Hey all,

I’ve recently been taking a close look at the most optimal valving-coil rate setup for the IFS.

Real world tested numbers can vary as a function of strut diameter. For example, 2” struts have for a while been using a high rebound-mid range coil rate setup, such as the D563 Bilstein/660lb/in King combination. This setup is 3400N:660lb/in at 0.52m/s. It has been used in FJ’s a lot by our good mate Jason at Sydney Suspension. Feedback from drivers is excellent. This setup is also very close to the OEM 150 spec combination defined by Jim at Quadrant, which uses the 24-173032-1/675lb/in H&R combination. This setup is 3200N:675lb/in. I also know that Ironman offer a 3700N:650lb/in combination for the Prado.

Larger diameter struts can use significantly different valving-coil rate combinations, for example, 5000N:960lb/in has been used in 66mm struts, with excellent driving quality.

The plot below shows the Valving-Coil rate connection for typical 2” diameter struts. The lower ellipse is for 120 OEM spec, the upper ellipse for 150 OEM spec. Note that by the time you add bullbars, batteries etc. to a 120, you end up in the 150 spec ellipse, so based on heavier kerb weights, it is actually better to run the 150 OEM spec combination in the 120 IFS.



You can see that there are several possible high rebound-coil rate possibilities in the 150 ellipse for 2” struts. I am trying the D563/H&R 675lb/in combination in my 120. I am trying the H&R cold rolled coils as theoretically they shouldn’t ever settle/set compared to hot rolled coils. The X/K 675lb/in coil from H&R is only 366mm in free height, significantly shorter than most other after market coils.

Due to the short length of the H&R X/K coils, I made the decision to put coilover sleeves on my D563’s so I could wind the preload up a lot and dial my ride height in to the millimetre. Even with an extra 10mm of preload past the OEM coil seat position, the X/K coils will still have less preload on them compared to a longer Dobinson or King coil. Below is a photo of the coilover sleeves I purchased from PSR at the Gold Coast.



The next step in building my struts was to spend an afternoon at Fulcrum to machine a new lower circlip groove, and to delimit the open length of the D563’s from 575mm back to 570mm. The reason for delimiting to 570mm is because I am running the new Superpro camber/caster adjustable upper control arms from Fulcrum, which have the ball joint binding up at 575mm. As such you need to be under 575mm open length on your struts to ensure the IFS droops on the struts and not the ball joint.

So the next step was to use a 5mm spacer to increase the top-out spacer to 5mm longer, which reduces the open length by 5mm. The photo below shows the D563 being pulled apart.



The 5mm spacer is then added to the top of the top-out spacer. This also works the other way around - to increase open length, you reduce the length of the spacer.



After putting the delimited D563 back together and re-gassing with nitrogen to 300kPa, they are ready to have new synthetic polyelastomer bushes pressed in, and a new lower circlip groove machined in for the coilover sleeve. Next photo shows the new circlip being machined.



After machining the new circlip grooves, the D563’s are ready to dry press the coilover sleeve on. The sleeve needs to be pressed on dry so there is good frictional contact between the strut body and the sleeve to ensure the sleeve cannot slip when you adjust the preload collars with a c-spanner.



Next photo shows the sleeves pressed on ready for a coil. I have also used the new synthetic polyelastomer material in the strut caps.



After pressing the the X/K H&R 675lb/in coils onto the D563’s, the final assembled struts are shown below.



Here you can see the collars getting rotated quite easily in-situ with vehicle weight off the struts, a breaker on the c-spanner makes it easy.



The BE5-D563-M2 struts I have used are M2 or modification 2 struts, which have slightly higher rebound than the M0 versions. My combination is 3667N:675lb/in.

So after all of this effort to build these struts, what do I think about them?

After field testing them with my new Superpro adjustable arms on Fraser Island, I can say that these struts have transformed my Prado. With my Prado over GVM, they performed exceptionally well in the rough stuff. Soft sand, soccer ball sized rutted out high tide stretches, bouncy inland tracks, I hit them all pretty hard, and the handling is a massive improvement. No more pitch or roll that won’t cycle out. I can waddle through the rutted out stuff (with soccer ball sized washout holes), or bang through them at high speed, either way the Prado sits very still through it, and the suspension cycles beautifully.

On road the low-mid range valving is surprisingly supple, which is more due to getting the correct coil rate. Speedbumps feel smooth at 40km/h. At higher velocities you will feel the higher valving kick in, so a big pothole can feel stiff on the bitumen.

This front end setup won’t be for everyone, as many won’t like the stiffer feeling of the high rebound on the bitumen. It is off road where this setup will easily outperform softer or mismatched setups.

I’d also like to add that this front end combination works brilliantly with my rear end setup, 80 series shocks with 3300N:600N matched to a 280lb/in 430mm coil. Everything is cycling very nicely now in the suspension, rather than the Prado lurching, pitching and rolling and requiring braking or slower speeds to control undesirable dynamics. I also have Superpro bushes in my front upper control arms and in my rear upper trailing arms. I believe this is also a big factor in allowing the suspension to articulate more freely. I’m very happy with this setup. My next step is to upgrade the rear shocks to 66mm monotubes to keep the shock fade down in my heavily loaded rear.

Big thanks to Bevan at Fulcrum who spent a Friday arvo putting these struts together.

Best

Mark