, I carefully measured closed-open lengths, and I can see that the open length is 553mm, spot on the quoted figure for a 712 strut, but my closed length has decreased by 5mm, and the shaft flange where the washer sits can now actually travel inside the shock body.
Not good!
If you add the shroud length of 8mm, my effective closed length should have been 427+8=435mm when the 712 strut was new. This shows that during my travels, it’s likely that repeated bump outs have destroyed the internal bump stop in my 712 strut. I can also see where the paint has rubbed off on the control arm from bump outs.
I’m guessing there must have been some flex in the lower strut bush and the top hat rubber for this to happen, and my lower strut bush is definitely off centre now. Other possible contributing factors to consider are the ageing of the bump stop rubber, and the softer valving in bilsteins, a common complaint across all types of 4wd’s.
So the moral of this story is that if a 712 strut with effective closed length of 427+8 = 435mm can get bumped out on an OEM bump stop and destroy the internal strut bump stop, then you can also bump-out a longer 433+8 = 441mm closed effective length on an extended travel bilstein. The longer closed length on an extended travel bilstein means it will always bump out before a 712 strut does.
Even though it is stated in the FJ thread that FJ’s with OEM bump stops and the same extended travel bilstein struts had no problems, I think a 5mm spacer (protecting 5x1.5 = 7.5mm strut bump travel = shroud cylinder height) on the OEM bump stop is a good idea for long travel bilsteins with closed length 433mm, particularly if you use the shroud. Over time, repeated bump outs may destroy the internal strut bump stop.
Maybe in the new extended travel monotube Bilsteins the internal bump stop is better engineered. Without actually eyeballing one I can’t say. The 712 strut is a very old design (I quote 30 years from a seller), and I’m suspicious the new Bilstein/TRD monotube is just a longer version of the 712. If anyone has some engineering knowledge I’d like to hear it!
This is also the reason why I very much lean towards a minimum closed length of 420mm (even less), which is what the OEM Prado 120 strut is. You would typically always get solid coil bind before you bump out 420mm closed length, so even if the bump stop completely failed, your strut will hopefully survive.
So putting Hilux struts with long closed length around 433mm into the Prado 120 or FJ is in my head not as straightforward as it seems.
If anyone has had some experience in this area I’d like to hear your thoughts. Hope this is informative to those with long closed length extended travel struts. Watch out for the big bumps!
Best
Mark
After reading of the problems that some FJ cruisers were having with after-market bump stops and extended travel struts with longer closed lengths, I thought it would be a good idea to see what the relationship is between strut travel and lower control arm-bump stop separation for the Prado 120.
Problems with FJ Cruiser and after market bump stops;
http://www.fjcc.com.au/f36/superbump...-details-1413/
I spent an afternoon, and pulled the coil out of one of my 712 struts and put the strut by itself back in. This way there’s no coil bind holding anything up, and I can freely measure all the way to the bump stop.
This is the relationship I measured;
As you can see from the graph, the ratio between strut travel and control arm-bump stop separation is 1.5:1. This is a substantial difference to the 2:1 assumption in the FJ thread. Note that I did not use the shroud for these measurements, this is from washer seat to ring-eye centre.
As discussed in the FJ thread, the after market bump stop was far too soft, and compressed so far that the struts bumped out, and actually forced the shock body into the shroud. This suggests the internal strut bump stop has failed also.
The Prado 120 and FJ Cruiser IFS geometry are very similar (identical?) as far as I can see from various discussions.
If you were to use a typical 433mm closed length extended travel bilstein, and then you also put a shroud on it, then both the closed and open lengths are shifted. On the top of the shroud there is a small metal cylinder which slips over the strut shaft. This cylinder is 7.8mm high (call it 8mm).
So by using the shroud, the strut is physically lowered by 8mm from the bottom of the washer, giving a new ‘effective’ closed length of 433+8 = 441mm.
The after-market bump stops used in the FJ thread compressed much more easily than the OEM bump stop, up to 25mm (possibly more). If you follow the extrapolation in the plot, you will see -25mm compression is at a strut length of 440mm, which is when the long travel strut (with shroud) will have bumped out in the FJ.
You can see for my Prado 120, I could get the OEM bump stop to compress by around -6mm with the jack under the control arm supporting the weight of the car.
So from these figures, I initially thought it was safe to use long travel bilsteins with closed length 433mm, and as far as I know, nobody has reported any bump-out issues with 5100’s so far?
Something else of note is that the FJ must have had solid coil bind, or very close to it. I also left the coil seat on my strut when I did the measurements without the coil, so I could measure the approximate coil working height. This is what the plot looks like;
For my ZC7296, I calculate a solid coil length of around 207mm.
The FJ user didn’t specify what coil lengths or coil type he was using, but even so, the coils must have been very close to full solid bind.
It must have been quite a hard hit to do this. We’re talking 3 times the normal corner car weight in Force to bind a coil solid at -25mm bump stop compression.
So after pulling out my 712 strut, I carefully measured closed-open lengths, and I can see that the open length is 553mm, spot on the quoted figure for a 712 strut, but my closed length has decreased by 5mm, and the shaft flange where the washer sits can now actually travel inside the shock body.
Not good!
If you add the shroud length of 8mm, my effective closed length should have been 427+8=435mm when the 712 strut was new. This shows that during my travels, it’s likely that repeated bump outs have destroyed the internal bump stop in my 712 strut. I can also see where the paint has rubbed off on the control arm from bump outs.
I’m guessing there must have been some flex in the lower strut bush and the top hat rubber for this to happen, and my lower strut bush is definitely off centre now. Other possible contributing factors to consider are the ageing of the bump stop rubber, and the softer valving in bilsteins, a common complaint across all types of 4wd’s.
So the moral of this story is that if a 712 strut with effective closed length of 427+8 = 435mm can get bumped out on an OEM bump stop and destroy the internal strut bump stop, then you can also bump-out a longer 433+8 = 441mm closed effective length on an extended travel bilstein. The longer closed length on an extended travel bilstein means it will always bump out before a 712 strut does.
Even though it is stated in the FJ thread that FJ’s with OEM bump stops and the same extended travel bilstein struts had no problems, I think a 5mm spacer (protecting 5x1.5 = 7.5mm strut bump travel = shroud cylinder height) on the OEM bump stop is a good idea for long travel bilsteins with closed length 433mm, particularly if you use the shroud. Over time, repeated bump outs may destroy the internal strut bump stop.
Maybe in the new extended travel monotube Bilsteins the internal bump stop is better engineered. Without actually eyeballing one I can’t say. The 712 strut is a very old design (I quote 30 years from a seller), and I’m suspicious the new Bilstein/TRD monotube is just a longer version of the 712. If anyone has some engineering knowledge I’d like to hear it!
This is also the reason why I very much lean towards a minimum closed length of 420mm (even less), which is what the OEM Prado 120 strut is. You would typically always get solid coil bind before you bump out 420mm closed length, so even if the bump stop completely failed, your strut will hopefully survive.
So putting Hilux struts with long closed length around 433mm into the Prado 120 or FJ is in my head not as straightforward as it seems.
If anyone has had some experience in this area I’d like to hear your thoughts. Hope this is informative to those with long closed length extended travel struts. Watch out for the big bumps!
Best
Mark
Comment