Piston Slap: To Love, To Hate Aftermarket Rimz (Part III)

Sajeev Mehta
by Sajeev Mehta

Austin writes:

Sajeev,

So, a while ago I asked Bark and community what to replace my broken Audi‎ wagon with. I ended up purchasing a 2010 Lincoln MKT Ecoboost. So far, it has meet all my needs and — more importantly — my expectations, with one minor but irritating issue. At speeds around 70 mph and up, I notice a constant “twitch” in the steering on some highway surfaces — a sensation of vibrating left and right just enough to be noticeable, but not enough to change the actual t‎rajectory of the car.

Thinking that it was an alignment issue, I ran to discount tire (free rotations and balance because of my previous purchase history), where it was pointed out to me that the aftermarket wheels the car came with (future note: check the width and not just the diameter) did not align with what their computer system said I should have.

A stock tire on an Ecoboost MKT is 255/45-20 Goodyear Eagle RS-A. The tires on my car are 235/60-18 Michelin Premier LTX. The gentleman explained to me that my tires were basically an inch narrower than ‎stock, and while the ratio for the tire and the wheel was correct, because of the prodigious power output of a front-wheel-drive Ecoboosted car, my front tire might be “slipping” on some surfaces at higher speeds due to smaller tire contact patches. It’s not enough to get the AWD system to kick in but enough to get the vibration sensation when driving at higher speeds.

He said the best solution would be to get wider wheels and tires. Now, I consider myself somewhat knowledgeable on the benefits of higher-profile tires in terms of ride, snow traction, and towing performance, I have no experience with tire width. The cost of a wheels and tires swap-out for a stock setup is about $2,000. Thus, I would like to know what the community and yourself thinks before I start looking for ways to part with a sum of money that, in theory, could get me into old but reliable Civic or Camry.

Sajeev answers:

“The gentleman explained…because of the prodigious output of a front wheel drive Ecoboosted car, at higher speeds because of the smaller tire contact patches, my front tire might be “slipping” on some surfaces.”

Never. Double the power and it’ll still hook up at highway speeds at throttle inputs soft enough to not activate the power take-off unit. Your vibration is related to aftermarket wheel quality, not throttle input.

The circumference difference is only 0.3 percent (91.2 inches vs. 91.4 inches), so the tire choice is valid. My relationship with aftermarket wheels is still love/hate, since my Fox-body Cougar is daily driven: the reproduction Cobra wheels vibrate at around 40-50 mph on certain paved surfaces, even after switching to factory lug nuts.

I’m sick of it, and I feel your pain. We both need higher-quality aftermarket, or factory, Ford wheels. I should grab some 15×7-inch factory turbine wheels and you should consider a set of “take-off” hoops from an MKT, MKX, or perhaps that newfangled Town Car thingy.

[Image: Shutterstock user Mikhail Leonov]

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.

Sajeev Mehta
Sajeev Mehta

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  • Jthorner Jthorner on Jan 11, 2017

    I had an America's Tire counter person try to sell me aluminum wheels for our daughters' steel wheeled Hyundai Sonata as a way to improve fuel economy. While there might be some theoretical gains from a slight reduction in rotating mass, I don't think anyone has ever measured it in the real world. Said counter dude also knew nothing about rolling resistance of tires, which can have a measurable impact on fuel economy. Sadly, many of the people working at parts counters, tire dealers and other repair shops either don't know what they are talking about or are just saying anything in hopes of making a sale. That said, I generally hate aftermarket rims for a host of reasons. Unless you are buying top-of-the-line stuff they are often not designed and built to the standards of OEM parts.

    • See 1 previous
    • Rpn453 Rpn453 on Jan 12, 2017

      @Scoutdude That might be a rule of thumb for the effect of unsprung weight on the suspension. In terms of rotating mass, a solid disc has an equivalent mass factor of 1.5, and the factor is 2 for a thin ring. So an automotive wheel/tire combination would have to fall somewhere in there. Tire choice can indeed have a much greater effect than wheel choice, unless you're dramatically changing the wheel mass. With tires, tread mass is the biggest factor when it comes to rolling resistance.

  • Bozi Tatarevic Bozi Tatarevic on Jan 11, 2017

    Sajeev is spot on and the tire counter guy is way off base. Your tire sizes are very close and even closer than a lot of winter setups that people run. My first hunch is that the aftermarket wheels may have a bigger center bore and if you are not running centering rings then one may be slightly off center on the hub. I would check that first and order some rings if none are installed. The next issue is the quality of the aftermarket wheels. They may not be of equal weight or equally balanced. Like Sajeev says, the easiest thing to do would be to swap to some 18 OEM wheels from another Lincoln. The original 20 wheels on your MKT were 20x8 inches with 39 offset The closest match in 18 is a 2nd gen MKX wheel which is 18x8 with 40 offset Here what specs look like when switch to the MKX wheel and transferring over your current tires: Spec | MKT 20 | MKX 18 Diameter 737.5mm 739.2mm Circumference 2316.9mm 2322.3mm Poke 62.6mm 61.6mm Inset 140.6mm 141.6mm Speedo error 0% -0.23% Reading at 30mph 30mph 29.93mph Reading at 60mph 60mph 59.86mph Ride height gain 0mm 0.85mm Arch gap loss 0mm 0.85mm As you can see, the offset and size of the MKX wheel is almost a perfect match and your current tire is only slightly wider than the stock MKX tire so it should fit without issue. If you need to swap tires then I would recommend going for the 245/60-18 tire that is found on the MKX as that will bring you even closer as far as speedo match. Diagram: https://goo.gl/YGpDBb

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