Unsprung weight and performance benefit

[JHZR2]

Hello,

Im thinking through the opportunity to get a set of 15x7.5" wheels for my 91 BMW 318i, to replace the 14x6.5" wheels I currently have.

My question here is, will there be any real or perceivable benefit to doing this... and what would be the downsides.

Original setup, p196/65r14 'H' rated tires on 14x6.5 wheels. The wheels weigh a hair under 14 pounds, and the current tires weigh 18.3 pounds. The alternate tires ive seen in that size are all about one pound more.

So the current setup has a weight of 32.3-33.6 pounds per wheel/tire assembly.

The wheels Id be getting, 15x7.5" ronal German-made basketweaves are 17.2 pounds each. Looking at tires in the p195/60r15 size, Im looking between 17 to 21 pounds each (the 21 is for potenza re960AS pole position, which are VERY highly rated at tirerack). Looking at p205/55r15 tires, Im seeing 19-20 pounds.

So, overall Im looking at 34.2-38.2 pounds for the tire/wheel assembly.

Now, let's take midline estimates, so 33 vs. 36 pounds at each corner, will I notice any real effect/degradation due to weight? Im not auto-xing, but spirited driving and going fast around offramps, hard braking, etc may occur.

More importantly, I'd be using a taller rim, and 5-10% less sidewall height. I'd think that this would benefit handling slightly, but given the weight increase, is there any real benefit?

Also, given that I drive on a lot of PA roads which have a lot of roughness and potholes, how big of a ride difference is this going to make? I usually run my pressures a bit higher than spec for fuel economy reasons, but I'd hate to make the car turn into a buckboard because of its light weight, slimmer sidewalls, and high tire pressure.

That said, does higher tire/wheel weight or unsprung weight in general help or hurt the ability to smooth out potholes and bad roads?

Id appreciate any insight on this potential decision that I can make, as it will help me out quite a bit.

Thanks and best regards,

JMH

 

[Kestas]

Going with low profile tires is a tradeoff. As a rule, tires are lighter than rims. When you increase the rim size - as in low profile tires - you have more rotational kinetic energy to deal with. This means that you will gain in cornering traction and stability, but you'll lose out in acceleration and braking.

With regard to potholes, low profile tires make this experience considerably worse. There's less sidewall to flex and take up the pounding. That is why I personally don't care for driving with low profile tires on the moonscape of SE Michigan.

 

[Lorenzo]

JMH, higher unsprung weight will very slightly hurt your suspension’s ability to absorb bumps but I don’t think that the modest change from 195/65r14 to 205/55r15 is going to be very significant. The smaller aspect ratio will help with a little sharper handling at the expense of a little comfort. It might make it slightly more prone to damage on bumpy roads, but you’re still far from a 35 or 45 aspect ratio that is becoming so popular so I wouldn’t worry.

The real thing to be careful of is an unintentional geometry change because of selecting a wheel with incorrect offset. If the offset is off (ha ha), the wheels can effectively be mounted out or in too far which creates a scrub radius and all sorts of handling problems. On most late model cars, a lower offset number actually pushes the wheels out wider. Beware that most buyers make their selection based on looks and go for a wider “racing” track that causes real-world handling problems and excess strain on suspension hardware.

You’ve already done your homework on the weights, now it’s time to find out your stock offset and make sure you select a new wheel that correctly adjusts offset by the right amount for your new wheel size. The tirerack people probably can help but I would start by searching a BMW forum for knowledge of your specific model.

 

[JHZR2]

Thanks! Fortunately the wheels Im looking at, Ronal LS, are the OE geometry and offset, so that is a non-issue.

Thanks!!!

JMH

 

[astraelraen]

While I imagine every car is different and the effects of more/less unsprung weight would be felt differently. I went from 235/60/18 to 255/45/20 wheels/tires on my Mazda CX7. Wheel+tire weight went from ~58 to ~62 per corner and I can definatly feel a difference in terms of steering input. Its nothing drastic, but it takes more effort to turn sometimes.