simon's learning what to do with the polo thread

[simon k]

I wonder whether it's worth burning $50 on a set of these - 2Pcs Adj. Front Camber Plate Plates Top Mount for BMW 3 Series E36 M3 318i 323i | eBay

It's safe to assume that the bearings are rubbish, but look replaceable...

I can draw up and get a top plate cut to suit the Polo, so I only need the plate that holds the bearing.... I could buy the bearing retainer and make a plate, but it's about the same cost as buying the above and throwing away what I don't want

[simon k]

ahhh, got me again, I take ages to type something because I'm working as well, then someone posts while I'm mucking about

I've been caught out a couple of times interstate without my camber gauge and used my phone instead. I just cut a piece of aluminium angle to the right length so it sits on the rim, stick the phone to it with some blue tack or double sided tape. I even used velcro once. Using that, camber is easy to measure, just correct for any ground slope.

For measuring caster, which is camber change over +20 and -20 = 40 degrees of steering angle, I get the wheel aligner to tell me how many turns of the steering wheel equals 20 degrees. Then I write that down in the book, that lives with every car I look after. So when I want to do an alignment at the track I only need to turn the steering wheel the appropriate amount (ie; not use turn plates). I do roll the car back and forwards a little to release any angle restriction from the turning. At home I have 2 pieces of 3 mm thick alloy sheet, greased in between, that I use as cheap (and lightweight) turn plates.

Of course these days "book" equals iPad with settings loaded in the cloud so that I can pull them down anywhere anytime.

Cheers
Gary

I went to bunnings yesterday and looked at their magnetic protractor/inclinometer, but they're $40 - I think I'll get a cheap digital one off ebay. My phone has buttons on the sides, and the back is curved, so it's a bit hard to be consistent with it!

And yep, from other stuff I was reading about the wheel angle, 20° is just an arbitrary angle they use, as long as it's consistent and repeatable it doesn't matter - a number of turns of the steering wheel is just as good.

I took your advice about the turn plates - I think you've posted it before - I had some bits of scrap stainless and put grease between them when I was redoing the toe after putting the balljoints in

What do you think of getting some of those cheap strut tops to just use the bearing carrier?

[simon k]

I've been caught out a couple of times interstate without my camber gauge and used my phone instead. I just cut a piece of aluminium angle to the right length so it sits on the rim, stick the phone to it with some blue tack or double sided tape. I even used velcro once. Using that, camber is easy to measure, just correct for any ground slope.

I discovered that the back of samsung galaxy tablets are magnetic - we're using one for dashboard in the Honda and it kept sticking to the roof when I was fiddling, I thought it was wet but it wasn't, then it twigged...

[Sydneykid]

I wonder whether it's worth burning $50 on a set of these - 2Pcs Adj. Front Camber Plate Plates Top Mount for BMW 3 Series E36 M3 318i 323i | eBay
It's safe to assume that the bearings are rubbish, but look replaceable...
I can draw up and get a top plate cut to suit the Polo, so I only need the plate that holds the bearing.... I could buy the bearing retainer and make a plate, but it's about the same cost as buying the above and throwing away what I don't want

Shoot $48 the Chinese repetition shops are taking over the world.

Once the profile master is stamped the bearings are the most costly part, so that's where the cost cutting kicks in. They occasionally use an obscure bearing size so try and establish the dimensions before you buy them. Then price a decent quality replacement from CBC etc. Especially check that the spherical ID matches the shock shaft OD.


Cheers
Gary

[simon k]

Shoot $48 the Chinese repetition shops are taking over the world.

Once the profile master is stamped the bearings are the most costly part, so that's where the cost cutting kicks in. They occasionally use an obscure bearing size so try and establish the dimensions before you buy them. Then price a decent quality replacement from CBC etc. Especially check that the spherical ID matches the shock shaft OD.


Cheers
Gary

got it - so maybe worth rolling the dice

I've asked the seller for some dimensions already - I doubt I'll get anything intelligent, but we'll see...

I think I'll be taking a shock assembly out tomorrow to do some measuring

[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

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