Polo 6R: Handling issues at high speed on highways

Hello,


"With my Polo WRC, I can easily drive straight at its maximum speed (which is officially registered as 243 km/h). However, once I exceed approximately 200 km/h in a curve, a noticeable lateral acceleration builds up, and the car becomes frighteningly unstable. It feels like being on a ship experiencing side sway with a frequency of about 1 Hz, or like driving in a zigzag pattern without any corresponding movement of the steering wheel. To me, it seems like something is building up momentum."

Although I drive my Polo without a rear seat and (show) roof spoiler, and with high 205/50R16 tires, it seems that I am not alone in experiencing this problem, see http://www.polo6rfreunde.de/index.php/Thread/14542-Zu-hoher-Adrenalinpegel-bei-190-km-h-Fahrwerkprobleme/.

The WRC Polo has the "soft" track-correcting rear hub bearings that are commonly used within the group, which I believe should stabilize the rear end rather than making it nervous (see attachment). Nevertheless, in the 6R friends forum, people are recommending upgrading to aftermarket suspension hub bearings (SuperPro is a popular choice), which suppress the track correction function.

As far as I understand the topic, wouldn't removing a stabilization function actually make the rear end more unstable? Or could converting to non-steering-correcting bearings still reduce or prevent that lateral roll we're talking about?
If I have misunderstood the track correction function, please correct me.

ulf wrote:

... but as soon as you're going above approximately 200 km/h in a curve... ...

Speeders!

Quote:

... As far as I understand the topic, wouldn't the rear end become even more unstable if a stabilization function is removed or "detuned"? ...

In principle, I agree with you on that point.
However... What if this corrective movement along the trend line escalates?
For me, your description sounds exactly like it.
Dynamic correction of the horizontal axis always has the disadvantage of negatively affecting something in some way.
For me, something like that would also be a definite no-go. I would make the housing rigid and that would be good enough.

Bertil wrote:

What if this corrective movement along the series axis escalates? To me, your description sounds exactly like that.

The comparison with my (sold) Ibiza 6L Cupra might confirm this: The 6L has stiffer rear suspension bushings, where the steering angle-activated lateral movement of the bushing mounts is also blocked by spacers - and it never behaved as "temperamental" as the Polo in fast highway curves.
The 6L tires were slightly narrower: 205/45R16 instead of 205/50R16, but I still have the original 215/35R18 wheels in the basement for the Polo. I want to do another test with those to hopefully rule out the tires as the cause.

Quote:

For me, something like that would also be a definite no-go. I would stiffen the suspension and that's it.

So, you are confirming the advice of the chassis specialists in the 6R forum. I actually thought that the OEM generally builds suspensions that are at least drivable under everyday conditions *) up to the standard maximum speed.

*) This also includes curves with a flat road surface and a constant radius, where the maximum possible lateral acceleration based on grip is clearly not yet being reached.

ulf wrote:

... in order to hopefully rule out the tires as the cause.

Shallower slope = harder surface -> Earlier use of "adaptive steering".
Whether this has a positive or negative impact...??

Quote:

So, are you confirming the advice given by the suspension experts in the 6R forum?

So, at least that's one approach as far as I can see.
I'm too conventional for that. In terms of driving style, I'm against that kind of "following along."

Quote:

I actually thought that OEMs generally build suspensions that are at least drivable under everyday conditions *) and up to the standard maximum speed.

And what do you dream about at night?

Hi!
There might be something to the story about the warehouse.
"Back in the days on the Corrado forum, everyone was crazy about the Superflex bushings, which were basically just plastic lumps. I didn't install them in my car though, because I want a car that drives well, not something that rattles."

But all the mechanics were enthusiastic about the bearings and even installed them in the front control arms.
Reading this, with a lateral roll of 1 Hz, that could very well be the issue. I've experienced this before in older BMWs or Vectras when the rear axle bearings were completely worn out; you start feeling like you're drunk and being thrown around on the road from about 80 km/h.

Quote:

So, at least that's how I see one approach. I'm too conventional for that. In terms of driving style, I'm against such "following" behavior.

I believe that the steering mechanism is used to counteract the twisting force on the suspension arms.

Possibly, the increased tire profile and twisting force create two components that amplify each other. One of these forces needs to be reduced.
The question is how the vehicle will react in the border area if things become too rigid.

Just like in motorsports, one should try out a few different wheel and tire combinations, as well as various sets of rear-wheel bearings, and thoroughly test them all.

ulf wrote:

The 6L tires were indeed flatter: 205/45R16 instead of 205/50R16, but I still have the 215/35R18 factory wheels in the basement for the Polo. I want to do another test with those to hopefully rule out the tires as the cause.

Hi there,

Regarding the sporty models with special wheels from the factory, I believe that the suspension (especially the damping) is also adjusted to these wheels.

What made me think of that:
When I put the lighter 205/55 R16 winter tires on my GTD, the suspension feels overdamped and tends to make a clunking noise when driving over bumps. With the standard 225/45 R17 tires, this problem disappears.

Therefore, it's possible that the IMO (Independent Multi-Mode) system could also experience over-damping in the WRC (World Rally Championship), especially if the wheel mass suddenly becomes significantly lower than what was originally calibrated for. This would then explain the high-frequency oscillation.

I'm curious to see what the installation of the 18-inch wheels will be like.

Bertil wrote:

Quote: I actually thought that OEM companies generally build suspensions that are at least drivable under everyday conditions *) and up to the standard maximum speed . And what do you dream about at night

? For example, that as part of the type approval process, a test engineer(?) drives in the maximum speed range for a few minutes, and that a driving behavior as "soft" as my Polo's is considered unsafe/not roadworthy.
That's probably not true, right?

Quote:

Therefore, it's possible that even in the WRC, over-damping could occur if the wheel mass suddenly becomes significantly lower than with the standard setup. This would then also explain the high-frequency oscillation.

Hmm, I wonder if a difference of 18.1 compared to 19.2 kg per wheel = a 6% deviation could have such a dramatic effect?
I would have suspected the lateral sway was caused by the high, flexible tire sidewalls: With the 205/50R16 tires, I can easily make the rear of the car oscillate laterally while it's stationary, just by wiggling the wheel well, and the oscillation lasts for about 3 cycles before settling down. With the 215/35R18 tires, there is practically no such movement; the rear feels almost rigidly fixed in place.
However, the oscillation frequency with the 50-profile tires, estimated at 2-3 Hz, does not match the "experience" on the highway with approximately 1 Hz .

"By the way, even when rocking the car vigorously while stationary with the 50-profile tires, I can't detect any lateral relative movement between the wheels and the body. Therefore, it seems that the rear suspension bushings are significantly stiffer than the tire sidewalls."

ulf wrote:

... For example, during the type approval process, a test engineer(?) might drive in the maximum speed range and through curves for a few minutes, and evaluate handling characteristics as "egg-like" (meaning unstable) and therefore unsafe/not suitable for approval.

Of course, the vehicles are tested. However, one person might find it acceptable and tolerable, while another might consider it life-threatening.

Quote:

... I would suspect the lateral sway is more likely due to the high, flexible tire sidewalls: With the 205/50R16 tires, I can easily induce a lateral oscillation in the rear of the car while it's stationary, simply by wiggling the wheel well. This oscillation lasts for about 3 cycles before settling down.

It might be.
You can easily rule that out. Try inflating the tire to a very high pressure (but do not exceed the maximum value specified for that type of tire). This will provide much more support to the sidewall.

Often, there are even restrictions on the types of tires allowed in the approval documents, as the suspension system may not be compatible with other types (of tires from different manufacturers).



I experienced something similar with a racing tire once.
With the 175/70R13 tires, the sidewall was extremely high, which caused it to collapse very quickly. The handling was quite adventurous. A similar tire from a different manufacturer didn't exhibit this problem. When we removed both tires from the rim, we noticed that the sidewall of the second model was extremely stiff. So, I drove with the first type of tire at 3.5 bar (which was the maximum allowable pressure) -> and it had excellent handling.

I was not allowed to change the tire size because I was competing in a class that closely resembled the standard configuration.

Well, if Ulf ever drives with the standard tires, we might get a different result.
Perhaps during development, the focus was also on the standard trim level, and not all modifications made by owners were checked.
I can imagine that all the components were carefully adjusted to work together within a certain tolerance.

When vibrations occur in more complex systems, interactions between different components are to be expected. Therefore, the currently installed wheel and tire combination might be the cause. Whether removing the spoiler would have an effect is theoretically possible as well... just like reducing the load on the rear wheels by "cleaning up" the car, which is likely already quite light at the rear, like many of its siblings, and which certainly wasn't improved by the rather large engine.

Perhaps it's a problem we created ourselves?

matthiasTDI96 wrote:

Perhaps during development, the serial trim was also considered, and not all modifications made by owners were checked.

Therefore, the currently mounted wheel/tire combination might be the cause. }At least with "my" tire size of 205/50R16 on a 6.5" rim, I am still quite close to an original equipment tire size: for winter tires, 195/50R16 on a 6" rim is specified, and their sidewall is not significantly flatter.
By the way, I'm already running the 205 tires with an uncomfortably high pressure for a car with an empty rear seat: about 2.5 bar.

Well, if I can really get going with those 35mm rollers, we'll see what difference the tires make.

Okay, so...

I was able to drive the Polo enough today to determine that the factory-fitted tires, size 215/35R18 (still without the rear seat and without the roof spoiler), don't make my heart race nearly as much as the 205/50R16 tires.
With the factory-fitted tires, it doesn't run like on rails in those well-known critical curves above 200 km/h, but even with intentional slight swerving, it doesn't show any tendency to become unstable.
Only when braking sharply from approximately 220 km/h (not an emergency brake) did the rear end slightly wobble – it looked a bit awkward, but wasn't really frightening.

So, for me, this initially means that I will only exceed 200 km/h with the 50-profile tires if I am safely within a non-critical speed range before the next curve.
I'm also going to try Bertil's suggestion of using a very high tire pressure.


The question remains whether it would be possible to use 205/50R16 tires with normal tire pressure, without any side sway during cornering, even with tuned hearing aid (HA) bearings that suppress self-steering...?

Hi,

Quote:

So, for me, this initially means that I will only exceed 200 km/h with the 50-profile tires if I am safely within a non-critical speed range before the next curve.

...and hoping that you won't have to dodge anything or anyone.

Have you ever calculated how much downforce the mentioned spoiler can generate at speeds of 200+ km/h? Getting the rear end to stay planted, especially in small, front-wheel-drive cars, is a crucial design goal at that speed range, and your "optimizations" completely contradict that. Presumably, the handling in that speed range would be even better with a hundred kilograms of cement in the spare tire well and the spoiler.

Best regards,

January.

Jan6K wrote:

The handling in that area will likely improve significantly with a hundred kilograms of cement in the spare tire well and the spoiler.

If spoilers and the weight of the rear seat contributed so much to safe driving, then the Polo with the factory wheels wouldn't drive quite as smoothly at speeds above 200 km/h.

I've been thinking again about the self-steering behavior of my home automation system... see attachments.

The head of the axle has very little room for lateral movement in the "negative steering" direction because the outer sides of the bearings are already almost touching the outer guides even without any side force.

2. In curves, in addition to the stabilizing lateral movement, the rear axle can also rotate in another direction: namely, towards the center of the curve, because the rubber bushings are also radially elastic. This change in track adds to the steering angle of the front wheels, meaning that the rear becomes more agile or exhibits oversteer, which fundamentally creates a tendency for oscillation.


Since the radial deformation of the rear wheel bearings is not limited by any hard stop, I now assume that the range of motion for oversteer (i.e., unstable handling) is greater and will therefore be more dominant than the understeer characteristic of the rear axle.
Perhaps the soft, high sidewalls of my 1950s tires provide enough lateral flex to allow those wavy patterns to develop... ?

Here's another test with the 205/50R16 tires: Following Bertil's suggestion, I really pushed them hard this time. Before, they were at 2.5 bar, now they are at 3.2 bar (the maximum limit is 3.5 bar).
And indeed, the tendency to pitch and sway in the test curves has practically disappeared. The Polo still feels a bit softer than with the 215/35R18 tires, but now I can drive it towards its maximum speed without feeling uncomfortable.

Thank you, @Bertil, for the tip .
However, the possibility of converting to non-steering rear wheel bearings is still being considered.