why 18'' wheels more popular on track?

Apollo8642

What he's describing is I believe centripetal force, and not radius of gyration in the moment of inertia. As I said before that answer would fail you in physics pertaining to the physics of a wheel.

This statement by synolimit is really a waste of time. The the avarage person wouldn't think of moment of inertia, or the radius of gyration of unless they are an engineer, or have and OCD compulsion issue. It's kinda pointless, and I doubt that anyone really gives a sh!t, even about this waste of time of a post, arguing this matter.
People are going to look at style, weight, build quality, and price before they look at the actual physics of the rim and how it fuctions on their car. If that was so important way aren't people running in board brakes and small wheels on there cars.. I truth they aren't, they are running large rims i.e. the Le Mans Corvette Racing Z1 is runs 19x10 and 20x12.

The real point I'm making is that synolimit is posting something again, and I have a raging hard on for this guy, specially when he spews pointless often off basis BS that often time brings, or adds nothing to conversion, in my opinion. Here's an example. The difference of the moment of inertia of a 18" rim to a 19" rim is very small, and really as no bearing on if you should or shouldn't run a 18" or 19" rim, if that's what he was refering to. It would have more to do with the size of the tire you ran on the rim, the compound of the said tire, and it's total circumference as a whole. But that's wasn't the question, the question was why are 18" wheels more popular on the track, so lets talk about more sh!t that doesn't really matter.

In my defence I don't know sh!t from a shoe, but I do like to pick on synolimit, cause he makes it so easy. Thanks synolimit.

Red__Zed

This is the tech section, not off-topic.

The physics behind the performance of tires is a valid component of why smaller wheels are used more frequently on the track. MOI is about 25-40% less on an 18 as compared to a 19 on the wheels I've examined in the past. You can check that pretty readily as a 10kg disk with a 9" diameter has an MOI about 11% lower than a disk with the same weight and a 9.5" diameter. It is a consideration for those that are using this subsection for it's intended purpose.

If you want to pick on the dude, that's between you, him, and the forum admins. But please keep it out of the tech section.

XiP

I failed physics but I know bigger is better because that's what she said

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TerribleONE

Exactly. I was thinking of buying some 22s for my track setup

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synolimit

He's not all there, just let it go. You're correct in everything though, not him.

Ps its easy for me too. The tard just made a post about average people not caring yet we're in the racing thread TALKING ABOUT RACING!! Dert dert durr!

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Volk Z

Word, but I never said a 19 would yield better grip.. I just said could help with track times as it may help have less flex in the sidewall and possible help with cornering (less understeer type of deal)

If we are talking grip then- tire brand, tread pattern for specific event, tire pressure, alignment, suspension, sway bars etc etc.

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andre12031948

I took my used 2012 base auto(1,200 miles) to Atco Friday night. Owned it for only 3 days but just had to find about traction/car.

My first completed run in a dry lane, 60' was 2.010 seconds & 13.137@106.12 1/4. At 1,500 rpm I let the brake go & at moderate rate stepped on the gas peddle. After that run, it looked like amateurs went into & over the watered ground & got everything right up to the stating line wet.

Even at idle or a bit more than that, my wheels chirped a bit & 60' & 1/4 times suffered. 60' 2.20's & 2.10's. I got the exact numbers in my post that I made Friday.

So if 2.00 60' can be done with stock base wheels/tires & leaving at only 1,200 to 1,500 rpm, why would I next spring have a problem doing 1.8's and better using drag radials??? It's an automatic All you need is a clean prepped dry starting lane.

Apollo8642

Let me try to explain this, for those who are still following.

A rim, and tire is technically an annular cylinder. You could calculate the moment of inertia by calculating the annular cylinder about the symmetry axis. 1/2M(R 1over2+R 2over2) I believe is the formula, kinda hard to put it in a post like this, but hope you get the idea. You have to keep in mind, you still wouldn't be correct even then, do to the altering forces of the vehicle it's self suspesion, weight, eta. then you have different road surfaces, geometry of the road surface, that act upon the annular cylinder (AKA wheel and tire) or in other words there are real world conditions that have effects in this equation.

Red_Zed figuring out the moment of inertia of a 18" and 19" rim is great if you're in a class room, and you're solving this in a controlled environment. So in theory yes, you are correct, in caluation I would need to do the math to make sure myself to say other wise. The reason you aren't correct though is clearly you're not including the full circumference, width of the tire on either given rim, their total mass, or any of the other factors for that matter in this that play huge factors. People don't drive in class rooms, or completelly controlled environments with prefect surfaces either, not to mention there is something attached to that wheel, call a vehicle.

Red_Zed look more at the big picture, and not just one part, that's why this Bull$sh!t really has no relevance to "why 18'' wheels more popular on track?" like I said before. This is a case of people being stupid, and aguring over something that is completely superfluous to the topic.

Red__Zed

The intertial differences are a large part of the motivation to use smaller tires, whether the average weekend warrior understands that or not is irrelevant to the physics. Regardless of whether someone can articulate that reasoning is irrelevant--it is a driving force for the use of smaller wheels on track.

Red__Zed

BTW, your math would not give the "exact" moment of intertia for a real rim, as you claim. You need to take into account the mass profile. The math isn't terribly tricky but getting the right numbers is challenging. It's a lot easier to measure.

b1adesofcha0s

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GSS138

gentlemen, there's only one way to settle all of this argument.

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Apollo8642

Now your putting words in my mouth, and I did say you have to take mass in account. It seems like your having an issue understanding this is not a disk, or a hoop. It's not just about the rim size but it's the tire size diameter and width, that will in the end dictate the moment of inertia. If your going to do it, do it right is all I'm saying.

That's a great and all, but why is it that race teams have been putting larger diameter rims on races cars, and every decade they seem to get just little bigger?

Red__Zed

maybe you misunderstood. I measured with a photogate. Hence the "it's a lot easier to measure"

I'm pretty sure I'm not the one struggling to understand the physics involved


brakes.

GSS138

Enough with the nerd talk, I think you guys need to race.

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