Wheel Size vs. Acceleration - Round 28 *ding*
 

Sorry to ask another question like this but... I have 18" wheels and realize that this increases rotational mass/inertia/drag so my car will accelerate slower. After doing some research I am still left with a few questions.

1.) If you compare a 16 wheel/tire setup to a 18" wheel/tire setup and they both weigh the same, how much slower will the larger diameter setup be? Significantly? How much slower if the 18" setup is lighter than the 16" one?

2.) Won't trap speeds be higher on the larger diameter combo even though 60' and 1/4 times will be slower?

3.) What's the difference, in terms of acceleration, from a rolling start? Would this change at higher speeds, ie. on the highway?

Just curious. Thanx in adv.

Re: Wheel Size vs. Acceleration - Round 28 *ding*
 

1)the 18s will be slower because the biggest weight of a rim is on the outside and on an 18" rim the outside (and most weight) is 2" away from where it would be on a 16" rim.

2)I don't think so because the slower start will mean you cross the line slower because it took longer to speed up

3)not as bad as a launch, but it will still be a little worse.

please correct me or agree where needed

Re: Re: Wheel Size vs. Acceleration - Round 28 *ding*
 

1) it will be one inch on EACH side for two inches total, so technically it's 1 inch

2) this is somewhat true but it you get a larger diameter rim/tire combo then stock you will be higher et's and higher traps, ie a different final drive, look at don's 12.2 at like 102. it was all the tires.

3)on the highway it will matter a little but not as much as around town. that's why i could beat a GS 400 around town but on the highway it will beat me. no exactly the same thing but on the highway weight matters a bit less then at slower speeds.

Hey man, I just wanted to take a quick second and say your video is inspirational. :D That was a quick a$$ run for N/A. Congrats long over due. I've showed it to some of my frinds who think I'm insane in saying the max is pretty quick. Anyway, I won't be running low 14's anytime soon with my 18's, unless I just... :devil:

Hmm, I run SLOWER with 15s than with 17s... Lower trap speed too.

If you look at the physics of it, a wheel with a larger radius will have a higher rotational inertia, I. For a wheel, I is proportional to M*r^2. So, the rotational intertia goes up as the radius squared. The higher the rotational interia, the more torque it needs to get up to a given speed. torque = I * alpha, where alpha is the rotational accleration. So for a given rotational accleration, the higher the I, the more torque you need. Now that was with a lot of simplifications. To get the I for a real life rim, it is complicated and will depend on how the mass is distributed throughout the wheel.

so would in general 17's be the best all around size to go with for acceleration. I understand there is alot more to it like weight displacement and such, but im looking to buy some new racing harts and im not sure whether to go for 17 or 18.

Re: Re: Re: Wheel Size vs. Acceleration - Round 28 *ding*
 

steve.. you never beat my cousin.. where did you get that idea from?

If you have EXCELLENT control of your launches, 15s are great. If you don't have such great control, use bigger rims, they tend to peel out less.

Catch 22..

smaller rims = better acceleration = less traction
bigger rims = slower acceleration = more traction

like greg said.. you might be a bad launcher with bigger rims and run quicker than a average launcher with small rims...

Re: Re: Re: Re: Wheel Size vs. Acceleration - Round 28 *ding*
 

:laugh:

...*thinking out loud* carry the one, divide by two, add 7 uhhh, yeah...yeah, what he said!

Well, I went from 17" to 15" steel wheels at the track.

ET went from a best of 14.75 to 14.59 and my Trap Speed went up from 93.7 to 95.0

Re: Re: Re: Wheel Size vs. Acceleration - Round 28 *ding*
 

Not all the tires. His rims and tires are smaller in diameter than stock. This allowed him to cut down the rotational mass conciderably so that his engine didn't need to work as hard on the low end. Just think of tires/wheels as another gear in the machine. If you alter the overall diameter then you change the output of the mechanism. The draw back is lower trap speed because, although the wheels are spinning very fast, he's not making up as much ground as he would with larger diamter tires/wheels.

When most of use are looking to upgrade our wheels, we need to maintain the same overall diamter of the tire to, amung other things, maintain acurate speed readings from our speedometer. So, larger wheels with orignial tire diamter will most always slow any acceleration, but they should "technically" never reduce top speed, unless you'r engine can't get up to maximum rpms because of the greater inertia of rest in the wheels.

Cool, so you're saying that his trap speeds would've been higher with a larger diameter setup right? I guess as well as his et's like dmbmaxima88 said. But, then how would you explain ejj5875's higher trap speed with a smaller setup?
Am I misinterpreting what you're saying, most always or always? Wouldn't a larger/lighter setup increase the amount of grip a 5sp could use to get a better launch and overcome or offset the larger rotational mass? Or at least enough so it doesn't really have a huge impact on acceleration? I think I'm just scratching at the wind. I guess I'll just accept the laws of physics and move on to something more constructive to try and disprove. Damn Newton, Einstein and those guys. :D

what about 205/65/15 (stock)vs 215/60/15(planning on putting it on stock rims soon) ? they're the same in terms of speed?

Ra1max: I think that if you got a 16/17" wheel and tire combo that's the same weight or lighter than whatever your present setup is, you can only be doing good. You'd get the handling and traction benefits of more aggressive tires, with no acceleration penalty (or even a gain if it's lighter). I get what you're saying though, and the balance between grip gain and weight increase (where they would cancel each other out) would be anyone's guess and would depend on lots of factors. This assumes you'd gain contact patch area in going from say 15" to 17". I think the generalized rule of more weight = worse acceleration is probably best, especially since some 15" tires are pretty good grip-wise.

costcowholesale: according to this useful page: http://www.miata.net/garage/tirecalc.html (bookmark it :) ) those tires are 1.3% too small, meaning it will slow down your speedo that much and increase your gearing that much. Not a big deal at all IMO. If the tire is only a bit heavier than the 205/65, then I'd probably go for it just to increase the tire grip. According to the "tire weights" chart though at the Miata.net garage, some weights vary wildly, so unless you are replacing it with the exact same tire make/model, you could be getting a tire that's lighter or heavier. (Normally shouldn't be that much difference between 205/65 and 215/60)

increase gear = good thing? when I tried cartest, I made maxima's final gear ratio to 4.00 instead of stock's 3.62 and I got .2 less in the 1/4 mile.

Yeah, well the short answer is "yes" with regard to acceleration, but then again it's no big deal in your case. The flip side is revs will go up. Like if someone in a '71 Hemi Cuda put a 4.88 diff. gear in his car, he'd increase his gear leverage, but he'd be spinning over 3000RPM on the highway. Same effect if you get tiny 13" tires (like those dumb cars with super wide but short tires that stick out of the fenders... uhg). The opposite happens with larger tires or taller gearing.

thanks, but will that size fit on 99 stock GLE rims?

ejj5875's overall tire diameter didn't change, just the wheels. Smaller wheel have less rotational mass and thus allow the engine to accelerate faster under the same work load.

Quote:

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Am I misinterpreting what you're saying, most always or always? Wouldn't a larger/lighter setup increase the amount of grip a 5sp could use to get a better launch and overcome or offset the larger rotational mass? Or at least enough so it doesn't really have a huge impact on acceleration? I think I'm just scratching at the wind. I guess I'll just accept the laws of physics and move on to something more constructive to try and disprove. Damn Newton, Einstein and those guys. :D

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Get frustrated. We all are. As you have noted here, there are many other factors that offset negative impact of one element with a positive inpact of another. Now you can uderstand why car manufacturers take years to develope a new car. :)

A person can theoretically get wheels of equal diameter, weight and identical tires, but one pair will allow for better acceleration than the other, because the weight distibution throughout the wheel is different. The biggest problem isn't directly related to wheel weight, but weight placement. When you put more weight farther from the center of rotation, you exponentially increase the inertia of rest.

To get great ET's there must be a perfect balance of wheel weight, properly positioned mass, rubber compound, track conditions, overall diameter and of course, lauch techniques, etc...

I think some of you forgetting that Don runs full blown tiny slicks which cause his ets to be a rather odd and low 12@102mph. Since Don can come off the line so freaking hard and fast, he has less time to develop the mph. Aren't Don's 60' something like 1.7s or something? Also, the tiny slicks are wide and create lots of drag after the first 1/8th line. Finally, the smaller slicks force a shift into 3rd well before the end of the track and we all know that 3rd on the auto Maxima isn't geared very well even with the extra torque multiplication of the smaller slicks. Don runs are huge shot of nitrous which creates a massive surge of torque. If he sprays off the line, he'll get a better et due to the awesome 60'. If he sprays further down the track he'll have slower et, but higher mph.

For the most, it's all in the first 60 feet. Our car's have enough torque to pull 1.9 60' stock...if you have the traction. Look at it this way:

89mph trapspeed = ~14.9
90mph = 14.8
91mph = 14.7
92mph = 14.6
etc.

I use to have a 46lb 17" setup on my 96 Max. On the 17s, I ran 15.08@93 with a 2.27 60'. On my lightweight 16s (39lbs) I've run a best of 14.73@94mph with a 2.28 60'. After viewing my timeslips, much of the gains from the lighter wheels/tires are seen in the first 1/8th mile. I suggest getting the lightest rims you can afford. The ride will be better and so will the performance. Rotational weight is far worse than static weight. At all rpms and speeds, it is always going to be harder for the motor to spin the heavier wheels.



Dave

Acutally, it is quadratically. If it was an exponential relationship, larger diameter wheels would really hurt acceleration. I don't want to be picky, but this is a huge difference. :)

Yes, they should. I don't know what the rim width is for 4th Gens, but the 215/60 sized tire is approved for 6-7.5" rims anyway (and I don't see your rim being skinnier than 6").

damn you and your voice of reason dave. i wanted some blingy 18's but now i have to save up for some nice light ones. what are some nice light 18's that i can afford(ie not volks)

nope, it's exponential
 

Larger wheels do really hurt acceleration :)


The moment of inertia of a massive body about an axis is the sum of all the products formed by multiplying the magnitude of each element of mass by the square of its distance from the axis.

Re: nope, it's exponential
 

Not necessarily true grasshoppa. If you get a 18" rim that weighs 10lbs less than the stock 17" rim you may still have a net reduction in rotational inertia. Until you go out and make some measurements of different rims I don't think you can say that all larger rims are going to slow the car down. There also is the little detail that we don't really know how the weight is distibuted on a rim.

Stereodude

follow up the tread a little.
 

I was replying to a simple notion, not to be concidered an absolute. On average, larger wheels will slow acceleration, but as I had already stated, location of weight from around the axis is the derterminant for acceleration loss or gain. Putting more weight farther from the axis will exponential decrease acceleration.

larger rims also hurt your miles per gallon if they are heavier. My 18x8's weigh about 48 lbs with tires on them, whereas my stock 15s were like 44 lbs. I think I lost about 30-40 miles per tank going up in size/weight.

So that's where my mpg went. I need to hurry up and put on my Y and CAI so I can get better mileage... :laugh: I average about 270 miles per tank, all city stop-n-go traffic. I'm not sure how much my setup weighs but when my wheels and tires were shipped it read 52lbs. I don't know how accurate that is though. Since my wheels weigh 19-20lbs my tires must weigh a hefty 32lbs, which seems like a lot. I'll check later. BTW, do you happen to know the weight of the stock 215/55-16 setup for a '99 SE?