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 Lowering amount on the front and back

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Hiace Master
Hiace Master

Number of posts : 716
Home City : Cambridge, UK
Model and year : Model: KD-KZH100G-MRPGT
Year: 1996
Colour: 4K1
Trim: FN42

Registration date : 2016-07-16

PostSubject: Re: Lowering amount on the front and back    Thu Jun 14, 2018 1:32 pm

OK some more numbers.

Squashed bushings from the cut end:
The Toyota bushing below the rear spring has a natural height of 21mm (under extreme force when the bottom collapses, so in normal usage probably about 29mm. That's 29mm from the spring pan base to the spring, full of Toyota rubber.

When Mr Style cut down the spring I expect he thought that the bushing would remain at 29mm, but due to the crushing point load of the cut end the rubber at that point compresses to 12mm only, a loss of 17mm.

The geometry of the trailing arm is 60cm with the spring base centred some 12cm from the axle end.
This gives a normal well seated spring a 48/60 = 80% leverage (or 1.25 going the other way).

So the spring's cut end would have slowly sunk 17mm into the rubber losing 17 * 1.25 = 21.25mm of ride height, coincidentally the amount that I've very keen to restore. Of course the geometry is not accurate with a cut spring because now we have a point load at some random point within the spring pan so the ride height will also vary depending upon the rotational position of the cut spring as to exactly where the point load end of it bears.

My initial measurements indicated that in use the spring needed to be 10mm longer. The initial simple plan was to add a 10mm rubber plate and it's mission accomplished. Now thanks to my Japanese friend's angle grinding antics I'm having to manufacture a custom seat from a spring assister to sit it properly at the right height.

Ideas for mitigating the disaster of cutting a square ended spring on the Hiace SC:
So now I'm thinking that I should give it a 20mm high seat rising to support the (now tangentally cut) spring properly as it comes out of the trailing arm. This will then give me a basis to say it' it's OK at that height or if it needs correcting enough to warrant putting a shim in there.

As my new mount will support the spring as it climbs out of the arm socket it needs to be the correct angle so as not to fully support it - to allow some of that spring to still move else I'd have effectively shortened the working spring length even more and will get an even harsher ride. Ideally I need to be around 3-5mm low of the high side of the mount when it's installed and loaded.

Custom spring alternatives
BTW I also did some spring rate measurements that while rough, are still useful.
The spring rate (of my cut spring, subject to measurement error) is 16.5mm / 100kg.
So a custom made spring for a nice usable low ride height might be:

110mm core diameter (i.e. with 15mm wire you get 95mm inner, 125 outer).
270mm length with 2 properly formed square ends (i.e. a spring you can stand on a table top).
16.5mm / 100kg spring rate.

This would be a proper custom spring to match the cut spring - i.e. slightly stiffer than stock (as it is shorter) and lower, I'd estimate around 15-20mm of bump-stop clearance, so no off-roading but usable around town.

Modern note
Note that these are my measurements and ideas, if you use them and it all goes wrong I'll be surprised and sympathetic only: i.e. at your own risk!
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Hiace Master
Hiace Master

Number of posts : 716
Home City : Cambridge, UK
Model and year : Model: KD-KZH100G-MRPGT
Year: 1996
Colour: 4K1
Trim: FN42

Registration date : 2016-07-16

PostSubject: Re: Lowering amount on the front and back    Thu Jun 14, 2018 7:11 pm

Ok, some more spring remediation work, ideally the final chapter for a while Very Happy.
The ultimate drop is around 35-40 mm on this wagon for me.

Interestingly the original Toyota bottom rubber mounts add around 28mm when new, so simply removing that and putting a 1mm rubber sheet in the trailing arm pan gives a 28 - 1 = 27mm * 1.25 = 35mm drop with only a little extra transmitted noise from the diff etc. Should I buy new springs this is what I will try first..

I'm detailing my work but only as a matter of curiosity - my only recommendation is to never cut the Hiace springs.
However due to the huge saucepan Toyota provided for them to sit in I suspect one can get away with more than on other setups, and why I only noticed the terrible ride and wear on the bump stops.

Spring diary:
As I'm currently a bit stuck with springs cut to 270mm I needed to make a tangental spring mount - which means a mount shaped like a spiral staircase. In the end I had to buy some monster spring assisters and cut them.  I'm hoping they will be strong enough, at least the load is distributed reasonably over the first half coil.

The length calculations were such:
Ideal final height of spring + lower bush = 230mm (for a 35mm drop).
Free length of spring = 270mm
Loaded length of spring = 202mm

Spacer therefore required = 28mm
Bush length circumference = 430mm (the spring assister was cut down to this).

Ramp calculations
Coils are 4 loops over 202mm = 50.5mm for the each loop. Remove 5mm to not shorten the effective spring length and we want a 45mm ramp. The rubber spring assisters were 50mm max, so I chose the half-way point of the circle to cut to and cut a wedge for 28mm to the edge over half the rubber length, which gives a 22mm ramp over half the coil which should provide some progressive springing as required.

Bear in mind the free (cut) end of the spring goes on the lower part of the rubber ramp. I cut the reverse side away to preserve the moulded channel for the spring to live in. I also used the uncut end of the assister for the small side of the ramp as that still takes the most load. Note that spring assisters are full of holes, I'm not sure how much of an issue this is but I did plug the ones near the cut end with spare rubber pipe - perhaps silicone sealant may be suitable - I don't know, I have decided to see how they get along as they are, if I find the van sinking I'll maybe do something. They were only £13 and pretty easy to cut so it's not a major expense if I need to buy some more and do it again.

Look how cute that spring looks in the new ramp mount, much better.

I decided to position the free end of the spring toward the axle, because that's the side that has the least load on it as it's almost directly holding the van up at that point. With that in mind you can see in the pics the spring sitting at a slight angle (due to the ramp allowance to provide progressive springing) but basically happy and without undue stress or balance issues.

Putting the suspension arms back is more difficult than disassembly because you can't drift the axle end bolts in, so careful balance, patience and care is needed. It also helps to have a ramp and a precision jack LOL, but it's still not easy - especially if like me you didn't take the wheels off. I left the wheels on because a: They are a faff to remove due to the centres b: they are really useful to hold up the axle, c: I used them to judge looks with the ride height.

What can help is that when it's all lined up and nearly in, a small woodworking hand-trigger operated clamp can simply draw them in - I found that very helpful. Similarly a clamp used to pull the arms toward the forward post is ideal for getting those ones in.

Required torque of these is unknown, I'll do mine up to 90NM as wheel-nuts seem to be about that.

Put back down the van has an immediate 40mm lowering which is I suspect due to general settling of the bushes + general calculation and measurement errors.
The bump-stops have what looks like over 25mm of gap which is what I was aiming for, so if it doesn't sink more than another 5-10mm it will all be fine, regardless I doubt it will ever sink as low as before though.

So the van still looks slammed and low, but you can see the suspension can work now and the springs are no longer digging holes and scraping the sides.

After a few days the height was sagging, probably due to the multiple holes in the rubber (they were assisters - not mounts!!) so I shimmed the bottom of the pan with a 130mm disc of solid 10mm rubber with an 85mm disc of 10mm solid rubber to keep the assister rubber from straying inwards. I then drilled the rubber and bolted it with a penny washer and M6 bolt and also drilled a drain hole to allow any trapped water out too. Currently it's sitting at a 30mm drop and looks good, I'd expect that to settle to a 35mm drop over time so we'll see how it works out!  Looks quite cool at 30mm in fact.

Had a quote for new Toyota springs, these can be sat on thin rubber at the bottom which will increase cabin noise but be a safe way to lower about the right amount - but with the original spring rate (which could be increased with some spring assisters used in the conventional way): £145 each. No time to try this now though.
Also it looks like NZ has another option:
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