Suspension mods

[Marc]

...are the 205 70's that are on the rear too tall in the side wall for good handling.
Is there an ideal tyre pressure...

Well, they aren't ideal, but they'll do. The problem is that a tire that big doesn't need a lot of pressure to carry the weight of a VW, so if you set the rear pressure low enough to maximize tread life (probably down around 22 psi) the sidewalls will have more squirm than desirable; raising the pressure to the upper-20s should result in better handling at the limit, but the ride will suffer and the tread will tend to wear more at the center. The wider the rims, the less that's a problem...recommended rim width for 205/70 is 5½-7½" - the ride will be more plush on 5½", the handling will be better on 7½".

[Steve Arndt]

205/70 are 26" tall. They have a rather balloon shaped sidewall. They are mainly used for the cal look stinkbug look (nose low ass high). Also 205/70 are heavy and usually in a touring tread and traction style. 205/65 are available in much better compounds and performance ratings.

[andy198712]

To what? Just the heavier-rated rear torsion bars? Alone, they'll make for a harsher ride (you'll feel bumps & potholes more). The stiffer spring rate will also aggravate the inherent "tucking" behavior that causes swingaxles to roll over in extreme cornering - that's why you'll also want to have the camber compensator.
The swingaxle design has some inherent problems which cannot be resolved by any means (for one thing, its rear roll center height is at the center of the differential and there's nothing you can do to change that - smaller-diameter tires and/or lowering the suspension/raising the transmission can help, but that only goes so far; the best you can do is to find a compromise which will make it the best that it can be for your application.
Camber compensators (and the stock `67/`68 Z-bar, once the slack in the links is take up) pass the springrate of the inner wheel over and subtract it from the outer, which reduces the load on the outer wheel/tire...that reduces the jacking (upward movement of the rear roll center) and positive camber of the outer wheel (which reduces tread contact area and tries to make the tire corner on its sidewall).
When the body rolls, mass doesn't move but the load on the tires changes. Take the case of a hard left-hand corner where the body is rolling to the right. On most chassis, one would typically add a rear antisway-bar which would couple the LR springrate over to the RR additively, stiffening the RR and reducing the body roll. When you do that to a swingaxle, though, bad things happen - that's why the camber compensator or Z-bar is used instead, it actually promotes body roll in order to help the RR tire tread stay in contact with the pavement. The mass of the car pivots on the imaginary axis from the LR to the RF; when you reduce the load on the RR you also reduce it on the LF and increase it on the RF, which increases the slip angle of the RF tire. Decreasing the slip angle on the RR and increasing it on the RF will promote understeer (or at least it'll reduce oversteer) - the same thing that a front anti-swaybar does - but if it's done through the use of a camber compensator or Z-bar it does it by making the body roll worse instead of better...the car may not look as "planted" but it'll actually get around a turn at a higher speed before it barrel-rolls.
Under certain conditions it can be desirable to add a rear antisway-bar to a swingaxle, but that's really only for track use IF the suspension is lowered enough to put it into negative camber under static conditions, and limiting straps (or short shocks) are used to prevent it from going into positive camber when unladen. The resultant short suspension travel is punishing on the street...if you want good handling and a pleasant ride, IRS is the only way to go.

cheers for taking the time to explain that

[Roflhat]

The wider the rims, the less that's a problem...recommended rim width for 205/70 is 5½-7½" - the ride will be more plush on 5½", the handling will be better on 7½".

Can you go into this some more please? I'm using 205/60 on 5.5J rims, would I be better off with 6.5 or 7.5" wheels and how? I'm after better handling, ride comes second

[Marc]

Wider rims increase the section width of the tire (by about 0.2" per ½" of added rim width) and, by "pulling out" the sidewalls make them stiffer => more responsive to steering input and less susceptible to rolling under when cornered hard. The reduction in sidewall flex also will make the tire transmit more road "noise" into the chassis, so although the ride may not suffer much (60 series still have a fairly generous sidewall compared to something like a 45-series performance tire) you will feel/hear things like expansion strips a bit more.
http://www.yokohamatire.com/tires_101/customizing/

[Roflhat]

Thanks for that, some really good information.
So how much difference does it actually make?

[PhillipM]

The wider the rims, the less that's a problem...recommended rim width for 205/70 is 5½-7½" - the ride will be more plush on 5½", the handling will be better on 7½".

Can you go into this some more please? I'm using 205/60 on 5.5J rims, would I be better off with 6.5 or 7.5" wheels and how? I'm after better handling, ride comes second

Handling-wise, I'd be running 6J to 6.5J rims with 205's, 185/195's on 6J's, 165/175 on 5.5's.

[petew]

Thanks for the info guy's.
...By the way are the 205 70's that are on the rear too tall in the side wall for good handling.
Is there an ideal tyre pressure.
Thanks G

40psi and drive it with the throttle.

Seriously.

I should say, I actually advocate this approach, to try and transfer to the front end of the car.

[FJCamper]

Hi Garrick,

Take a look at this series of handling posts--

FJC

http://www.shoptalkforums.com/viewtopic ... 1&t=127619