Whitey
Tweet
Hey all,
There’s a lot of confusing misinformation and poor terminology about how strut and coil spacers for the IFS work, so I thought a fresh perspective was needed to physically understand what they do, and why they can be potentially very damaging.
I’ll break this into sections about each.
Strut Spacers:
The photo below shows the location of the strut spacer;
Adding a strut spacer above the coil actually adds a small amount of mass to the vehicle sprung mass, and this is the more physical perspective from which it should be understood.
When you calculate the effects of this extra sprung mass using a ride height calculation, the effects on the coil working height are very small, meaning that there is no obvious change in the final coil working height under vehicle sprung mass load. For example, a 10mm steel spacer would weigh around 0.45kg, and the resultant decrease in ride height would be 0.1mm, barely noticeable.
This means that there are no significant measurable changes in the shaft position, as the coil working height has not changed length due to the minor extra sprung mass of the strut spacer. Any change in coil working height means a change in strut length, according to;
The rigid nature of the solid strut spacer and the lack of change in coil working height means the entire strut assembly is forced to a lower position by the thickness of the strut spacer.
This in turn means a new effective open length, longer by the thickness of the strut spacer;
New strut open length (effective) = Old strut open length (normal) + strut spacer thickness, as shown in the diagram below;
Note that for 120 OEM strut caps you may also need longer strut cap studs to accommodate the strut spacer.
Any change in strut open length forces the IFS lower control arm to a new position, lowering the wheel relative to the guard, and creating a new ride height according to;
Due to the leverage mechanics of the lower arm, out at the ball joint/hub position, the arm lowering/bottom rim to guard has increased by the motion ratio multiplied by the spacer thickness, or 1.89 x spacer thickness, roughly doubling. This means a strut spacer will increase your lift by roughly twice the thickness of the spacer. As there has been no change in the remaining down travel on the strut shaft at the increased ride height position, this means you won’t lose any droop with the use of a strut spacer.
These two inescapable geometrical functions of the IFS are what dictate the nature of strut spacers and their resultant effects on vehicle ride height.
So a strut spacer forces the lower arm to a new position and creates a higher ride height with no loss of droop. How and at what geometrical numbers can they become dangerous and cause mechanical catastrophes such as CV’s being ripped out, struts being bent/sheared on their lower ringeye connecting posts, or upper arm ball joints being bent or ball joint posts sheared?
Both of the geometrical limits of the IFS must be considered, bump and droop.
Strut spacer at bump;
For the Prado 120 IFS, the geometrical bump limit is defined by when the lower arm contacts the chassis mounted bumpstop, at a strut length of 468mm. However, this is not the strut length which is important for determining maximum strut spacer thickness.
When considering the potential thickness of a strut spacer, 100% compression of the bumpstop must be accounted for as shown below. Rubber bumpstops will undergo extreme deformation and pancake to very thin dimensions under high acceleration/G-out/bump out conditions. This has been observed with high speed video/go pros etc.
The 120 Prado front OEM bumpstop is 25mm thick, which means that 468mm bumpstop contact length reduces to 468-(25 x 1.322) = 435mm in a severe bump out condition. The extra factor of 1.322 is due to the motion ratio between the bumpstop/arm gap and the strut shaft travel.
This motion ratio between the bumpstop gap and the strut shaft length is what determines the allowable thickness of a strut spacer. Many will not be aware/neglect the motion ratio and do a simple calculation like 468-25 = 443mm, and assume a 10mm strut spacer would be ok. This is very wrong.
This also means if you go and add in a strut spacer, you also change the effective closed length by;
New strut closed length (effective) = Old strut closed length (normal) + strut spacer thickness
OEM and most after market struts are 430-433mm or longer closed length, demonstrating that 100% bumpstop compression has been built into the closed length. Your typical Bilstein for a Prado is 433mm closed length.
As an example, if you added say a 10mm spacer, this means an effective closed length of 433+10 = 443mm. At 443mm closed length, the bumpstop has compressed by ca. -19mm, leaving another 6mm of bumpstop compression which cannot be accessed as the strut will have already bumped out.
There are even more extreme cases, such as 35mm thick spacers being used on top of OEM struts on a Hilux, shown below;
This 35mm spacer would have pushed the effective closed length to 430+35 = 465mm, so in this case, the OEM bumpstop has been compressed by -2.35mm before the strut has bumped out, and the resulting damage is extreme (note that the Hilux IFS bump geometry is the same as the 120, but that the droop geometry is longer due to a different upper ball joint geometry).
Even without a strut spacer present, it is still possible for a strut to be bumped out. Typical annular ringeye bush thickness is in the 8mm range, and this can be compressed during a severe bump out, allowing for the strut to travel 8mm below the 435mm safety length which defines 100% bumpstop compression. In this case, you would need a closed length of 435-8 = 427mm. There are some after market struts with 420mm closed length, which can be considered bump proof.
Some will say to restrict the maximum strut spacer length to 10mm, but this anecdote is usually quoted with absolutely no knowledge whatsoever of the IFS bump geometry, and it is dangerous to say the least.
The maximum safe strut spacer thickness in bump geometry is 0.00mm.
Strut Spacer at droop;
As you gain lift but don’t lose any droop (due to strut lowering) with a strut spacer, there are just as many potentially damaging events that can occur.
Due to the effective strut open length lowering effects of the strut spacer, you can now potentially infringe upon the binding limits of the ball joint in the upper arm, or even the CV binding limits to the extreme case of pulling/popping your CV shafts out from the diff housing.
The first case is the binding of the upper arm ball joint which occurs at 575mm open length on the Prado 120. With the knowledge that effective open length = strut open length + strut spacer thickness, it’s a simple matter to determine the geometrical limits.
If you had for example a typical BE5-A712 Bilstein strut on your Prado 120, with open length = 561mm, if you added a 15mm strut spacer (to give you 15x1.89 = 28.35mm extra lift), the new effective open length would be 561 + 15 = 576mm. Just this extra 1mm means you are now hanging the IFS on the upper arm ball joint rather than the strut. The strut is designed to cope with the ca. 120kg worth of unsprung mass hanging off it, but the upper arm ball joint is not.
Under these circumstances where the effective open length is in between upper arm ball joint bind and CV bind, then the front suspension hangs/tops out on the ball joint, which can result in repetitive impact damage and/or complete shearing of the ball joint post that connects to the spindle arm.
In either case, the potential exists for catastrophic ball joint failure. The photo below shows repetitive top out damage that has occurred on the lip of the ball joint seat. It would not have been observable underneath the ball joint boot.
The more extreme case can be observed when pushing the effective open length beyond CV bind which occurs at 589mm open length. This case occurs when very thick spacers are used, 25mm or more. Imagine adding a 25mm spacer to that BE5-A712, giving 561 + 25 = 586mm. Straight away you’ll feel the CV binding up in the 585-590mm range. Under high load acceleration, you can easily disengage the CV tripod at around 560-565mm open length. With even thicker spacers, CV’s have been pulled completely out of the front diff housing.
Another problem with drooping to long open lengths is the OEM upper arms will bind on the coils. This can be alleviated by rotating the coils, however it is technically illegal. You’ll need after market upper arms to fix this.
The lesson from this is clear, know the geometry and all of the motion ratios present in your IFS before you dare think about using a strut spacer!
In my opinion, no strut spacer is sensible. If somebody is telling you that you need a strut spacer, then you have purchased the incorrect open length struts. Further, I have not yet met a retailer of strut spacers who is aware of geometrical bump limits or bumpstop gap:strut shaft motion ratio on any vehicle. I regard this as highly irresponsible considering the damage that can result.
Coil Spacers (preload spacers):
Coil spacers (or preload spacers) can be understood easily from the perspective of ride height changes, and the relationship between lift and droop. The photo below shows a coil or preload spacer;
A coil spacer should only be considered functional once it has preloaded the coil enough to shorten the preload height in the assembled strut state beyond the OEM coil seat position.
Once your coil spacer shortens the preload coil height past the OEM coil seat length, then you will both add lift and reduce droop on your IFS. The effects of preload are discussed at length here;
http://www.pradopoint.com/showthread...-we-do-with-it
The diagram below shows the effects of the preload spacer;
As there is no change in coil working height, the preload spacer forces the strut to a longer length and subsequently a new lower arm position, giving you a new lifted ride height. However, for the preload spacer, this means the shaft must lengthen, and therefore some down travel of the shaft is consumed, subsequently lowering your droop.
A coil/preload spacer works in a similar manner to a Bilstein circlip. Some manufacturers such as TJM utilise quite thick coil spacers, such as 20mm or more on quite stiff and short free height coils and short open length struts to achieve vehicle lift. This is a recipe for topped out leaking struts, as seen by the oily strut in the photo above. Under top-out conditions at full droop, the piston head impacts hard on the shaft seal/rod guide, and eventually after repetitive impacts, the oil seals suffer bypass. This doesn’t have to happen with a wheel in the air. With minimal droop, you’ll do this just driving your Prado down the street.
The biggest danger in using coil spacers is in using short open length struts. The plot below shows what can happen for a 10 and 20mm spacer for several different open length struts. This example uses a 600lb/in coil. Any lift you gain is lost in droop in a ca. 1:1 ratio.
With short open length struts on large coil spacer thickness, repetitive top outs will occur due to minimal droop, generally leading to leaking struts. In more severe cases, piston heads can be cracked, and top out spacers smashed into pieces. These broken pieces can jam themselves in the piston bleed circuit, and also in between valve shim plates. You can see that even with only a moderate 50mm lift, you would only have around 55mm of droop on a 555mm strut. It’s very easy to quickly run out of droop when you lift too far on the IFS, particularly with short open length struts. Damage can occur easily.
The obvious and careful solution in using coil spacers is to a) ensure the strut is maximum open length possible, so as to minimise droop loss, and b) use the longest coil free height possible to ensure the coil spacer is minimal thickness. Keeping your lift under 50mm will also give you maximum droop and minimal chance of top-outs. This applies to Prados, FJ’s and the Hilux.
Avoid the use of strut spacers altogether, and use coil/preload spacers conservatively with long open length struts and you’ll minimise potential damage to your IFS.
Best
Mark
There’s a lot of confusing misinformation and poor terminology about how strut and coil spacers for the IFS work, so I thought a fresh perspective was needed to physically understand what they do, and why they can be potentially very damaging.
I’ll break this into sections about each.
Strut Spacers:
The photo below shows the location of the strut spacer;
Adding a strut spacer above the coil actually adds a small amount of mass to the vehicle sprung mass, and this is the more physical perspective from which it should be understood.
When you calculate the effects of this extra sprung mass using a ride height calculation, the effects on the coil working height are very small, meaning that there is no obvious change in the final coil working height under vehicle sprung mass load. For example, a 10mm steel spacer would weigh around 0.45kg, and the resultant decrease in ride height would be 0.1mm, barely noticeable.
This means that there are no significant measurable changes in the shaft position, as the coil working height has not changed length due to the minor extra sprung mass of the strut spacer. Any change in coil working height means a change in strut length, according to;
The rigid nature of the solid strut spacer and the lack of change in coil working height means the entire strut assembly is forced to a lower position by the thickness of the strut spacer.
This in turn means a new effective open length, longer by the thickness of the strut spacer;
New strut open length (effective) = Old strut open length (normal) + strut spacer thickness, as shown in the diagram below;
Note that for 120 OEM strut caps you may also need longer strut cap studs to accommodate the strut spacer.
Any change in strut open length forces the IFS lower control arm to a new position, lowering the wheel relative to the guard, and creating a new ride height according to;
Due to the leverage mechanics of the lower arm, out at the ball joint/hub position, the arm lowering/bottom rim to guard has increased by the motion ratio multiplied by the spacer thickness, or 1.89 x spacer thickness, roughly doubling. This means a strut spacer will increase your lift by roughly twice the thickness of the spacer. As there has been no change in the remaining down travel on the strut shaft at the increased ride height position, this means you won’t lose any droop with the use of a strut spacer.
These two inescapable geometrical functions of the IFS are what dictate the nature of strut spacers and their resultant effects on vehicle ride height.
So a strut spacer forces the lower arm to a new position and creates a higher ride height with no loss of droop. How and at what geometrical numbers can they become dangerous and cause mechanical catastrophes such as CV’s being ripped out, struts being bent/sheared on their lower ringeye connecting posts, or upper arm ball joints being bent or ball joint posts sheared?
Both of the geometrical limits of the IFS must be considered, bump and droop.
Strut spacer at bump;
For the Prado 120 IFS, the geometrical bump limit is defined by when the lower arm contacts the chassis mounted bumpstop, at a strut length of 468mm. However, this is not the strut length which is important for determining maximum strut spacer thickness.
When considering the potential thickness of a strut spacer, 100% compression of the bumpstop must be accounted for as shown below. Rubber bumpstops will undergo extreme deformation and pancake to very thin dimensions under high acceleration/G-out/bump out conditions. This has been observed with high speed video/go pros etc.
The 120 Prado front OEM bumpstop is 25mm thick, which means that 468mm bumpstop contact length reduces to 468-(25 x 1.322) = 435mm in a severe bump out condition. The extra factor of 1.322 is due to the motion ratio between the bumpstop/arm gap and the strut shaft travel.
This motion ratio between the bumpstop gap and the strut shaft length is what determines the allowable thickness of a strut spacer. Many will not be aware/neglect the motion ratio and do a simple calculation like 468-25 = 443mm, and assume a 10mm strut spacer would be ok. This is very wrong.
This also means if you go and add in a strut spacer, you also change the effective closed length by;
New strut closed length (effective) = Old strut closed length (normal) + strut spacer thickness
OEM and most after market struts are 430-433mm or longer closed length, demonstrating that 100% bumpstop compression has been built into the closed length. Your typical Bilstein for a Prado is 433mm closed length.
As an example, if you added say a 10mm spacer, this means an effective closed length of 433+10 = 443mm. At 443mm closed length, the bumpstop has compressed by ca. -19mm, leaving another 6mm of bumpstop compression which cannot be accessed as the strut will have already bumped out.
There are even more extreme cases, such as 35mm thick spacers being used on top of OEM struts on a Hilux, shown below;
This 35mm spacer would have pushed the effective closed length to 430+35 = 465mm, so in this case, the OEM bumpstop has been compressed by -2.35mm before the strut has bumped out, and the resulting damage is extreme (note that the Hilux IFS bump geometry is the same as the 120, but that the droop geometry is longer due to a different upper ball joint geometry).
Even without a strut spacer present, it is still possible for a strut to be bumped out. Typical annular ringeye bush thickness is in the 8mm range, and this can be compressed during a severe bump out, allowing for the strut to travel 8mm below the 435mm safety length which defines 100% bumpstop compression. In this case, you would need a closed length of 435-8 = 427mm. There are some after market struts with 420mm closed length, which can be considered bump proof.
Some will say to restrict the maximum strut spacer length to 10mm, but this anecdote is usually quoted with absolutely no knowledge whatsoever of the IFS bump geometry, and it is dangerous to say the least.
The maximum safe strut spacer thickness in bump geometry is 0.00mm.
Strut Spacer at droop;
As you gain lift but don’t lose any droop (due to strut lowering) with a strut spacer, there are just as many potentially damaging events that can occur.
Due to the effective strut open length lowering effects of the strut spacer, you can now potentially infringe upon the binding limits of the ball joint in the upper arm, or even the CV binding limits to the extreme case of pulling/popping your CV shafts out from the diff housing.
The first case is the binding of the upper arm ball joint which occurs at 575mm open length on the Prado 120. With the knowledge that effective open length = strut open length + strut spacer thickness, it’s a simple matter to determine the geometrical limits.
If you had for example a typical BE5-A712 Bilstein strut on your Prado 120, with open length = 561mm, if you added a 15mm strut spacer (to give you 15x1.89 = 28.35mm extra lift), the new effective open length would be 561 + 15 = 576mm. Just this extra 1mm means you are now hanging the IFS on the upper arm ball joint rather than the strut. The strut is designed to cope with the ca. 120kg worth of unsprung mass hanging off it, but the upper arm ball joint is not.
Under these circumstances where the effective open length is in between upper arm ball joint bind and CV bind, then the front suspension hangs/tops out on the ball joint, which can result in repetitive impact damage and/or complete shearing of the ball joint post that connects to the spindle arm.
In either case, the potential exists for catastrophic ball joint failure. The photo below shows repetitive top out damage that has occurred on the lip of the ball joint seat. It would not have been observable underneath the ball joint boot.
The more extreme case can be observed when pushing the effective open length beyond CV bind which occurs at 589mm open length. This case occurs when very thick spacers are used, 25mm or more. Imagine adding a 25mm spacer to that BE5-A712, giving 561 + 25 = 586mm. Straight away you’ll feel the CV binding up in the 585-590mm range. Under high load acceleration, you can easily disengage the CV tripod at around 560-565mm open length. With even thicker spacers, CV’s have been pulled completely out of the front diff housing.
Another problem with drooping to long open lengths is the OEM upper arms will bind on the coils. This can be alleviated by rotating the coils, however it is technically illegal. You’ll need after market upper arms to fix this.
The lesson from this is clear, know the geometry and all of the motion ratios present in your IFS before you dare think about using a strut spacer!
In my opinion, no strut spacer is sensible. If somebody is telling you that you need a strut spacer, then you have purchased the incorrect open length struts. Further, I have not yet met a retailer of strut spacers who is aware of geometrical bump limits or bumpstop gap:strut shaft motion ratio on any vehicle. I regard this as highly irresponsible considering the damage that can result.
Coil Spacers (preload spacers):
Coil spacers (or preload spacers) can be understood easily from the perspective of ride height changes, and the relationship between lift and droop. The photo below shows a coil or preload spacer;
A coil spacer should only be considered functional once it has preloaded the coil enough to shorten the preload height in the assembled strut state beyond the OEM coil seat position.
Once your coil spacer shortens the preload coil height past the OEM coil seat length, then you will both add lift and reduce droop on your IFS. The effects of preload are discussed at length here;
http://www.pradopoint.com/showthread...-we-do-with-it
The diagram below shows the effects of the preload spacer;
As there is no change in coil working height, the preload spacer forces the strut to a longer length and subsequently a new lower arm position, giving you a new lifted ride height. However, for the preload spacer, this means the shaft must lengthen, and therefore some down travel of the shaft is consumed, subsequently lowering your droop.
A coil/preload spacer works in a similar manner to a Bilstein circlip. Some manufacturers such as TJM utilise quite thick coil spacers, such as 20mm or more on quite stiff and short free height coils and short open length struts to achieve vehicle lift. This is a recipe for topped out leaking struts, as seen by the oily strut in the photo above. Under top-out conditions at full droop, the piston head impacts hard on the shaft seal/rod guide, and eventually after repetitive impacts, the oil seals suffer bypass. This doesn’t have to happen with a wheel in the air. With minimal droop, you’ll do this just driving your Prado down the street.
The biggest danger in using coil spacers is in using short open length struts. The plot below shows what can happen for a 10 and 20mm spacer for several different open length struts. This example uses a 600lb/in coil. Any lift you gain is lost in droop in a ca. 1:1 ratio.
With short open length struts on large coil spacer thickness, repetitive top outs will occur due to minimal droop, generally leading to leaking struts. In more severe cases, piston heads can be cracked, and top out spacers smashed into pieces. These broken pieces can jam themselves in the piston bleed circuit, and also in between valve shim plates. You can see that even with only a moderate 50mm lift, you would only have around 55mm of droop on a 555mm strut. It’s very easy to quickly run out of droop when you lift too far on the IFS, particularly with short open length struts. Damage can occur easily.
The obvious and careful solution in using coil spacers is to a) ensure the strut is maximum open length possible, so as to minimise droop loss, and b) use the longest coil free height possible to ensure the coil spacer is minimal thickness. Keeping your lift under 50mm will also give you maximum droop and minimal chance of top-outs. This applies to Prados, FJ’s and the Hilux.
Avoid the use of strut spacers altogether, and use coil/preload spacers conservatively with long open length struts and you’ll minimise potential damage to your IFS.
Best
Mark
Comment