MKIV suspension - Why Lowering is bad for camber

[enginerd]

The MKIV suspension geometry has been modeled using cad software, and some dynamic motion software. Pyce from the vortex was so kind as to post the results from this geometry study. There is an exhaustive study of the geometry here http://forums.vwvortex.com/zerothread?id=1363022

I have taken only the Camber details from that thread and reposted in a simplified form for you elitists

Figure 1.



This figure shows the camber of the front wheel as it is moved through it's travel. As you can see there is a point where the camber begins to become positive. Negative camber is necessary to compensate for the tires tendency to deform under cornering forces, and prevents the tire from riding on the outside edge with a small contact patch.

As a car is turning the outside tire does most of the work. The tire is responsible for the majority of the cornering force, and has considerable weight transfered to it. This weight transfer compresses the suspension and you move through the camber curve shown. If your car is lowered, the more you compress the suspension, the more positive camber you have and the less grip you have.


of the 2 curves shown 1 is for the a stock MKIV vw, and the other curve is for the audi TT, audi S3, and the VW R32. The difference between the 2 suspensions is found in the spindle. The spindle on the Audi has a lower mountign point for the lower control arm.


This changes the arc for the suspension allowing for a lower ride height because you need to compress the suspension much more before having a poor camber angle. There is still a limit and these cars shoudl not be dumped to the ground either, remember more camber is the goal here,a dn up to 3 degrees negative woudl make for a great track setup.

[enginerd]

Also covered in that thread is how camber changes when the car rolls, or when the steering wheel is turned. The angle of the strut relative to vertical causes caster on teh front suspension. As the wheel is turned, it must also rotate about the strut axis and creates a camber change. This is good, as turning the wheel creates negative camber on the outside wheel.

figure 3.


This is the same as the figure above, however it includes the addition of vehicle body roll. If you take the car, and tilt it on an angle, everything being equal, you will tilt the wheel on an angle, a positive camber angle (bad)

Figure 4.



This figure shows how the steering angle effect the camber of the wheel. This is a good thing. The more you turn the wheel the more negative camber you get on the outside tire. But you need to add everythign up.

Starting Camber + Body Roll angle + caster induced camber angle + suspension travel camber = actual camber in a corner.

Everythign works together, balancing this for optimum negative camber through a corner = maximum grip. You will need to check tire temperature to be sure you don't have too much negative camber, but for a MKIV the goal should be to add as much as possible. :driving:

[Kix]

mybe a stupid question but will ask anyway...
can you install the TT spindles on a Beetle? (MK4)

[enginerd]

Kix,

You can buy and install the TT parts on a MKIV. There are a few issues.

The VW ball joint is different that the TT
The TT ball joint won't fit the VW lower control arm
The TT lower control arm fits the VW, but the swaybar mount is missing
The tt mounts the sway bar to the strut.
The TT parts are expensive new, and used it's hard to find a good set with good bearings/etc.

Your best bet is to get some H2 sport spindles. H2 sport is a canadian company that made MKIV spindles with a low ball joint position. Even lower than the tt. I have this kit on my GTI, and I must say it has transformed the car completely. Far less body roll, much more negative camber. It's an awesome mod. This is why car magazines rave about the TT yet poo on the GTI/jetta even though we think it's "the same chassis"

These parts are avaialbe with bearings, hubs and sway bar extensions for a very good price. Better than you can buy TT parts.

http://www.h2sport.com/   for details

The only problems I have run into were related to my big brake kit. I have ECS 13" brake upgrades that shift the rotor inward alot, and they hit the lower ball joint area on the spindle. If you upgrade and use TT brake kits, they do not interfere with the ball joint. (some may be very close, so you will want to investigate before you buy) all TT sized rotors will fit perfectly.

[Kix]

Thanks a lot.  I asked because I have driven Both the TT roadster and the Leon Cupra and they feel much MUCH better when dialing power at the apex, no weird twiching  no surprises on the point and shoot department.

I changed my  suspension setup and now have sway bar and front sway bar  tensors also Bilstein coilovers with minimum drop.  It has performed a lot better and the car is very neutral at the limit and progresive but my "limit" is 3 to 6 mph lower than a TT so Ill def, will ask for those and check em..

[sciroccoR]

nice analyses.

camber changes due to droop and compression are quite important.  however, probably the main benefit of having taller spindle (like on the tt) is where it causes the roll center to be.  if a car is lowered and the spindle length (talking macpherson here) is kept constant, the roll center will migrate downward.  as the migration of the roll center occurs more quickly than the migration of the center of gravity, there is less resistance to roll.  the car rolls more, invoking your discussion of camber changes in compression. 

[enginerd]

Yes the roll center below the ground just exacerbates this problem as you roll more and lose more camber. 

Typical "street performance kits"  lower the car, add progressive springs that are no stiffer than stock.

Combining the two end up with a car that handles worse than stock, with a "cool" lowered look.

Some spindles and real stiff front springs and you will learn real fast that the twist beam rear end becomes the limit not the mac strut front end.

[sciroccoR]

i am curious as to why you write that with stiff springs and good spindles, the limit is the rear.  i agree, in principle.  especially since my rear is that of a MK1 which has the pivot point the flex beam virtually colinear, meaning the roll center there is on the ground.  i think in mk2s it's the same but i don't know.  i that in mk3 and mk4s they moved them apart to increase the height of the roll center.

anyway, one of the problems with the rears is that camber is basically body roll.  so, the body rolls, the camber goes bad.  nothing you can do to fix it either. so, we shove stiffer springs in.  the roll's less and the camber stays better than if there were wussy springs.  makes you want to use springs instead of anti-roll bars, huh? 

another issue is track length change.  if you lower the car, the trailing arm moves up around the pivot point shortening the wheelbase according to cosine of the angle.  now cosine of an arc doesn't change much when it's flat (horizontal, zero, whatever you want) but it changes pretty quickly when we talk about short arc lengths and when it's already well past zero (the car is lowered).  so, now, with the wussy roll resistance of above, the car rolls a lot and the outside wheel (relative to the pivot axis) moves more than the inside wheel.  essentially, the track on the outside gets shorter more quickly than the inside wheel!  this, clearly, hurts the handling of the car tremendously. 

so, this is just another reason why lowering your car but just throwing in some springs with not so stiff springs is just a bad idea.  use shine's real street stuff instead if you're not going to do things all the way.

[enginerd]

My comment regarding the rear suspension is to its inferior design. I love Vdubs, but this rear suspension has no business on a modern vehicle over 20 grand. A little body roll, suspension compression and the geometry is all messed up. The "poor" front suspension design is to balance out the weak rear for an understeer bias.

You commend regarding wheelbase is another good point. I think the MKIV's are better somewhat as they seem to increase wheelbase initially. Of course this only applies to cars with some suspension travel. Mine still has a dune buggy look despite the coilovers.



All last season I was fighting oversteer with this car. Sure easier to balance at the limit, but i realized quick that the rear end was going to be difficult to tame. Im looking into some camber plates for the rear to try and help, but i sure wish there was a multilink rear back there.

[sciroccoR]

i have some shine camber shims in my scirocco.  they work well.  they are so simple, i can't imagine them not working well!

i moved the pivot point of twist beam up inside the car (it's mounted off of the roll cage now) to get rid of the badly self adjusting wheelbase problem about which i wrote.  i haven't driven the car yet with that mod so i don't know how much it did.

dick shine has done an interesting mod to the twist beam where he extended the trailing arms forward of the beam and mounted a tubular roll bar quite a bit ahead of the twist beam.  this raises the roll center from the ground (as discussed before).  (you can't do this if you have your gas tank sitting right there, though).

i'd like to put a the setup from older mazdas (which had the passive semi-four wheel steering similar to the weissach rear on the 928) into the scirocco some day but it would screw with the diffuser.  i think i'll just increase the spring rate and suck it up.