CT TL

Every time I read threads about people bringing their cars to the track to get their times they always mention that they had larger tires on there car, thus giving them slower times.

Well, I was watching Junk Yard Wars the other night "Dragster". When JYW illustrates the teams ideas on their Phineas J. Whoopee 3-D Bulletin Board, they noted the following.

Smaller tires will produce quicker acceleration.
Larger tires will produce faster top end speed.

The thought is.. will the TL get up to speed fast enough for the larger tire to become effective at higher speed. Thus having a higher speed and/or a faster time.

Essentially it's a gearing effect of a larger tire operating as a larger gear.

Bitium

The smaller tire is quicker because of the weight. The lager the diameter the faster. But the larger it is the more power it takes to turn.

CT TL

I'm just trying to start a logic discussion - kinda like the one you would have at a bar after a dozen beers.

Ok, yes - i'm sure the weight has some influence - but what is the weight difference between 2 or 3 inches in diameter (x 4)? (it can't be that much - is it?) But I think it has more to do with the engines ability to spin a smaller tire faster at start-up. I would think that the difference would be made up from the circumference of a larger tire over 1/4 mile.

Ok let's try some simple math for illustrative purposes

16 rim + 8 (two 4" sidewalls) = 24 24 x 3.14 = 75.36" circumference

18 rim + 8 (two 4" sidewalls) = 26 26 x 3.14 = 81.64" circumference

That means that every rotation of an 18" wheel is going 6" further. Over a 1/4 mile (1,320ft or 15,840 in.) a 16" wheel needs to turn 210 times while an 18" wheel needs to turn only 194 times. (that's 7-1/2% less rotations)

For illustrative purposes let's say a TL reaches 60 in 7.5 seconds, which is roughly half the elapsed time of a 1/4 mile. That means that it's got more than 1/2 the time, and definately more than half the distance for the larger wheel size to overtake the quicker acceleration of the smaller tire.

VDo

This calculation is not true. When people on the board upgrade to larger wheels, they inturn purchase tires that have a smaller sidewall so that they keep the total diameter of the wheels and tires as close as stock as possible. Here are some examples of tire/wheel combinations that some of the TL owners here are using and their corresponding diameters:

205/60-16 --> 25.68"
215/50-17 --> 25.46"
235/40-18 --> 25.40"
235/35-19 --> 25.47"

So even tho people are putting larger rims on their cars, the "wheels" are pretty much the same size. Therefore, the only real significant factor that'll affect how fast the car will run is the wheel weight.

I saw that episode of Junkyard Wars too, and the two dragsters had significantly different total wheel/tire diameters. That is the differnce they were talking about. The V8 would wooped ass if it had a working tranny.

RAdams

Your logic assumes that the car will be able to reach the same speeds with different tire sizes.

You mentioned earlier that any wheel/tire mod that changes the OD of your tires will alter your gearing -- and that's the key. The car simply will not perform the same way with gearing that is effectively taller. Yeah, it has the potential to pull any given gear to a higher speed, but since we're talking same engine (and same power) on a fixed length (the 1/4 mile) -- all else being equal the shorter effective gearing is going to be quicker. You need more power to pull a taller gear. Compare the gear ratios of a Civic to those of a Corvette and you'll see what I mean.

Of course, we're also leaving out total wheel/tire weight, polar moment, inertia, etc.

I'm all for in-depth technical discussions... but in the end it's all theory. The timing lights 1320 feet down the track will tell you the whole story.

CT TL

VDo - Oh yeah, i guess you're right - I'll drink

Still, what is the wheel weight difference?

Again this is a discussion that takes place in a vaccuum - so we can forget about variables like friction, air pressure, polar moments. What exactly is a polar moment.

Anyway, I'll rephrase the question. IF the TL (and that's a big IF) had larger diameter tires would it have faster times in the 1/4 mile, or racing in general. Do you think our cars have the power to pull the larger gear? It would be slower off the line, but would it recover.

Any yes, the two tires illustrated in the show were significantly of different sizes, but a small increase may make a difference when we are talking about people who strive to take .1 second off their time

Severin

FYI:

circumference = 2 * 3.14.. * radius,
not 3.14.. * radius....

Thus,
16 rim + 8 (two 4" sidewalls) = 24 24 x 2 x 3.14 = 150.72" circumference

18 rim + 8 (two 4" sidewalls) = 26 26 x 2 x 3.14 = 163.28" circumference

it makes a difference....

CT TL

No, no, no, no, no

circumference is 2(pi)(r) or (pi)(d), 2(r) is the diameter. Your foumula says our tires have a 13 foot circumference.

Glastonbury, huh - I work in Glastonbury

Severin

Those are big ass tires! My bad; I thought from a glance that you spec'd radius - I never even thought about whether the numbers made sense...

Yeah; I'm in Glastonbury- Not much work in Glastonbury unless you're a lawyer, financial advisor or insurance person.