Dartiness?

[evildronepilot]

I came from a slew of FWD cars (pretty gross things, like a Saturn SC2, Scion TC, and C-Max Hybrid), and I do a ton of freeway driving. Those cars tended to be pretty planted to the road and didn't need to be babysat to stay in the lane as much.

My EB PP seems to be "darty." I don't know if that's the the right term, but I seem to have to constantly input steering to keep it going straight. I usually keep it in "sport" steering mode as it helps a bit and I like the feel.

Is this a common thing for this car or RWD cars in general? It tracks straight on flat (un-cambered) roads, but it's almost like it responds too well to road camber.

I'm not complaining though. I LOVE LOVE LOVE this car!

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[JeffreyDJ]

I've driven mostly RWD cars, except for a '14 fusion. I don't really notice what you're describing - but it *could* be because it's behaving as I expect it.

 

[minicobra]

I've noticed this slightly with my GT as well. There are several things that can attribute to this, which I have corrected in the past. Generally it had to do with the toe/camber/alignment adjustment, and sometimes even the tire can cause tramlining. I'm not a suspension expert by any means, but If it is something that you feel needs improving, I would take it to a really good alignment shop, and let them know your concerns. Make sure that you take it to a place that is not afraid to go out of factory spec in order to get you what you want. Usually a place that works on performance cars or track cars.

 

[evildronepilot]

Thanks, both of ya! I'm not incredibly concerned; I was just wondering if it's more common with RWD or this particular suspension setup, being that this is much more of a driver's car than I have had previously.

 

[JimmyTwoTimes]

I've driven a mix of RWD and FWD cars in my life, and, yes, what you're noticing is an inherent trait of RWD cars.

The easiest way to think about it -- tires have 100% available traction. That traction can be applied laterally (turning or road-holding) or longitudinally (accelerating and braking). In FWD cars, some of the front tires' grip is going towards pulling the car along at all times; this lessens available lateral grip and means that accelerative force is always being applied in the direction the front wheels are turned.

In a RWD car, the front wheels are applying no accelerative force, meaning that 100% of available tire grip goes to lateral forces -- meaning you're more likely to catch road camber and paving ruts changing your heading than in a front wheel drive car. The "active nibble control" baked into the electric power steering is supposed to mitigate this.

Similarly, all of the car's accelerative force is along the car's center line, rather than in the direction the front wheels are pointed, making it feel subjectively more like car is darting around because the car's acceleration will be in a slightly different direction than your wheels and eyes are pointed in turns.

This, by the way, is the main benefit of RWD (aside from weight transfer during acceleration giving more grip to the rear wheels). Seperating the drive wheels from the turning wheels gives each set of tires more traction available to do their job.

 

[evildronepilot]

That was a really great explanation of the difference of FWD and RWD. I just never thought to apply it to going in a straight line. Thanks!

 

[Ecobeast]

I noticed on my f150 when I switched tires from a smooth road tire which had a rounded edge to a off road tire that has flat edges my truck wandered in the grooves a lot more. Just sayin :-)

 

[evildronepilot]

Hrmmm...I think you just solved my problem, Ecobeast, even though you didn't know it!

I just checked my tire pressure via TPMS, and they're at 36. It's about 40F right now, and they should be about 32. I think I'm going to let some air out tomorrow morning before I go anywhere and check with a real pressure gauge, but this now seems to be a case of over-pressured tires (from the dealer).

 

[OppoLock]

Likely to do with toe and caster, and the fact that a FWD car is actively driving/pulling the tires in a forward direction. I would assume that any variation of the S550 has more aggressive toe out and negative camber than the cars you listed in the OP, which would create less straightline stability, relatively speaking. A wider series of tire will also create more tramlining.

 

[evildronepilot]

Likely to do with toe and caster, and the fact that a FWD car is actively driving/pulling the tires in a forward direction. I would assume that any variation of the S550 has more aggressive toe out and negative camber than the cars you listed in the OP, which would create less straightline stability, relatively speaking. A wider series of tire will also create more tramlining.

Awesome. Do you think the over-pressure condition actually has anything to do with it?

 

[Tony Alonso]

A wider series of tire will also create more tramlining.

I would attribute the OP's experience to this. On my previous 2010 GT, which had 245 mm wide tires, I would also notice some tramlining. On my current GT with 235 mm wide tires, I do not. The 255 mm width, in my opinion, is a contributor to that feeling. Of course, the EPAS system is supposed to help mitigate that.

 

[Grimace427]

I agree with the others recommending to have the alignment checked if you feel there is an issue. If the front is toe-out it can definitely cause sensitive steering feel.

Awesome. Do you think the over-pressure condition actually has anything to do with it?

36psi is what I run my tires in cold temps because as temps drop so do tire pressures(law of thermodynamics). If they were 45psi+ then that would not be good, but only 4psi over recommended is fine.

 

[Phlyguy]

What tires come stock on the EB with Performance Pack? Are they Pirelli P Zero's? The Pirellis are sensitive to road temperatures and tire pressures. Think you have a combination of minor alignment and tire pressure/compound items going on plus the adjustment to center line thrust rear wheel drive sensitivity. Add to this the electronic EPAS and you're feeling every undulation and imperfection on the road surface...different tactile feel

 

[Hack]

I don't notice that at all on my GT, but I've only driven V8 powered, rear wheel drive cars as dailys for years and years.

I wonder whether if you turn off the sport mode that will reduce it? You could turn it off on longer trips to reduce fatigue and turn it back on if you're going to be driving rush hour.

 

[doulos4jc]

Mine tracks straight and true. Steering is always in sport mode. I am running A/S tires not the P Zeros.

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