Sam's build thread

I remember Ortiz mentioned that relationship between roll centre and COG with the VW twist beam and gave me some pretty detailed instructions on how to draw it out but I got the feeling he was referring to the rabbit/golf Mk 4beam which is admittedly very similar to mine. I can't remember if he said the roll centre was located 1/4 of the way down from the twist beam or a 1/4 of the way up toward the twist beam from the ground. Like you say its going to matter a lot what I do with that so I'll have to dig out his emails and get super familiar with it. I took the car for another good drive today and there are no lurking vices (yet) which I'm happy with. Its not doing anything funny or strange or unexpected it just feels right, so I cant wait to run it on the track soon and get a better gauge on how its affected things.
One thing I forgot to mention is that once my current fronts die I'm going to go back to running 205/50/15 at the front but stay with 195/55/15 at the back. Later on I may go to the 205's all round but I've always wanted to try running like this.
Yeah it'd be great to catch up somewhere - are you doing rounds of the NSW state series or club stuff next year? What car do you run in if you do hillclimbs - maybe I've seen you before.

Quote:

---

Originally Posted by sambb

I remember Ortiz mentioned that relationship between roll centre and COG with the VW twist beam and gave me some pretty detailed instructions on how to draw it out but I got the feeling he was referring to the rabbit/golf Mk 4beam which is admittedly very similar to mine. I can't remember if he said the roll centre was located 1/4 of the way down from the twist beam or a 1/4 of the way up toward the twist beam from the ground. Like you say its going to matter a lot what I do with that so I'll have to dig out his emails and get super familiar with it. I took the car for another good drive today and there are no lurking vices (yet) which I'm happy with. Its not doing anything funny or strange or unexpected it just feels right, so I cant wait to run it on the track soon and get a better gauge on how its affected things.
One thing I forgot to mention is that once my current fronts die I'm going to go back to running 205/50/15 at the front but stay with 195/55/15 at the back. Later on I may go to the 205's all round but I've always wanted to try running like this.
Yeah it'd be great to catch up somewhere - are you doing rounds of the NSW state series or club stuff next year? What car do you run in if you do hillclimbs - maybe I've seen you before.

---

Mark's wording for calculating the CoR for a twist beam takes a bit of interpreting, I had to draw it several times before it clicked. It would have been quicker if he had drawn it. I'm not going to attempt to better his words, but if/when we catch up I'll draw it for you.

I'm sure you know, but just in case, I'd suggest keeping an eye on the welds for the reinforcing of the twist beam. They work a lot, so they will eventually crack, you can count on it.

Personally I'm not a fan of different sized tyres front to rear on a FWD. Firstly for practical reasons if you, say, flat spot a front you can't just quickly swap for a rear. Mostly though for handling reasons, it's really a band aid, if it helps then there is something else wrong. I'd always suggest fixing what it is that's wrong not sticking a band aid over it. In long, high speed, high lateral G force corners with positive longitudinal G force the required amount of rear grip is pretty close to the front. Not so much in short, low speed corners though.

I've been doing mostly circuit race engineering for a few teams for the last 6 years, Sports Car 12 Hour (Porsche and Audi), Production Car 6 Hour (BMW), Formula 3 (Dallara) and Improved Production (FWD and RWD). Plus data review, driver coaching, long distance race strategy and overall team management. So I haven't had much spare time to work on my own car. I've almost completed a long term upgrade to the R32GTST, added about 1.1 litres of engine capacity which should add 300 bhp or so, which of course meant a larger gearbox, fuel system upgrade, change to E85, bigger winged sump, oil cooler, intercooler, bigger brakes, waste gate, titanium exhaust etc etc. It's just about ready for a dyno run, the oil is in the sump, there's E85 in the fuel tank and the water is in the radiator. I don't have the schedule for the race teams next year as yet but I want to, at the very least, fit in a Bathurst Hillclimb weekend and maybe a round or 2 of the Nulon Nationals.

Cheers
Gary

Yep I am a bit paranoid about those welds. The fabricator was saying that you can hang a lot of weight off a stitch weld but what it will cope with in torsion is another thing. After the first run I got back under there and checked them over and will continue to - and i'll be hanging onto the other beam and RARB just in case.
Re the tyre stagger isn't the thing that's wrong that I'd be band aiding the fact that its a FWD?! ha ha or that its F 64%: R 36% weight split. I will give it a shot mostly because I can only afford new semis in pairs and will have the wider tyres on the front in the interim. I hear what you saying though.
Sounds like a weapon. If you are going to Bathurst for Round 1 and 2, PM me

Quote:

---

Originally Posted by sambb

Yep I am a bit paranoid about those welds. The fabricator was saying that you can hang a lot of weight off a stitch weld but what it will cope with in torsion is another thing. After the first run I got back under there and checked them over and will continue to - and i'll be hanging onto the other beam and RARB just in case.
Re the tyre stagger isn't the thing that's wrong that I'd be band aiding the fact that its a FWD?! ha ha or that its F 64%: R 36% weight split. I will give it a shot mostly because I can only afford new semis in pairs and will have the wider tyres on the front in the interim. I hear what you saying though.
Sounds like a weapon. If you are going to Bathurst for Round 1 and 2, PM me

---

I had quick look at the regs for 2018 and it should fit OK into Time Attack 2WD. It's totally OK for Improved Production except no restrictor.

If I remember rightly the first round is usually at Bathurst in early March which might be OK, depends on the race teams' schedules. I don't have any commitments for the 12 Hour next year, so early on it's only going to be the NSW State Championship race meetings that might get in the way. I'll let you know if its going to happen.

The welding will be subject to working metal fatigue, I wouldn't expect it to just give way without warning. More like little cracks appearing that will gradually grow. A quick check after each meeting should be enough and just reweld any cracks as soon as they appear.

Cheers
Gary

yep round one is saturday up the esses and round 2 is sunday up mountain straight. It was march 4th and 5th this year I think but basically the first weekend of march and always just after the 12hr. Its then 6 more rounds after that spaced out about a month and a half until mid September. Who knows maybe you'll get into road reg if you have enough engineering certificates! Dejan Bunetta was in our class in 2016 in his "street reg" time attack supra which was pushing the rules a bit.
I was having paranoid visions of the car sitting on a bit of a slope and one weld letting go and them all of them just going ting, ting ting in sequence. Good to hear its unlikely to happen like that - I can stop worrying about every little sound now!

Nothing to add except that I'm enjoying the reading. I've always run/driven rwd cars, and it's interesting just how different a lot of the approaches are and why, all makes a lot of sense though. Not sure that I'd ever looked at/taken notice of a torsion rear beam before! That's a huge weight split you're dealing with too, which makes sense, but again isn't something I'd considered before. My Vette is slightly rear weight biased (which I think is pretty damn good considering!), it's just way too heavy overall. I run square tyre sizes all round (most people run staggered sizes, much larger rear, on Vettes) and I see no reason to change that at this stage. I actually have more room available for tyre width on the front than rear.

Do you run the same neg camber settings on the street as at events? I run -1.5 front and -0.5 rear at all times, and have been wondering about adding a little more to the rear as it does go positive on the outside rear (based on photos) when pushing hard. With droop, the front outside goes a bit positive on corner exit too, but is pretty heavily negative under braking/corner entry, so I figure I probably don't want to add any extra neg camber on the front. I'm sure I would benefit from stiffer springs/shocks, but I am already riding fairly rough on the bumpy country roads around my house, and it is still primarily a street car. I would like more rebound stiffness, but am running non-adjustables (Bilstein Sports).

Anyway, I'm finding the discussion very interesting, carry on! Nice work on the modifications around the front for cooling/cold air too. I have some work to do in that area myself, I plan to make up a cold air tray to bring fresh air over the radiator to the intake.

yeah I've been running between 2 to 2/1/4 degrees neg at the front street/track for the last year. Outside tyre wear wasn't too bad on the fronts like that. The rear is standard with 1.5 degrees neg. which was half a degree more than the front had from the factory! making for a pretty awesome understeerer until I got the front end sorted with that ball joint mod.
When I swap from Bilstein B8 struts to MCA coilovers I'll be able to get neg adjustment at the top as well as the bottom. My choices are to leave the ball joints where they are and be conservative at the top which means running my existing rim offset (ET40) or I could slide the ball joints back in which will reduce the track width and neg camber gotten at the bottom and get it back at the top, but then I'd have to go back to positive scrub with say ET20 rims to get my track width back. The coilovers might actually be a while in the fit up. The strut towers have these domes that sort of enclose the top nut. These will have to be cut off for access to the adjusters so I'm still struggling with the best way to tackle that.

On a strut car that's used for both street and track I set up the front camber on "the bottom" at the mid point of where I want it for the 2 disciplines. Let's say 1.5 neg for the street and 3.5 neg for the track, so I set it at 2.5 neg. Then I use the top adjuster to switch between the two settings. This gives quick and easy camber adjustment at the track, just mark the strut top for each 1/4 degree of camber adjustment. The only trick is confirming that there's minimal toe change when the top adjuster is moved. On later generations strut cars there isn't generally a bump steer change, but it is worth checking that as well. The top adjuster should give you more than +1 or -1, so there's scope for more drastic settings if required. I can't recall the exact movement ratios of your model Polo, but around 10 mm either way should give you enough adjustment for + or - 1 degree.

I suspect that 2.25 degrees may be OK for the Esses Hillclimb but almost certainly won't be enough for the Mt Straight due to the high lateral G forces for longer. Of course front camber is always a compromise for acceleration and braking traction on a FWD car.

In regards to the strut tower top (dome) what you do is more a regulations question than a practical one. On a strut car I would always run an inverted damper hence the adjuster should be on the bottom of the leg. If the shock pin is protruding too far then I'd dimply trim it so that it fitted under the dome and allowed sufficient movement to adjust the camber. To maximise the adjustment it may be necessary to fit an alloy spacer between the strut top and the strut tower, move the pin down to clear the dome. The alternative is to simply cut the dome off with an angle grinder, but that could result in class regulations issues. Both should give you enough space for the camber adjustment, so no handling difference.


Cheers
Gary

Ok cool that's a good rule of thumb. My Audi TT ball joints had a bit more than 10mm adjustment so I'll set them at the midpoint (that should be about -1.75 deg) and then get the rest at the top. The issue I have with the MCA's is that the actual camber adjusters are shrouded by the hat that sits atop the strut tower. Guys have set them first and then fit the struts with a sort of stab in the dark at what camber they'd get, and then worn the fact that they can't adjust camber with them fitted. The damping adjuster is at the top and then needs a hole drilled through this hat for access. So to get to damper adjuster but mostly so that I can get to the camber adjusters i'll have to slice the whole hat off. You can see the hat I'm talking about that the strut brace straddles. Yeah its a proper visible change to the car that probably needs certification really but I'm doing it anyway.
I'd definitely like to be able to move the camber from day to day with different events or between street and track so I'll have to get a much more reliable toe setting measuring tool than a tape measure between treads on the front and back of the tyre. I can actually get the total toe bang on but usually I'm off to the left or right with both tyres by a whisker.
While I've got you Gary I was hoping you might be able to verify something for me. The pic below which is on its side shows the tie rod sitting behind that FARB with the lower control arm (black) below that. The witness marks on the FARB suggest that at full lock, the tie rod is brushing against the FARB. The reason for this is that the Audi ball joints + the rear lower control arm bushes (castor position) have pushed the hub forward relative to the steering rack and FARB and the tie rods (which are now angled even further forward than OE) are now able to hit it at certain angles. What compounds this is the design of the OE tie rod ends. They are offset with the ball/pin sitting behind the line drawn down the tie rod which pushes the tie rod forward of the pivot and even closer to the FARB which it is hitting. The easiest solution I can see is to just swap the tie rod ends from left to right to change the location of the ball/pin on the tie rod end to in front of the tie rod. There's absolutely no reason why I can't do that yeah? I may need to re toe it afterwards but that's fine to do isn't it??




Attachment 30563Attachment 30564

From memory the offset on the tie rod end is for clearance to the drive shaft boot. You might want to check that before you do the swap. The swaybar will take a bit of contact without damage but a spinning drive shaft boot won't.

It might pay to remove the spring from the struts, put the struts back in and work the suspension through its full travel and steering lock. That will confirm what hits and what clears.

Cheers
Gary