simon's learning what to do with the polo thread

[sambb]

the caveat though that my car is sprung stiffly so will show up any softness in the chassis in warp, whereas your car is a little softer so there's a bit more give in the suspension which maybe wont translate into as much chassis warp in your car? I'd guess that I'd notice those braces in my car moreso than in a soft car if you get my drift.

[simon k]

no problem opening/closing my rear hatch when crossed up in the driveway with 3-4" of air under the back wheel, even with the back seats pushed forwards - no creaks or groans when rocking the car with the hatch open and my hands on the taillights...


I can't do the sprint that's on in a couple of weeks - scheduling conflict. The next event is at Winton in mid-March... maybe I'll try to do a turbo dump/exhaust before then

[sambb]

Simon did you do the sprint on the 17th?

[Sydneykid]

thanks - I'll give him a shout. It'll give me a way to use the NT01s, even if it works out that I run 7" wheels at the front and 6" at the back (or some combination)
I assume running wide tyres on a narrow rim makes the sidewalls flex and roll-over more than desired - more air pressure if that happens?

Several things are going on there. Firstly an R tyre is not as susceptible to flex as a road tyre. Then the amount of flex in the tyre has an effect on its temperature and therefore how much pressure it gains between cold and hot. So a more flexy tyre and rim combo may well need less starting (cold) pressure to achieve the same running (hot) pressure. What pressure the tyre runs at is far more important than what it starts off at and the only way to ensure that is to check it. If the pressure is too high A050's will blister and it doesn't seem to matter what rim size they are on. We run 215/50/15's on 7" rims at 30 psi (hot) the same as 225/45/16's on 8" rims, 245/40/17's on 8" rims and 265/30/18's on 9" rims. The target is always 30 psi, but they start off at various (cold pressure) depending on the conditions on the day.

Hope that made sense.
Cheers
Gary

[Sydneykid]

I've corner weighted a lot of cars in my time and a sure sign of chassis flex is constantly chasing the balance. Move one corner 10 mm and that moves 20 kgs, move it another 10 mm and it moves 30 kgs, huh? Take the last 10 mm back out and it only moves 15 kgs, WTF?

There are a couple things to keep in mind. Controlling the suspension geometry is far more important than overall chassis rigidity. Limiting unwanted camber change for example. But trying to selectively add chassis braces can be hit and miss and in general there are better more effective ways to spend the limited Budget.

Based on my experiences a strut tower to strut tower brace on the front is very worthwhile on a macstrut car. Basically it doubles the resistance to camber change and a small amount of movement at the top of a strut can mean a lot of camber change at the wheel. Is 100% increase enough? It varies from chassis to chassis and then by the age of the chassis and how many K's and accident repairs. The triangulation back to the firewall often actually doesn't add that much, plus it depends on how strong the firewall is in that location. On a car designed for a triangulated strut brace then it's likely to do more. But generally if they are designed for a triangulated strut brace then they already have one.

I could spend a couple of hours on this subject, so I'll keep it short. The firewall on a production car is a hinge point, where the chassis in front of it can flex vertically/separately from the cabin area. This is why we have roll cage members that extend from the A pillar bars to the strut towers, generally 2 each side, triangulated vertically. This lessens the hinge effect at the firewall. A triangulated strut brace may well help in that regard, but it puts a large impact load on the centre of the firewall. Which if not designed for it will flex, fatigue and eventually crack.

In summary a strut tower to strut tower brace on a macstrut car is a no brainer. But spending the extra on a triangulated one may not be good value for money.

I really wouldn't bother on the rear of a twist beam/trailing arm car like a Polo, they have no effect on the geometry. So really all they can possibly do is add minutely to the coverall chassis rigidity.


Cheers
Gary

[simon k]

In summary a strut tower to strut tower brace on a macstrut car is a no brainer. But spending the extra on a triangulated one may not be good value for money.

that's excellent Gary - that makes a lot of sense - thank you very much. I'll see what I can find in my scrap pile to make one up

I understand what you mean - I also had a look at the firewall on the Polo and it's very hard to get to, triangulating a brace back to it would mean disassembling all sorts of things and probably pulling the dash apart.

Thanks for the info on the tyres too

stealing this from Sam's thread...

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.

...

Around 10 mm of negative rake is OK, measured at the jacking points just behind the front wheel and in front of the rear wheel. I confirm that and then translate it to the wheel to guard measurements for easier checking. Beware of damaged jacking pints of course.

I'm doing a Winton sprint this Sunday, so on Saturday I'll put the 15" wheels on and take 1" or so out of the rear and set up the front as you've said there. When I test fitted the 15" wheels there was a LOT of clearance between the tyres and the guards... I'll machine up some bits of nylon to put under the rear springs when the big wheels go back on

[sambb]

yeah the front rides pretty much at stock ride height to have "by eye" good control arm angles for roll centre. With 15's it looks ridiculous. When I have a full tank of fuel it is noticeably bum down (neg. raked).

The 'firewall' on our car is just a flimsy piece of tin. There is a huge gap behind it (houses all the wipers assemblies, air intakes and motors etc) and the real firewall which is right under the windscreen. Like Gary said it would give you zero gain to triangulate to it. So nice and easy straight across tower to tower for that one.

[h100vw]

then there's the rear, do things like this make any difference?

I have one like that, you could tell if the bolts came loose, it creaked going on and off the drive. Whether thats an indication of stiffness or not, who knows. Not sure I have the senses to pick it on the street...

It's modified from a mk5/6 GTI one. M8 nutserts into the body seat still fits etc

[Sydneykid]

A job for the machinist, what I'd suggest is a spacer (alloy) that enables the front outer ball joint to sit on top of the control arm instead of below it. That won't fix the roll centre (too low) problem but what it will do is assist in optimising the dynamic camber change. The pivot points will still be in the same place relative to each other hence the roll centre will still be at the same height. But the control arm will be lower on the outer than it is on the inner, hence as the suspension compresses (in roll) the negative camber will increase maintaining the tyre contact patch.

The above shouldn't affect the bump steer as the steering arm and lower control arm ball joints will still be in the relative positions. But I'd check it just to be sure,

The only way to really fix the roll centre problem would be a change in the upright to move the ball joint pivot point lower. Have you guys looked at an equivalent Audi upright? I recall on the early TT's there were 2 or 3 different uprights with different roll centres to compensate for the different models ride height. The one we ended up using was 18 mm or so lower, which raised the roll centre about 30 mm so well worth doing.

This also shouldn't affect the bump steer as the source vehicle should be OK in that regard, but again always worth checking.

Cheers
Gary

[simon k]

That won't fix the roll centre (too low) problem but what it will do is assist in optimising the dynamic camber change. The pivot points will still be in the same place relative to each other hence the roll centre will still be at the same height. But the control arm will be lower on the outer than it is on the inner, hence as the suspension compresses (in roll) the negative camber will increase maintaining the tyre contact patch.

that's a good one, should be quite an easy modification - on minis they use a balljoint extender that's about 1" thick, is that the amount of change you're looking at?

A job for the machinist, what I'd suggest is a spacer (alloy) that enables the front outer ball joint to sit on top of the control arm instead of below it.

Since the balljoint is currently sandwiched inside the control arm, is what you're suggesting there as simple as making a plate to sit inside the arm (just so it doesn't crush), then another plate to sit between the arm and the balljoint (and using longer/thicker bolts), or is it something more substantial?

edit: maybe like this...

The above shouldn't affect the bump steer as the steering arm and lower control arm ball joints will still be in the relative positions. But I'd check it just to be sure,

The only way to really fix the roll centre problem would be a change in the upright to move the ball joint pivot point lower. Have you guys looked at an equivalent Audi upright? I recall on the early TT's there were 2 or 3 different uprights with different roll centres to compensate for the different models ride height. The one we ended up using was 18 mm or so lower, which raised the roll centre about 30 mm so well worth doing.

This also shouldn't affect the bump steer as the source vehicle should be OK in that regard, but again always worth checking.

Cheers
Gary

very interesting - no upmarket wrecking yards near me, go Sam go!!

googling around I see what you mean about different uprights, here's a comparison from one SEAT to another