simon's learning what to do with the polo thread

[simon k]

I wrote a long post about springs and whatnot last night and then somehow the browser tab closed on me, so I went to bed

Today I found out something pretty cool - Nissan (S13 Silvia etc.) strut tops are the same pattern as the Polo, but reversed - they both have the same 105mm PCD with 3 studs, the Polo has the single stud towards the wheel, the Nissan the single stud towards the engine. Handy!

I found this out because I was at another mechanic friend's workshop and asked him if he had any random coilover bits spare - he gave me a pair of spring top hats and a single S13 strut top mount plate, he says the other one is kicking around somewhere and will find it for me.

So I have a pair of spherical bearing mounts, they're cheapies and the bearing is very stiff, but I think they'll do the job - will replace the bearings if I need to. The camber adjustment screw spacing is the same as the strut top mount plate above, also handy!

The spring top hats do not have bearings in them, but I can machine a bearing recess into them to sit a thrust bearing between the top hat and the spherical bearing.

so here's what I've scrounged so far (I have a pair of the black & red bits, the pair for the orange bit is hopefully coming)


So the next issue is the spring length. I didn't get to take one of the struts out of the car, but I took a wheel off and did some measuring. The image below shows the strut at full droop and with my jack under the balljoint, so probably not compressed to static ride height. Should I do that? maybe even drive one wheel up on to a ramp to get more compression going?



The line 20mm below the domed top of the damper is the position of the top of the current spring. I got the 20mm by measuring down from the mounting plate where the tower bolts drop through the tower.

I'm not sure whether I calculate spring compression at the spring, or at the wheel. My logic is often flawed, but I think it's at the spring because the body of the car is pushing down on the spring. When the wheel hits a bump, the lower control arm is acting as a lever to apply more load to the spring, but I don't need to worry about that right now.

So, going with what I have there, at full droop there's 250mm between where the top of the standard spring and the sway bar link mount, 150mm of that is the damper. With the jack under it the spring is compressed 60mm - if the stock spring is 2.55kg/mm then 60mm compression is only 153kg, which kinda sounds OK knowing that I only had a jack under the lower control arm.

From some other threads, the Polo weighs 1190kg, and is 60/40 weight distribution. I make that 357kg on each front corner.

If that's the correct way to look at spring compression & rates, then the next thing to do is take Gary's recommended spring rate of 250lb/in (4.46kg/mm) and find out how much it compresses with 357kg on it - 357 * 4.46 is 80mm.

This page suggests that an 8" (203mm) spring has 5.2" (132mm) of free travel. If I put 357kg on that spring, then it'd compress to 132-80 = 52mm of free travel left before coil bind.

This page says that a 6" (152.4mm) spring has 4.2" (107mm) of free travel. If I put 357kg on that, then I'd be down to 27mm before coil bind - that's not much!

But I guess right now we're more interested in how much room there is for a spring, rather than how much travel we want? or maybe we want as much travel as possible that will fit in the space available?

If I was to assume that the spring adjuster nut is 20mm tall, it'd reduce the free travel of the damper at full droop to 230mm, which means the spring will be flopping around with 30mm spare. Putting the car on the ground will take that 30mm out, and then squash the spring another 80mm which takes us down to (220 - 30 - 80 + 20) 140mm between the top of the spring and the sway bar link mount plate.

Going back to the stock spring and putting 357kg on it, it's meant to compress by 140mm. So if I have my 250mm and take 140mm off that then we're sitting at 110mm between the top of the spring and the sway bar mount plate.

An 8" spring will add on 30mm extra ride height? even if fully un-adjusted?

I'm sure there's a flaw in my logic somewhere...

[sambb]

Nice find re the adjustable tops. So you're thinking that you'll position them in the +- caster plane yeah. Simon I'll dig out the stock springs tomorrow and measure their free length for you. flick me a text to remind me if you haven't heard from me by 7.30pm.

simon these were Gavs numbers with tools/spare removed and half a tank on MCA's scales.

Also if it helps, I can give you the free length + compressed length of my 7kg springs if that helps you back calculate something useful.

[simon k]

Nice find re the adjustable tops. So you're thinking that you'll position them in the +- caster plane yeah.

no, in the camber plane - the slots in the orange plate you see there run east-west across the car. The way the plate is sitting, you're kinda standing near the left-hand door mirror. That plate may be no good but it'll do for a mock-up at least. I may need a thicker plate so I can mill the slots wider for the screw heads to sit below the surface... will have to see

Simon I'll dig out the stock springs tomorrow and measure their free length for you. flick me a text to remind me if you haven't heard from me by 7.30pm.

I think that'd be handy Sam, I am fairly confused... not sure that knowing the free length of the spring helps that much, but can't hurt

[ATTACH=CONFIG]35487[/ATTACH]
simon these were Gavs numbers with tools/spare removed and half a tank on MCA's scales.

Also if it helps, I can give you the free length + compressed length of my 7kg springs if that helps you back calculate something useful.

I saw that picture in the thread where front/rear weight distributions were posted, there were a couple of different distributions thrown around, Gav's there is 65/35

Your front springs should be compressed between 51 and 55mm - I'm keen to know how far the bottom of the spring is from the bottom of the strut, if you don't mind crawling under the car that is. They are 8" springs though, right? do you have 'helper' springs? the soft springs that keep them from falling out under droop/jacking up

[sambb]

Oh ok in the camber plane. Very very good find then. I think if you remove the rear seats and run only about 5 litres eg for a motorkhana you'd see that shift a bit forward all those things are on the back wheels circa 35kg. You can certainly feel the change when you do it. I had 180mm fronts initially and now run 200mm so that I could get a bit more bump travel before coil bind. I'll measure the 200's and I think I wrote down the numbers for the 180's. No helpers on the front. They are 5mm off being captive at full droop.

[Sydneykid]

Quite a lot of over thinking going on, so let me try and simplify it.

The top spring seat sits on the shock shaft, above the shock body. As such it doesn't need a bearing, it moves (angularity) with the shock shaft. You just need a solid spacer between the spherical (in the strut top) and the spring seat to allow for the angularity changes, so the spring seat doesn't hit the strut top (that would break the shock shaft).

If the spring rate is, say, 7kg/mm (~ 400 lbs/inch) and the movement ratio is 0.9 and the leverage ratio is the same 0.9, then the effective rate (at the wheel/tyre) is 400 x 0.9 x 0.9 = ~325 lbs per inch. If the front (one side) weighs ~840 lbs (380 kgs) then the spring will compress 840/325 = 2.6" (66 mm) just to hold the car up.

Rule of thumb, we aim for around a minimum of 100 mm of travel (on the front of an FWD race car), usually 70 mm bump (compression) and 30 mm rebound (extension). A combination road and track car should be a fair bit more than that, say 130 mm and 40 mm. The bump (compression) measurement is before bump stop contact.

Inverted Bilsteins have internal bump stops. Take the shock body out of the strut and measure the bump stop length, commonly 50 mm or 75 mm.

You shouldn't have to use helper/tender springs on the front of a FWD car, there is enough weight (from trapped) to compress the spring to ride height. The rear is of course a different story as there is less weight to compress the spring and the spring rate should be much the same as the front.

Common FWD front spring free height is 7" or 8", occasionally 10". Looking at your diagrams you would need to move the bottom spring seat up around 70 mm to keep an 8" spring trapped (or use a spacer). Coil bind height is a spec provided by the spring manufacturer. It varies according to free height (obviously) but it also varies according to the number of coils and the wire diameter. FWIW, a 400 lb spring for example can be designed with less coils and thinner wire to give more travel or thicker wire and more coils to give a lower frequency. That's the spring designers choice depending on the objective. Obviously there is limit as to how few and how thin as that affects the spring durability which is also dependant on the material used and the winding method. High quality spring steel, cold wound is more durable and hence can be designed with more travel than lower quality spring steel hot wound.

Hope that helped.
Cheers
Gary

[simon k]

Quite a lot of over thinking going on, so let me try and simplify it.

thanks Gary - over thinking is normal for me, as is going around in circles trying to solve the same problem the same way and expecting a different result...

The top spring seat sits on the shock shaft, above the shock body. As such it doesn't need a bearing, it moves (angularity) with the shock shaft. You just need a solid spacer between the spherical (in the strut top) and the spring seat to allow for the angularity changes, so the spring seat doesn't hit the strut top (that would break the shock shaft).

I was thinking a bearing between the spring seat and the spherical bearing to allow the spring to rotate with the shock body rather than twist or grind on the seat or make the spherical bearing rotate. Same as this setup - I gather a shim does the same thing, but over-engineering goes with over-thinking... I'll leave that idea out

If the spring rate is, say, 7kg/mm (~ 400 lbs/inch) and the movement ratio is 0.9 and the leverage ratio is the same 0.9, then the effective rate (at the wheel/tyre) is 400 x 0.9 x 0.9 = ~325 lbs per inch. If the front (one side) weighs ~840 lbs (380 kgs) then the spring will compress 840/325 = 2.6" (66 mm) just to hold the car up.

Rule of thumb, we aim for around a minimum of 100 mm of travel (on the front of an FWD race car), usually 70 mm bump (compression) and 30 mm rebound (extension). A combination road and track car should be a fair bit more than that, say 130 mm and 40 mm. The bump (compression) measurement is before bump stop contact.

Cool - this answers some questions - my spring compression calculation was on the right track, but I was leaving out the effective/wheel rate. A 250lb spring ends up with 87mm compression just to hold the car up. With a 200mm spring that only leaves 113mm of bump - I don't know how much I have now, but there is a certain bump on my way home from work where the tyre scrapes on one of the screws holding the mudguard liner... I'd guess at 60-70mm of compression at the tyre, so 80% of that at the damper. I think I really do need to measure how much travel the damper has at the ride height

However, with that second diagram showing damper compression with my jack under the balljoint there's only 90mm of damper travel anyway... I don't think 130mm of bump travel is even possible without > 20kg springs and 6" of clearance between the tyre and mudguard!

Inverted Bilsteins have internal bump stops. Take the shock body out of the strut and measure the bump stop length, commonly 50 mm or 75 mm.

understood - the instructions that came with the Bilsteins said to use the stock bump stop, so I think I installed that. I also used the dust guard, so I can't actually see it... regretting not taking some photos while I was putting them together

You shouldn't have to use helper/tender springs on the front of a FWD car, there is enough weight (from trapped) to compress the spring to ride height. The rear is of course a different story as there is less weight to compress the spring and the spring rate should be much the same as the front.

Common FWD front spring free height is 7" or 8", occasionally 10". Looking at your diagrams you would need to move the bottom spring seat up around 70 mm to keep an 8" spring trapped (or use a spacer). Coil bind height is a spec provided by the spring manufacturer. It varies according to free height (obviously) but it also varies according to the number of coils and the wire diameter. FWIW, a 400 lb spring for example can be designed with less coils and thinner wire to give more travel or thicker wire and more coils to give a lower frequency. That's the spring designers choice depending on the objective. Obviously there is limit as to how few and how thin as that affects the spring durability which is also dependant on the material used and the winding method. High quality spring steel, cold wound is more durable and hence can be designed with more travel than lower quality spring steel hot wound.

Hope that helped.
Cheers
Gary

It does help, but gives me more questions I guess... a mate has offered me some 2.25" x 5kg/280lb springs, not sure what length but I'll grab them anyway, apparently one of the members in our local club has a spring tester he doesn't mind people using...

[sambb]

remember simon I've got some 7in (180mm) 7kg springs you can use if you get stuck. Now that I'm running that front rate on a stock FARB its actually quite streetable.

For what its worth my 200mm springs are compressing to 155mm fitted (seat to seat) = 45mm compressed.

[Sydneykid]

understood - the instructions that came with the Bilsteins said to use the stock bump stop, so I think I installed that. I also used the dust guard, so I can't actually see it... regretting not taking some photos while I was putting them together

I'm not sure that the OE bump stops would fit inside the strut tube. On an OE non inverted shock the bump stop sits on the shock shaft, at the top, so they tend to be larger in diameter than the bumps stops designed to fit inside strut tubes. The rear Bilsteins being non inverted could use the OE bump stops (if they have them) and the OE dust covers.

Cheers
Gary

[Sydneykid]

FWIW, 111 mm at the shock is 111/0.9 = 123 mm travel at the wheel.

Cheers
Gary

[simon k]

I'm not sure that the OE bump stops would fit inside the strut tube. On an OE non inverted shock the bump stop sits on the shock shaft, at the top, so they tend to be larger in diameter than the bumps stops designed to fit inside strut tubes. The rear Bilsteins being non inverted could use the OE bump stops (if they have them) and the OE dust covers.

Cheers
Gary

ah - yeah, you're right. If I used them, I would have put them at the top of the shaft... so thinking about it that way it's probably impossible for me to have used them...

[simon k]

put in my SEAT intercooler today, all ready to go but I'm one hose clamp short

I had to take a little break though, because I dropped it on my big toe!

of course I was wearing my best safety-thongs...



hurts like a mother-f*cker...

[MattSA9N3]

Ouch, plenty of swearing I'm assuming?

[simon k]

I've been mucking around the last few weeks machining my rear plates to give a bit more camber and put a couple of mm toe out into it. I CADed it all up and worked out the difference in thickness the plates needed to be at each mounting bolt

The 3/8" plates are installed in the car, I figured that if I used one of the 1/2" plates it'd give me plenty of room to make mistakes. The 3/8" plates don't give a lot of room for error. Turned out that was a good move because it did take me 3 goes to get the angles right... first I had them all upside down so I would have had massive toe in and positive camber, then I had the toe right but the camber was upside down, then I got it right...

So, first I made a machining boss to hold the plates, it's just a bit of solid round bar with a plate welded to it. I drilled and tapped a hole in the bar and drilled holes in the plate before welding.

Then I machined the face of the plate flat

and mounted one of the plates to it to drill a hole in the centre of the plates, not necessary, but easier than doing it on the drill press

(spinny action shot)

so then the plate can mount on the machining boss using the centre bolt

next I welded some little bits of rod into the holes to locate the plate, and mount spacer shims (as shown - shims cut out shim stock and drilled)

Spacer shims and measurements (in mm) shown for each hole. Hard to get the shims to exactly the right thickness but it's close, within ~0.03mm. I aimed to get them all a little bit thicker than the correct measurement, so all of the errors were at least in the same direction

so then I mounted the plate, with the shims making it sit at the angle I wanted

then machine it flat - here's the finished product for the left hand side against the original. It ended up about 9mm thick, down from 12.5

the knob in the middle where the bolt was holding it to the boss fits neatly inside the stub axle

I installed the left hand side this afternoon, then took the plate from there and machined it to suit the right hand side... Machining and installing it only takes half an hour, just a bit of ****ing around up front. Was correct first time on the right hand side

I don't know what the camber is, but it looks even. I ended up with less toe than I wanted, it's more-or-less straight ahead, 0.1° on the left hand side and 0.02° on the right, totaling 0.6mm. Though that's just measuring with a stringline, the error margin is probably more than that....

[HaydEn]

That’s sick.

Good work man


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[Sydneykid]

Nice, I like it

From memory they have around 2 mm toe in standard, so whatever taper you have on them, I'd double it.

Cheers
Gary