Thoughts on rotating mass.

[sketchman]

So I did the Rio swap and gained about 7.5lbs at each corner. Went up 10mm in tire width and nearly nothing in diameter. Verified both with an online calculator and a tape measure.

The majority of that weight is in the wheel/tire package. It went from about 21lb 155/80/12 on steelies to 25.8lb 165/60/14 on 11.8lb alloys.

I've lost about 5 mpg consistently after this. It feels slower to accelerate to me, but I'm willing to believe that's in my head and quite possibly just feels that way because it's smoother now.

The fuel economy decrease, though, has held steady with every tank after the swap. I even use the same pump at the same station every time.

I read the following and started to think some.

http://www.w8ji.com/rotating_mass_acceleration.htm

The simple thing I take away from it is that with the swap, I added weight in the worst place I could have. In the tires.
Old tires weight approximately 10.5lbs each. New tires weigh 14lbs each. Old wheels are 10.5lbs. New wheels 11.8lbs.

So 1.3lbs in the wheel but 3.5lbs at the tire. The place that'll store the most energy before it moves.

Now that's got me thinking. What would happen if I stuck some 12lb 13s on with 155/70s? The TINY bit of added weight in the wheels moves toward the center of rotation a touch to offset it, and the diameter and width of the tire goes back down to almost stock, along with surely loosing weight in the tire too.

Thinking when it's time for more tires, it'll be time for more wheels too.

Thoughts?

[Spike]

Have you checked your speedometer via GPS, or just measured your wheels? My speedo is 4MPH off at 60, showing about 56 on the gauge. 155/80-R13s.

Plus, the wider tire would have an affect, but I don't think it'd be that much.

-Joe

[Advancedynamix]

Wider tires also provide an increase in rolling resistance. I would try running as little toe as possible. If you can get negative camber then this will help with stability at little to no toe. Also, try running more air pressure. 155-70-13's may help because of the narrow tread patch, but I can't stand driving on this size because the handling feels mushy and numb.
With negative camber and zero toe, your reducing tire scrub to a minimum.

[sketchman]

Hmm, well I had it aligned, but I don't remember exactly what the specs were.

I also don't know what, if any, is a typical drop in MPG with a RIo/Aspire swap and nearly stock sized tires. I run 40psi all the time. I guess it's also possible that now that the car is nicer to drive, I'm allowing the right foot more leeway.

[Advancedynamix]

Hmm, well I had it aligned, but I don't remember exactly what the specs were.

I also don't know what, if any, is a typical drop in MPG with a RIo/Aspire swap and nearly stock sized tires. I run 40psi all the time. I guess it's also possible that now that the car is nicer to drive, I'm allowing the right foot more leeway.

I think you are onto something there!

Alignment shops love to set cars up toed in. I have fixed hundreds of alignments that were done at "state of the art alignment shops". I think everyone who owns a car should own toe plates and a camber gauge. Toe in will drastically effect your mileage, and your tire wear. All of my cars are set to 0 toe.

[TominMO]

Sketch, I'm guessing you are doing a lot of city driving. On the hwy I doubt you would see such a MPG decrease, if the old and new tire diameters are about the same.

Edit: Since you are in WV, hills might also be a factor. Flat hwy driving should give you little or no change to mileage, again assuming the same tire diameter.

[blkfordsedan]

It doesn't take very much increase in diameter to feel the difference in acceleration....especially when you only have 50-60LbFt of torque. I've noticed the difference between 145r12 and 155R12's. Plus, rolling resistance depends a lot on tire construction, even if they're the exact same tire size. Combined with the increase in weight, I would say the slight loss is believable.

[sketchman]

Most driving I do is 5th all the way and between 55 and 75. Was getting 48mpg consistently before and now 43mpg max.

If I can find a level place I'll check the alignment myself. I normally would do my own, but I was paranoid about the new tires.

Oh, also speedo is accurate. Checked with GPS a few times.

[zoom zoom]

It doesn't take very much increase in diameter to feel the difference in acceleration....especially when you only have 50-60LbFt of torque. I've noticed the difference between 145r12 and 155R12's. Plus, rolling resistance depends a lot on tire construction, even if they're the exact same tire size. Combined with the increase in weight, I would say the slight loss is believable.

Yea I noticed a BIG difference swapping to aspire knuckles and 175 70 13's, car didn't accelerate nearly as fast, engine didn't rev up as fast... but when I swapped back to 145 12's and festy knuckles it accelerates nice. I just swapped to 13" 175 50 13's(festy knuckles) which are smaller overall diameter than 145 12's, but they still don't accelerate as fast as a 145 12, it's not nearly as slow as it felt with aspire 13" steelies and 175 70's. At the time I had the aspire knuckles on I also had a TON of positive camber, I have since kicked them out and have a good bit of negative camber and also adjusted my caster as much as I could with energy suspension bushings on the sway bar.

I would try a different alignment, I try to get mine to 0 toe like Charlie always says but I'm also doing it in my driveway so I can't tell you exactly what it's at right now other than decent - camber alot of caster and close to 0 toe.

[sketchman]

Well apparently I can get 155/65/13 too easily enough, which would be just that little bit better than the 70 series. Still gotta check my alignment. Wish I could find the printout they gave me. If I add caster it should pull the toe out, right? Just asking because with the level of accuracy I'm capable of (nearly shot in the dark and then road feel to correct it) with the place I have to work it would be more reliable than adjusting the toe directly at the tie rod ends. I can generally feel anything too far out of 0 toe, but getting absolute 0 probably won't happen if I adjust it directly.

I appreciate all the input. I'm still leaning towards going to close to stock diameter 13s, but probably just because my brain wants it to be the big obvious change that did it. We'll see. 13s won't happen till I get 4x100 in the rear so I can run deep dish racing steelies, and that won't happen till/IF I can make the civic hubs work, so there's a lot of wiggle room to try things till then.