Rear end instability

[dsitum]

Hello all,

I've recently been on a course of safe driving where we simulated various situations which might occur on the road.
There was one exercise that I remember the most - and it was something like moose (overtaking) test, but on the wet.
We were supposed to accelerate up to 50 kph (~31 mph) and then overtake an object, following by a quick return to the original lane.
Usual street/city cars do it effortless at that speed (at least that was what we've been told). But I tried it twice, and my car simply couldn't do it without rear end sliding. My rear tires (Nokian Powerproof) were at 6mm, and are regarded as good tires in the wet. I know I've been driving 50km/h because speed limiter was on. Also, ESP/ASR was on, as it should be when driving on the road.
I'm also driving non-cup and non-trophy version of the car.

Now, I know that our lovely 197 has stiff suspension, but the fact that it can't handle simple change of turn disappointed me a bit.

Do you have any similar experience?

 

[R20BTG]

Doesn't sound normal. Could be a number of things, I'd be checking:

- Age of tyres
- Rear tyre pressures
- Rear dampers (any leaks?)
- Rear springs (anything broken?)
- Get it 4 wheel aligned

Was there anything on the road? What were the weather conditions?

 

[dsitum]

All 4 tyres are DOT 2019 or 2020 (not sure) Nokian Powerproof XL (hardened sidewalls) with standard dimensions of 215/45R17. Front tyres were at 8mm, rear at 6mm.
Tyre pressures were okay, as I was checking them a day before the course (this was a requirement).
Nothing was on the road but the water (surface was butter smooth), and again, the instructor told us that the car should have no troubles passing the test at 50km/h (his regular Golf and the other car at the course did that without an issue).

Car passed MOT a month ago. Rear shocks were at 80-90%, while front were a bit more than 50% (if that means anything). There was no issue passing the MOT. I guess that if they noticed any leaks, they would have told me. The springs were probably not broken either.
Front wheel alignment (geometry) was done last year after replacing lower control arm on the left wheel.
Could rear wheels be out of alignment too?

I should check that, the springs and dampers in the back, just to be on the safe side.

 

[Ciapata]

Did you pass the maneuver in the end? Renault Sports are very tail happy by default, compared to other cars.

 

[dsitum]

Unfortunately not, as we had only 2 tries. Yeah, weight distrubution on that car is something like 64:36%, so that could also be the reason.
I can only remeber the guy saying me that this could be due to stiff suspension, and that I'll need to be very careful with that car on the road.
I am going to visit a workshop to recheck suspension and perform wheel alignment. Will definitely let you know how it went and if something was off.

 

[turkie172]

To be absolutely respectful and in order to help forget all the data, tread depth, suspension % stuff go and practice the maneuver somewhere safe your either lifting off the throttle (or the car is under cruse mode via steering angle sensor) or oversteering the car and upsetting the balance. If the car is not standard in respect to suspension standard shocks/springs and ride height then that's the problem get the instructor to do the maneuver or they must have a teaching car to do it with. It comes full circle to ability which is the reason of the training is to realize your limits or vulnerability and to behave and act accordingly on the road a fast/safe car does not make a fast/safe driver. The only thing I would check, through my steering rack replacement experience is get the wheel centered and check how many turns from center to left stop and the same to the right if they are not equal then you have a mechanical problem I had 8 steering standard genuine Renault racks that were not center and to get the wheel straight left unequal turns from center to full left and right stops.

 

[Wozzer]

That’s not normal and I’d definitely get someone to look at the car. 30mph in my RS200 does not displace the rear end i’d have to be going considerably faster.

 

[av4625]

Sounds very strange. I find that these cars (when standardish) are more likely to push the nose than whip the tail out even at higher speeds on track

 

[dsitum]

Unfortunately I can't reproduce this situation anymore. But maybe the problem was that I was using speed limiter. Maybe it cut engine power just at the moment I did sharp turn. And that cut of power caused rear end to slide. There is a phenomenon called "snap oversteer" or "lift-off oversteer" which explains just that.

Anyway, seems it's not possible to do rear wheel alignment on our cars. Only front wheel alignment is adjustable.

 

[suj]

Unfortunately I can't reproduce this situation anymore. But maybe the problem was that I was using speed limiter. Maybe it cut engine power just at the moment I did sharp turn. And that cut of power caused rear end to slide. There is a phenomenon called "snap oversteer" or "lift-off oversteer" which explains just that.

Anyway, seems it's not possible to do rear wheel alignment on our cars. Only front wheel alignment is adjustable.

You can adjust rear alignment by purchasing shims, they sit between the beam and the stub axle, so you check what your current alignment is, then you buy the shims for camber and toe for each side (they are sold separately in different sizes).

 

[Marsch]

This sounds strange. You normally can't produce lift-off oversteer at such speeds. Maybe with an absolutely shot suspension but I'd guess that's not the case here.

Was there any braking involved? If yes I'd check if one of the rear calipers is gone.

 

[av4625]

I know you mentioned your tyres being good. I have no idea what brand that is, but one thing that happened to me years ago. I had a 1.6 mk2 megane and put new tyres on. They where middle range tyres, so pretty cheap. But it through the tail out on me at low speeds in the wet and just all round felt dodgey going round corners. I swapped those tyres to the front and didnt have the issue.

Ever since having RS cars I’ve only ever run PS4’s on the road. They are just so good.

 

[burrellbloke]

You can adjust rear alignment by purchasing shims, they sit between the beam and the stub axle, so you check what your current alignment is, then you buy the shims for camber and toe for each side (they are sold separately in different sizes).

@suj is right. I had my car aligned in January 2021 and it showed that there was a different camber on my rear wheels. If you insert shims you can adjust the camber and toe even though the default set up has no way to tinker with camber and toe.

 

[bedoef]

Silly question, but what tire pressure are you running? Might as well rule out the simple stuff first.

 

[dsitum]

I was running 2.3BAR (~33.4PSI) front, 2.1BAR (30.5PSI) back at a time when it happened (I checked pressure the day before).

Now (after completing the course), I'm running 2.5BAR (36.3PSI) at the front and 2.3BAR (33.4PSI) back, as we were told that the rule of the thumb is to inflate tires to 0.2 bars above the manufacturer's recommended specifications to have the best contact between asphalt and the car.
That is of course for road use - not sure about the pressures for track use, as tires are heating up considerably more there.

 

[Wozzer]

I've never heard the need to inflate the tyres 0.2 bar above manufacturers stated level. Tracks a different matter given the heat generated. I typically run at 33psi on road but I've run at 30 on the rears and still never experienced the slide you had not at such a slow speed.

 

[Wozzer]

I can recommend Northampton Motorsport on the wheel alignment which I think would be a good start - if you tell them what you experienced they can at least look for potential issue as they do the job. I think I paid £100 for my Clio only a couple of weeks ago and they did a great job. Main think is get someone reliable don't assume that everyone with a Hunter/Pro-Align alignment tech to know what they're doing (I've made that mistake in the past).

 

[dsitum]

You can adjust rear alignment by purchasing shims, they sit between the beam and the stub axle, so you check what your current alignment is, then you buy the shims for camber and toe for each side (they are sold separately in different sizes).

Don't bash me for asking, but what's the purpose of adding shims if rear wheels have fixed alignment? Wouldn't that add more issues than it would solve? Correct me if I'm wrong, but if wheels cannot be aligned, then it means that rear wheels can't be out of alignment.

Anyway, I scheduled wheel alignment check this Monday. I'll let you guys know how it went.

 

[burrellbloke]

Don't bash me for asking, but what's the purpose of adding shims if rear wheels have fixed alignment? Wouldn't that add more issues than it would solve? Correct me if I'm wrong, but if wheels cannot be aligned, then it means that rear wheels can't be out of alignment.

The rear wheels can be out of alignment even though there is no OEM way to change the alignment. I am sure through general wear and tear, a few knocks here, a small kerb crunch there, could cause some minor alignment issues.

When my alignment was done in January 2021 the camber for my left rear wheel was at minus 1 degree and 56 minutes and my right wheel was at minus 1 degree and 31 minutes. The garage said it was nothing to worry about but I put a 30 minute shim on the rear right wheel so that both rear wheels sat at the same minus 2 degrees camber.

You can also add in shims to change the toe for the rear as well.

 

[R20BTG]

Don't bash me for asking, but what's the purpose of adding shims if rear wheels have fixed alignment? Wouldn't that add more issues than it would solve? Correct me if I'm wrong, but if wheels cannot be aligned, then it means that rear wheels can't be out of alignment.

Anyway, I scheduled wheel alignment check this Monday. I'll let you guys know how it went.

The shims aren’t flat, they are angled in very small increments (like a mini triangle). If you want more negative camber you install the shim with the thicker bit at the bottom and the thinner bit at the top which then changes the angle of the stub axle. Same with toe out/in, or you can combine camber/toe shims to achieve your desired geometry.


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