simon's learning what to do with the polo thread

[simon k]

Back to Gary's list!

haven't done these

#3 Front negative camber for obvious reasons - I'd like to do Sam's TT balljoint mod, but I'm worried about wrecking the strut top bearings
#5 Lower the rear roll centre - How do we do this?
#7 Front roll centre correction and bump steer elimination - ditto, lower the bottom ball joint?
#8 LSD - one day
#9 Front swaybar
#11 Front springs, ~1.5 times the standard rate

these are in progress

#2 Front positive caster, the more the better - I have a set of superpro bushes in the garage, I wanted to do them at the same time as the dampers but decided one thing at a time, maybe next weekend
#4 Rear toe out, ask Sam - rear stub axle spacers installed, eibach shims on my desk

these are done!

#1 Rear swaybar (or beam stiffener) nothing else comes close, bang for buck - done, massive one
#6 Dampers, buy the best you can afford - Bilstein B8,
#10 Rear springs, at least triple the standard rate - probably triple, they're f*cking stiff, need to measure them

I'm also piecing together a SEAT FMIC and dump pipe and exhaust

[sambb]

To do the rear roll centre, all you can do is lower it. I think from memory it moves downward at 1/4 the rate of the rear COG which moves down linearly with ride height. So a 10mm ride height drop COG by same amount and roll centre by 2.5cm - numbers may not be exact but you get the idea.
You can do the front ball joint mod and get the roll centre correction benefits without going excessive on the front camber. The TT ball joints have elongated slots in them. I made them even longer but you can leave that side of it alone get sy 1/1/4 degrees neg front which is still an improvement, without over stressing the strut top bearings.

[simon k]

To do the rear roll centre, all you can do is lower it. I think from memory it moves downward at 1/4 the rate of the rear COG which moves down linearly with ride height. So a 10mm ride height drop COG by same amount and roll centre by 2.5cm - numbers may not be exact but you get the idea.

gotcha - cos nothing else made any sense...

You can do the front ball joint mod and get the roll centre correction benefits without going excessive on the front camber. The TT ball joints have elongated slots in them. I made them even longer but you can leave that side of it alone get sy 1/1/4 degrees neg front which is still an improvement, without over stressing the strut top bearings.

sweet, thanks Sam

[sambb]

Even with minimal travel left, torsion beam cars still cant get their rear roll centre anywhere near as low as an IRS car. An IRS car could run a proper amount of rake and still have a much lower rear roll centre.

[simon k]

I went hunting for some info about the springs I have in the back of my car now, best I could get was from someone putting them in the front of an L series subaru, they'd called King Springs and asked for the info...

(Civic) KHFS-58 Diameter=65mm, Height=380mm, 11turns, 280lb/in

however, I measured the springs and have the following

wire diameter: 12mm (0.47")
spring OD: 95.6mm (3.76") - edit: I assume 65mm above is ID, but I'm 5mm off, the spring tapers a bit... ?
active coils: 7 (not sure how to tell active/inactive)

I put that in an online calculator and it told me 281lb/in which is scarily similar to what the King spring says above.

Here's the picture of the springs - 2nd from the right is the spring that's in the car, far right is the bit that was cut off it



Unfortunately I've no idea what the standard spring rate is in the back of the Polo, so I don't know if I'm anywhere near 'triple the standard rate'. However, from the feel when driving, I don't know if I'd want to go much stiffer, it's on the stiff side of comfortable.

[sambb]

Stock rear spring rate is 25 N/mm which google will convert to lb/in for you. To work out the number of free coulsvi normally look at them fitted to the car. You can see where the coil first peels away from the coil bound/captive ones. Mark that with a marker and then count how many complete coils +?whatever fractions of coils until the free coils bind again near the spring seat. You can get a fair idea of that off the car though so don't stress too much over that. You can often see where the seated parts of the springs bind up as there will be witness marks on them where they rub against each other. All the above gets very muddy trying to work out springs that are progressive or designed to be coil bound at rest like the H and R's.

[simon k]

Stock rear spring rate is 25 N/mm which google will convert to lb/in for you.

well that's 143lb/in so I've doubled them.... maybe

[sambb]

If you are still running that beast of a rear bar it might be enough.

[simon k]

If you are still running that beast of a rear bar it might be enough.

lol, yep, still there... rethinking having a similar thing on the mini though

[sambb]

Which springs did you end up with in the front, the stockers?

[simon k]

Which springs did you end up with in the front, the stockers?

yeah, I had a dig around last night for something the same length but a bit stiffer, but I don't really know what I'm looking for...

[sambb]

the fronts are 25 N/mm too, so 143lb/in, x 0.9 motion ratio and 0.9 again for leverage ratio (gary) and you are looking at 115lb/in at the wheel. The rears will be 143 x 0.8 x 0.8 = only 91lb/in due to different linkage ratio in the rear beam. By having 7kg front and 8kg rears my spring balance at the WHEEL ends up being square all round (based on those same calcs) , not inc bars. From what I remember Gary had suggested 7kg fronts as a starting point for me given how rough our tracks are etc and considering also that I was on 15in/205 mediums and after a track focussed setup. It could be that the tyres you intend to run would never develop the lateral G's to make running anything that stiff worth it. But on the flip side Gary also said that the spring rates you have in there now (same as what I'd had previously) were barely sporty street rates. The problem with the rear beam is that it is just so uncomfortable with stiff springs in there. You can remove the rear anti roll bar for the street to help make it tolerable and then beef it up for when you attack a track as a compromise but then you are left with an understeerer on the street.
Overall I think achieving that square balance in the wheel rate spring rates is a god starting point, but whether that model can work with soft spring rates the way its worked with my stiffer rates (340lb/in ish square at the wheel) I don't know,

[sambb]

I can send down my front weitecs 27N/mm and 8-10mm lower riding than the OE ones so roll centre still in a good spot. They measured only slightly stiffer than stock (in a press) but did feel a tad stiffer.
So with my rears, I went from 143 lb/in to 450 lb/in from memory which is about 3 times. I can tell you it is freaking hard, like stupidly rough on some roads. I bought the kids some icecream the other day and the poor things ended up with more on their faces than in their mouths the way the car jiggles around in the back. I dont know what your threshold for pain is but its barely streetable. You could aim to come back from that with say 7kg rears and then set the fronts up relative to what you can tolerate at the back. But having said tha, following what Gary has suggested I've definitely learnt that it is all in the back when it comes to going fast in a FWD. The transformation when you play with rear damper rates (that was worth 2 seconds to me at the dub nats), rear bar and proper spring rates is insane. You could even run the real stiff springs at the track and then the H and R's for the street because finding a happy comfort medium sort of compromises both if you know what I mean. Its a bit one or the other with the rear beam.

[h100vw]

I like the ride of my MCA coilovers, fronts are 10kg and the rears are 8. I do have some 6s for the back. For "comfort" but tbh, they arent that much more comfy and I'm too lazy to swap them in and out.
I added 50 series tyres to soften it up a bit though.

[sambb]

yeah I found the same Gav. I had some 6kg rears to use for the road. They didnt handle as well as the 8's with 7kg fronts but the discomfort was nearly the same. Thats why I think with the rear beam you either need to go focussed or comfort and its hard to find an area in between that will do both. Same as you gav I changed to 195/55/15's on the street to take the edge off Sydney potholes.