Sam's build thread

[Sydneykid]

I'd suggest running a couple of layers of race tape over the top of the threaded sleeve, that'll stop the crud getting in, then wrap the tape around the threads. You may have to replace the tape occasionally, on the race cars I do it about every 3 or 4 race meetings. You could put the tender/helpers at the top, but if you can keep the main spring trapped at full droop I'd just leave them off.

Measuring to bump stop contact, it'll take me longer to write this than for you to do it. Count how many coils are in the main spring, then measure the diameter of the wire. For example, 8 turns of 11 mm wire = 88 mm height at coil bind. With the springs out of the car and the shocks bolted up, jack up one side. What you need is for the bump stop to contact long before there is 88 mm between the spring seats. Well over 100 mm as a minimum, so the bump stop has to be compressed a fair bit (say half its free height) before the spring gets anywhere near coil bind.

Rule of thumb, the front roll centre needs to be somewhere around the crankshaft centre line, so on a strut based car that means lower control arm inner bush pivot points higher than outer ball joint pivot points.

On a FWD car the rear roll centre (CoR) needs to be as low as possible, ground level is not a bad place to start, which is hard (impossible) with torsion beam suspension (without engineering it for a Mufford link). Since you can't get your CoR anywhere near where we would like it, the next best is to lower the CoG. So to answer your question, negative rake is the go as long as there is enough suspension travel. That's why adjusting the rear ride height is a good handling tuning tool.

With a higher spring rate the rear will settle less under load than it will with a softer spring rate. Consequently it will rise less when you take the load out.

I think I may have suggested it before, but just in case, stick a small note book and a pencil (pens run out of ink at inconvenient times) in your glove box and write down everything that you measure. Ride height (centre of wheel to guard) is an important one. So next time you know what height did what last time. These days I carry an iPad with me, with everything on every car from every session loaded into individual spreadsheets that are uploaded to the cloud. That way I can look stuff up anywhere anytime with my iPhone, iPad or on the Mac at home.

I hope that answered all of your questions, if not ask away ................
Cheers
Gary

[sambb]

I'm stoked you said that regarding negative rake. I've tried that many different springs/heights to run the car raked, level and slightly bum down and always found that the latter gave the best handling despite everyone telling me its not right. Ive always run the front ride height at what would give level control arm piviots. Of course I can go up a tad and will if you think it'll help. One thing I've always been unsure of is working out the LCA pivot point to work off when comparing its height to the balljoint. The OE setup has anti-dive built into it as the front LCA pivot is much lower than the rear. So do I measure the rear LCA pivot height, the front, and then take an average. Or once I know the LCA inner pivot axis, do I work out a point along it that lines up in top view with the balljoint and that becomes a virtual inner pivot - make sense? Because at the moment I'm just working off the front inner pivot/bolt centre. I do remember Ortiz somewhere saying that the only thing on a rear twist beam that would limit how low you should/could go is ground clearance - alright i'll whip out the lowest collar initially and see what it gets me.
Today I jacked the car so that it was 2wheeling. The inner rear main spring was wholly over the threads and two of the helper coils had started to unbind on the back edge so maybe keep the helpers in?

[sambb]

Gary (and anyone else who's interested) here are the NSW hillclimb 2018 dates and series rego:
CAMS NSW Hillclimb Championship >> NEWS: 2018 Racing Calendar - Panorama 3/4th March
CAMS NSW Hillclimb Championship >> NEWS: New Registration Procedure for 2018 - 50 bucks

[metalhead]

Oh wow. Those dates are terrible for me! Bathurst is on my wedding anniversary weekend, and Grafton the weekend of my wife's birthday... Hmmm.

[sambb]

she's going to be very disappointed

[Sydneykid]

I'm stoked you said that regarding negative rake. I've tried that many different springs/heights to run the car raked, level and slightly bum down and always found that the latter gave the best handling despite everyone telling me its not right. Ive always run the front ride height at what would give level control arm piviots. Of course I can go up a tad and will if you think it'll help. One thing I've always been unsure of is working out the LCA pivot point to work off when comparing its height to the balljoint. The OE setup has anti-dive built into it as the front LCA pivot is much lower than the rear. So do I measure the rear LCA pivot height, the front, and then take an average. Or once I know the LCA inner pivot axis, do I work out a point along it that lines up in top view with the balljoint and that becomes a virtual inner pivot - make sense? Because at the moment I'm just working off the front inner pivot/bolt centre.

The inner (front) lower control arm bush pivot point and the ball joint pivot point are the references. So the first job is to identify exactly where the ball joint pivots. The bush pivot point is easy, it's the centre of the bolt. Depending on the car and what's in the way I use a steel rule, a piece of string or a laser pointer to determine the roll centre. If the engine is out, with nothing in the way, it can be exact, but that hardly even happens, so approximations are fine. In a production based car it's not like we can adjust the roll centre in tiny increments like we can in an F3 or a V8Supercar anyway.

FWIW, we call the rear lower control arm bush a caster bush, because that's what it mainly controls, the amount of caster.

We really want the front lower control arm pivot points pointing upwards towards the centre line, around crankshaft centre is a good rule of thumb. That's not the actual roll centre, it's a bit lower than that in a MacStrut car, but it's a pretty good indicator. Plus you want the car to gain camber on suspension compression, not lose it as would be the case with control arms parallel to the ground or even worse pointing downwards towards the centre.

The problem with anti dive (under brakes) geometry is that it creates lift on acceleration. Taking weight off the front wheels when they most need it. Lowering the rear caster bush is something we do on Golfs and WRX's for example, that's why there are aftermarket kits for it. You might want to have a think about how to achieve that on your Polo.

I do remember Ortiz somewhere saying that the only thing on a rear twist beam that would limit how low you should/could go is ground clearance - alright i'll whip out the lowest collar initially and see what it gets me.
Today I jacked the car so that it was 2wheeling. The inner rear main spring was wholly over the threads and two of the helper coils had started to unbind on the back edge so maybe keep the helpers in?

Yep, at that ride height keeping the helpers would be my suggestion.

It would be hard to get the rear too low for roll centre purposes, your limit is ultimately going to be retaining enough suspension travel. The revised thinking around FWD race cars and rear suspension geometry mostly came from SuperTourers, at best they ran zero rake and that was with adj roll centres. We just don't want too much weight transfer from the rear inner to the front outer tyre. Plus less roll helps keep the inside front wheel on the ground for more drive off the corners.

Cheers
Gary

[sambb]

Ok cool. Before I installed my audi TT ball joints I did the line of intersection thing and worked out their pivot centres. Working off the front LCA inner bolt will be easy. I'll have a look at what I can achieve with some front ride height fiddling. On my rear inner LCA bushes I did install offset bushes but not to take away some of the anti dive - I put them in the castor position thinking that was what the car was lacking most and with the subframe eccentric bolts gained a tad over a degree of positive castor - up to 6 and a bit degrees now.
I'm sending back the unused 8kg/mm fronts on Monday. I'm really torn between going for 7kg/mm or 8kg/mm for the rears. I could run 6kg/mm on the street and then swap in the 8's for the track but I'm thinking I could try the 7kg/mm to be in the car always and just drop the RARB when I'm on the street. Still to'ing and fro'ing over that one but have the weekend to think about it.
When I got my quick toe and go done the other day after I fut the coilovers I found that I have zero toe on my left rear and +3mm on the right rear so I'm going to have to get the eibach shims in to sort that too. There was also a bit of preload on the rear bar with the ride heights set so I'll have to adjust out one of my droplinks to counter that.

[sambb]

No I've decided to not be a pussy and just get the 8kg/mm rears so I'll get that happening today.

Got another question for you Gary concerning rear track. Once I get it the front aligned and can see what will fit under the arches ill basically space it out as much as I can. I think with the extra camber I can get now, 12mm spacers on ET40 rims will fit plus the car has extra track from the ball joint mod.
So how wide can you go on the front before you must start following it with the rear end especially considering how rear stiff the car will be? Just maintain the OE front:rear relationship or is there a rule of thumb to follow eg F:R track widths based off say F:R weight splits or roll stiffness distribution? Do you normally set and forget rear track once you've decided and tune with bars/dampers/ride height instead or do you use it often to tune the car? I have used it in the past but that and tyre pressure was all that was available to me but a few more things have opened up to me now.

[sambb]

Gary got another one for you: the 8kg/mm rear recommendation, does that still hold true considering I do primarily hillclimbs. While it might be right for circuit stuff I'm just wondering whether that'll be too aggressive for hillclimbs with dirty stone cold tyres. Just final checking before I order them.

[Sydneykid]

No I've decided to not be a pussy and just get the 8kg/mm rears so I'll get that happening today.

Got another question for you Gary concerning rear track. Once I get it the front aligned and can see what will fit under the arches ill basically space it out as much as I can. I think with the extra camber I can get now, 12mm spacers on ET40 rims will fit plus the car has extra track from the ball joint mod.
So how wide can you go on the front before you must start following it with the rear end especially considering how rear stiff the car will be? Just maintain the OE front:rear relationship or is there a rule of thumb to follow eg F:R track widths based off say F:R weight splits or roll stiffness distribution? Do you normally set and forget rear track once you've decided and tune with bars/dampers/ride height instead or do you use it often to tune the car? I have used it in the past but that and tyre pressure was all that was available to me but a few more things have opened up to me now.

The maximum front track is pretty much always determined (limited) by the scrub radius, which for a FWD car usually means around 25 mm positive (the pivot point ~25 mm inside the tyre centre contact patch). On a RWD we can get to as much as 75 mm, but that much creates too large a load on the steering in a FWD car, hit a ripple strip or track edge and with engine torque it will pull the steering wheel out your hands.

Once I have the front track established I then equal it with the rear if possible. Having the rear track wider than the front can cause issues with front tyres missing the ripple strips or track edge (nice apex) and then rear tyre jumping over it. With a combination of a high (relative) rear spring rate and low (relative) rear of chassis weight it can pitch a lot of weight transfer off the inside rear tyre and onto the front outside tyre. Having the rear track noticeably narrower than the front makes it easier for diagonal weight transfer under lateral G. So my default is to keep the front and rear tracks pretty much the same.

Keeping in mind that widening the track softens the effective (at the wheel) spring and anti roll rate.

Cheers
Gary