Twitchy Rear End
Discussion
Guys,
Can any of you suspension and handling gurus give me any advice on why the rear end of my 350z might feel a bit twitchy when I'm driving over really shiny strips of tarmac / paint in the wet?
I notice it when I'm going over roadwork repairs and large sections of paint on the road but only when grip is a bit lower in the rain. The tarmac generally has to be the sort you get in small strips with either very little or no visible aggregate at the surface so it's really shiny.
I can be driving in a perfectly straight line on a flat smooth road and as I drive over these strips at >50mph in heavy rain the car twitches enough for it to feel like my body is moving in the seat in response. This doesn't feel particularly fun and in the back of my mind I'm wondering about the day the rear aquaplanes and the last and first bit of grip it has during the 'flight' will be with some form of sideways force...
I can post up the full details of the last full geometry check I had done, the problem though was that my car is sitting slightly lower than standard and is on the optional 19" rims - both of which weren't an option as default settings on the geometry machine to compare with.
My front camber is ~1°30' on both sides, the rear is ~2°40'on both sides. The front toe is ~0.2 on both sides, the rear is ~1.8 on both sides. Last two times I've changed the rear tyres they have been evenly worn, I'd guess at no more than ~1mm difference across the width (the tyres are 275/35/19).
From my noddy understanding, if the rear tyres have a positive toe then this is toe out and will help when cornering (not sure how?) but straight line stability will be reduced. Am I correct?
I don't track the car so I'm not that bothered about eking out every last drip of cornering performance and I'm not keen on the wiggle in the wet so should I have the rear toe dropped back to ~0.8?
Any advice appreciated!
Can any of you suspension and handling gurus give me any advice on why the rear end of my 350z might feel a bit twitchy when I'm driving over really shiny strips of tarmac / paint in the wet?
I notice it when I'm going over roadwork repairs and large sections of paint on the road but only when grip is a bit lower in the rain. The tarmac generally has to be the sort you get in small strips with either very little or no visible aggregate at the surface so it's really shiny.
I can be driving in a perfectly straight line on a flat smooth road and as I drive over these strips at >50mph in heavy rain the car twitches enough for it to feel like my body is moving in the seat in response. This doesn't feel particularly fun and in the back of my mind I'm wondering about the day the rear aquaplanes and the last and first bit of grip it has during the 'flight' will be with some form of sideways force...
I can post up the full details of the last full geometry check I had done, the problem though was that my car is sitting slightly lower than standard and is on the optional 19" rims - both of which weren't an option as default settings on the geometry machine to compare with.
My front camber is ~1°30' on both sides, the rear is ~2°40'on both sides. The front toe is ~0.2 on both sides, the rear is ~1.8 on both sides. Last two times I've changed the rear tyres they have been evenly worn, I'd guess at no more than ~1mm difference across the width (the tyres are 275/35/19).
From my noddy understanding, if the rear tyres have a positive toe then this is toe out and will help when cornering (not sure how?) but straight line stability will be reduced. Am I correct?
I don't track the car so I'm not that bothered about eking out every last drip of cornering performance and I'm not keen on the wiggle in the wet so should I have the rear toe dropped back to ~0.8?
Any advice appreciated!
Daaave said:
Guys,
Can any of you suspension and handling gurus give me any advice on why the rear end of my 350z might feel a bit twitchy when I'm driving over really shiny strips of tarmac / paint in the wet?
Dunno. I've never driven or worked on a 350Z, unfortunately (nice cars...), but thread here suggests that perhaps TADTS?Can any of you suspension and handling gurus give me any advice on why the rear end of my 350z might feel a bit twitchy when I'm driving over really shiny strips of tarmac / paint in the wet?
Wet paint markings and bare tar are pretty lethal on bikes, too – they offer much less grip than general tarmac, so the twitchyness isn’t that surprising if the car is a bit tail-happy to start with?
Multi-link suspensions from the major manufacturers can do subtle and clever stuff, though, so unless you are confident that you're getting sound advice from a real expert on the specific manufacturer & model, I'd be inclined to suggest you just check that the geometry is exactly to manufacturer's settings and tyre pressures are spot on. You could easily make things a whole lot worse by tinkering.
Sam_68 said:
Dunno. I've never driven or worked on a 350Z, unfortunately (nice cars...), but thread here suggests that perhaps TADTS?
I'm not really experienced enough to say, but I'd like to drive the same section of road in similar conditions in another Zed to confirm it.Sam_68 said:
I'd be inclined to suggest you just check that the geometry is exactly to manufacturer's settings and tyre pressures are spot on. You could easily make things a whole lot worse by tinkering.
I agree, but should I follow the standard settings even though my wheel size and ride height are different? 
Firstly, I know nothing about the 350Z.
So I could be talking bllx.
When you change the ride height, the usual problem is that the suspension settings were set for a different position. So that if (say) your suspension is designed to increase camber and toe on bump, then if you lower the car without adjusting the settings, then in steady state you will have more camber and toe than designed, and in bump, or the out side of the car in a turn, you will have even more.
The point I'm trying to make; is that if the ride height is altered, in the absence of any guide settings for the new ride height, make sure that the settings at the ride height are the same as specified for normal ride height. It won't necessarily be correct, but it's a reasonable starting point.
AS to your back end 'squirming' over slippery bits of road, it doesn't matter what speed you are doing, but how much throttle input is there? Surely it's fine if you are just cruising?
Even in my E36 325 I used to expect a shimmy over whitelines etc if I had lots of throttle in the wet.
So I could be talking bllx.
When you change the ride height, the usual problem is that the suspension settings were set for a different position. So that if (say) your suspension is designed to increase camber and toe on bump, then if you lower the car without adjusting the settings, then in steady state you will have more camber and toe than designed, and in bump, or the out side of the car in a turn, you will have even more.
The point I'm trying to make; is that if the ride height is altered, in the absence of any guide settings for the new ride height, make sure that the settings at the ride height are the same as specified for normal ride height. It won't necessarily be correct, but it's a reasonable starting point.
AS to your back end 'squirming' over slippery bits of road, it doesn't matter what speed you are doing, but how much throttle input is there? Surely it's fine if you are just cruising?
Even in my E36 325 I used to expect a shimmy over whitelines etc if I had lots of throttle in the wet.
Paul Drawmer said:
AS to your back end 'squirming' over slippery bits of road, it doesn't matter what speed you are doing, but how much throttle input is there? Surely it's fine if you are just cruising?
Even in my E36 325 I used to expect a shimmy over whitelines etc if I had lots of throttle in the wet.
That's exactly why I thought it was a bit strange - it happens under constant throttle so I'm not accelerating or decelerating.Even in my E36 325 I used to expect a shimmy over whitelines etc if I had lots of throttle in the wet.
HRG said:
Ride a bike, then you'll learn not to cross white lines/tar in the wet. IMO you're feeling a loss of traction, it's nothing to do with an inherent geo problem as such. Make a habit of doing it and sooner or later you'll end up inspecting the hedgerows
The tar I'm talking about is strips that cross the width of the road, so short of performing an emergency stop and calling the AA to get them to lift my car over them - they are pretty hard to miss 
Fair point with the paint as that is mostly avoidable.Paul Drawmer said:
Daaave said:
The tar I'm talking about is strips that cross the width of the road, so short of performing an emergency stop and calling the AA to get them to lift my car over them - they are pretty hard to miss
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