Piston Slap: Torsion Beam Bump Steer Solutions?
Sam writes:
I have a 2015 Hyundai Elantra SE. It has 25,000 miles and serves its purpose of a street parked commuter car that is comfortable enough for the occasional 600-mile round trip on weekends. The only real issue I have with the car is the cheap-car torsion beam rear suspension. Over large bumps on one side of the car the rear of the car feels like it oscillates extremely in each direction.
In a straight line it is controllable but I worry that in a off ramp taken a little too fast that a unseen bump could actually upset the car enough to lose control (or surprise a fiancé who is used to the handling of a 200S). The 2007 Honda Fit and 2015 Chevy Volt I drove I believe have similar setups but didn’t feel at all like this, and neither did the 2011 Crown Victoria I drove. I’ve read online that some aftermarket replacement shocks would help with this.
Is this really the case? Would a lighter set of wheels also help?
Sajeev answers:
Of course the Crown Vic won’t skip about: Panther Love for dat Watts Link, Son!
Seriously, the number of reasonably adjustable variables (i.e. keeping factory pick up points) provide a staggering number of end results to vehicle performance. And just because a suspension expert journalist gives you two cents doesn’t mean you or your fiancé agree. Perhaps only a tire pressure change is needed?
If not, the Hyundai Elantra’s torsion beam bump steer (more precisely roll steer) could merit a spring/shock upgrade, or a bolt-in upgrade from Pierce Motorsports or Progress Technology. I’ve enjoyed the latter: the ancient TTAC review has yet to sour after 9 years and 60,000+ miles, though I’d love to dial out some sway bar to add spring and/or shock firmness.
Such theories are only great in vacuums. However, owner feedback from Hyundai forums suggest aftermarket shocks also fit the bill: there’s a KYB Gas-A-Just performance rear shock available. Will a higher performance rear shock adversely affect ride/handling with original, 3+ year old dampers up front?
Only YOU know what’s right. I’d tinker with rear tire pressures first: try ’em on the same bumpy turn at the same speeds. If you still demand satisfaction, keep the factory suspension intact and slap on those aftermarket torsion bars. Even better, this is such a well known problem you can likely sell it on Craigslist and claw back some cash after you sell the Elantra!
BONUS! A Piston Slap Nugget of Wisdom:
We’re overlooking the handling band aid nanny available in damn near every late model car: standard electronic stability control. Provided you’re driving to the road conditions, not sawing at the steering wheel back and forth (i.e. rocking the car into body sway), and have good tires with plenty of tread, this problem is a non-starter.
[Image: Hyundai]
Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.
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Rear twist beam suspensions have a couple of problems when it comes to steer over single wheel bumps. One is that the whole axle steers in response to the inevitable longitudinal force as it hits the bump, the other is that twist beams always have some roll steer, and in a single wheel bump you get half the roll steer, if you see what I mean. These can be reduced (I hesitate to say eliminated) at the design stage by an appropriate choice of hardpoint (usually unfeasible) and rather fancy bushes. Between those two some manufacturers manage to keep it together. Some don't. Altering the vertical forces is a roundabout way of coping with a steer problem, but once you've got the geometry and bushes defined that's your only tuning knob. The reason it is disconcerting is that there is little feedback that it is occurring via the steering wheel, whereas front suspension single wheel bump kicks back through the wheel, so at least you know where it is coming from.
I won't buy a car with Torsion Beam Rear suspension. Even my Base 1.4 Jetta has IRS, although the much hyped new MQB version again goes back to sold rear axle, VW assuming that if you care, you'll buy a Golf.