Sam's build thread

[HaydEn]

thanks for that mate. yeah I see what you mean. Is that your pic - are you considering doing the same?

Yeah I thought i better take a picture of what i was poorly trying to explain.

Will eventually do it.

As rear sway bars for mk3's are far to expensive.

[sambb]

cool good stuff. keen to see how you weld it and what sort of result you get.
for a cheap add on bar from a wrecker, apparently the 1990's corollas had a bar internal to the beam - the type that bolts through the beam at either end. If you look up autospeed.com Julian edgar added one of them to the beam on their Honda insight project car.

[Sydneykid]

yeah definitely became aware that 200lb/in was too soft for the softs/mediums that I've run - Just a road reg competitor though so all on a shoe string budget. Yeah the damper mount point is well behind the spring seat and hub position on my car.

I'd have to measure it to be accurate but my guess is around 0.8 to 1 movement ratio and 0.8 to 1 leverage ratio, so the 200 lbs/inch at the spring would be 200 X 0.8 X 0.8 = 128 lbs/inch at the wheel (what the tyre feels). That's a slightly sporty road car rate.

The damping intricacies that result from that I leave in the capable hands of Josh. I will get on to him re the relationship between bump and rebound and each click setting. I'm guessing he's a tad busy with the WTAC car at the minute though so that one can wait at least until I get the fronts in.

Not that you can change the ratio, but it is important to know, so that you can give him feedback. In simple terms if you adjust say the fronts to give you more rebound (so that the weight doesn't transfer as fast off the front when you lift off the brakes) you need to know how much that affects the bump (too much of which may make it understeer on corner exit, which is what you are trying to overcome).

As it is the MCA's are 8kg fronts, 6kg rears. He initially wanted me to run 9kg fronts and 8kg rears but with the standard 20mm front bar and no rear bar. When I explained to him that I wanted to run a rear bar (cos I just couldnt see that not happening) he came back and suggested the 8:6kg split. Of the 8:6kg split he said that despite the different motion ratio on the rears that further soften the rear spring rate, that relative to the front and the weight that it is carrying that it is still a rear stiff setup especially when you factor in the rear bar. As it is rear springs are 100 bucks for a set so I can up the rear spring rate cheaply/easily down the track if need be so I was happy to take his lead.

The front struts on the Polo (like most VW's) have ~0.9 movement and leverage ratios, and I think 9kg's (504 lbs/inch) is far too high a spring rate (408 lbs/inch at the tyre). There's not any R /semi slick that likes that much spring on our rough tracks, especially on the front of a FWD car. Even 8kg/mm (362 lbs/inch at the tyre) is still a bit high, personally I'd run 7kgs/mm on the front.

I know Josh (and Murray) have a preference for higher spring rates and lower anti roll rates, but that comes from the vast rally car experience plus the days back when we weren't allowed to change the swaybars on Production race Cars, so we had to use the spring rates instead. Pretty much every race car (where it's allowed in the regs) have driver adjustable swaybars, that's because they are the easiest method of tuning the handling balance. It's pretty hard to pull a pit stop in the middle of a race to swap spring rates. This is the same philosophy as us being at a hillclimb, it's much easier between runs to adjust the swaybars than swap springs.

Keeping the above in mind, I wouldn't be running 8/6 springs rates. I'd be looking at something like 7/8, that should give around 317 lbs/inch on the front and 286 lbs/inch on the rear (at the tyre of course). Obviously it's important to confirm the movement and leverage ratios first though.


Cheers
Gary

[sambb]

I'm glad you said that because its worried me from the start that the front will be too stiff even at 8kg/mm. But my mates W172 clio is on 400lb/in fronts and it felt great which makes me think 450lb/in on the front wont be far off - just not sure. I'm easy with the back because that's so easy to change but if the front is too stiff then I have to commit to another 100 bucks for more coils and another strut removal job/alignment etc or changing down to the stock FARB at minimum (which is also a big job and might still be too stiff!). Whats your opinion of 8kg/mm on the stock 20mm bar vs the 22mm whiteline - still too front stiff? In that case I can still tune the rear bar and the damping.
My current BALANCE is great on 200lb/in spring front and rear (rising rate) springs with FARB on soft and RARB on hard and needing a bit more bar at the back. So if balance can be adjusted up in relative terms I agree with the 1:1.14 split you mention.
Got a question for you re FARB's and inside front wheel droop. I've always imagined that a softer front bar would allow the inside front to droop more easily - be pushed into the road more easily/independently by its strut. I asked Ortiz about that and he said that's a theory that has gained a lot of traction in Oz/NZ but which is totally false - the inside front tyre doesn't know if its a spring or an ARB or both pushing it into the road, if the roll resistance is the same then the inside front will do the same thing. I still can't get my head around that. Whats your take?
I think Ill see if he can send down a 7kg/mm pair for the front (exchange).

[sambb]

sorry another question Gary - I always thought you just times the spring rate by the motion ratio. I didn't know about leverage ratio factoring into that same equation to get your wheel rate. Whats leverage ratio and how does it differ to motion ratio?

[sambb]

Gary this is MCA's scales from when they did the first Polo. That was with the spare/tools and back seats out and half a tank of fuel. In terms of weight split my car may be a tad lighter than that in the rear eg no muffler saved 6kg right at the back and I'd run with no more than 5-10 litres of fuel so maybe 8kg less weight just in front of the rear beam. I reckon my numbers would be very close to that though. I don't think they simulated driver weight though. So given those numbers does that change the 7kg/mm:8kg/mm recommendation you made earlier or does that still hold?

edit:to my calcs it looks like a 66:34 weight split

[sambb]

http://hillclimbnsw.com/pointscore.pdf


The pointscore or the years Hillcimbing finally went up. Despite missing rounds 3,4 and 5 because of the crash and not finishing in the points on day 2 (Rd2) at Panorama, I finished 5th/20 in class helped by a 2nd and two 3rd's. Initially I was filthy that the crash killed off my chances of doing the whole series but my results at the back end of the year and the fact that the car is way better now because it, have me pumped for next year where I can hopefully get up to Grafton and Kempey.

[sambb]

Ok so given 1191kg total, front:rear split of 776:415 kg and 66:34%. I'll work off 7kg/mm front spring rate coils with 317lb/in wheel rate. Notsoswift's rule of thumb says make the rear wheel rate 50%(1/2) higher again than the front wheel rate relative to the weight it carries on the rear.
The front then has 317lb/in wheel rate supporting 776kg = 0.4. The "50% higher again" ratio is 0.4 X 0.5 =0.2. 0.2 + 0.4 = 0.6. So for the rear 0.6 X 415kg = 249lb/in rear wheel rate.
To get that into spring rate 249/.8/.8 = 389lb/in (roughly 7kg/mm) rear spring needed. That's right in the pocket with Gary. I'll see about getting stiffer rears for sure then - split the diff. and get 7.5kg/mm ! bloody good. thanks guys for getting me a bit of clarity and schooling me on spring selection. thx

[simon k]

this all very good reading... since I only have detailed knowledge of classic minis, spring rates aren't in my vocabulary. I've only driven 1 polo, it's sitting in my backyard, and I only drove it around the block in 1st and 2nd gear before I bought it...

[Sydneykid]

sorry another question Gary - I always thought you just times the spring rate by the motion ratio. I didn't know about leverage ratio factoring into that same equation to get your wheel rate. Whats leverage ratio and how does it differ to motion ratio?

Movement ratio first, with a 0.9 to 1 ratio, if the wheel moves 10 mm then the spring only moves 9 mm. The spring rate is kg/s per mm, so less mm = less spring rate.

Next leverage ratio, the wheel/tyre has a lever effect on the spring, like a wheel barrow or your hand on a spanner, a longer spanner has more leverage. Again with a 0.9 to 1 leverage ratio that means a 10 kgs spring feels like 9 kgs to the tyre. The spring rate is kgs per mm, so less kgs = less spring rate.

A simple way to look at it is the movement ratio affects the distance (mm) and the leverage ratio affects the rate (kgs). So added together 10 (at the spring) x 0.9 x 0.9 = 8.1 (at the wheel).

Not all cars have the same leverage and movement ratios, the front of a Falcon for example, the leverage ratio stays the same through the travel but the movement ratio changes slightly due to the angle of the shock/spring unit.

Hope that makes sense.
Cheers
Gary