So confused about How smaller tires help speed

[chubbychu]

Hi, ive been on car forums for a while now, and everyone knows the smaller the tire, the better the accel.

but my teacher and i were talking. and this is where im confused, when you turn an axel thats connected to a wheel, small to big, it would only take one turn of the axel to get a complete turn of the wheel, and depending on the circumference..say 10 ft, than wouldnt it seem that having bigger tires at the drag strip is better?
and his thing was "thats why all those funny cars have big slicks" or something like that, im confused...help!

[Peking]

I remember reading a study a long time ago. That basically stated that the smaller diameter would rotate quicker then a bigger wheel. Revolutions per minute (or second, damn my memory). Also smaller wheels as you know are usually lighter then bigger ones.

[denmah]

the friction of a larger tire slows it down. quite simply that is why you put skinnies on the front of a RWD drag car.

[oompaloompab0i]

isnt friction what you want? so you stick to the ground and not all peeling out between gears stuff and lose traction?

[denmah]

well the friction does help for traction yes but it has to be balanced... you cant put funny car slicks on your turbo SI and expect it to get good times.

all things aside, when launching a car friction is key, getting grip traction etc off the line is key. while accelerating (or on a roll) you really dont want a ton of friction, think of how your breaks work.

[chubbychu]

Originally posted by Peking
I remember reading a study a long time ago. That basically stated that the smaller diameter would rotate quicker then a bigger wheel. Revolutions per minute (or second, damn my memory). Also smaller wheels as you know are usually lighter then bigger ones.

iuno...it would seem true except that because the circumference on a bigger wheel is longer.

if you had a 5 foot track, and the circumference of your tire was 5 feet, it would only have to turn once.

if you had that same track, but your tire had a circumference of 1 ft, than your tire would have to turn 5 times.

so...5 turns? or 1 turn? see what im getting at?

also, im not talking about launching or what not, but good point. i just wanna know why it would seem more logical that bigger wheels make your car faster.

[pedzola]

Heres how I see it... it comes down to a matter of simple machines.

Think of it like this.... you have a rod (axle) attached to a wheel. One person is holding the wheel, and one person is holding the rod. It would be very easy for the person to turn the wheel no matter how hard the other guy is holding the rod. However, if the guy turned the rod, it is likely that the guy on the wheel end wouldn't have to exert much pressure to keep the wheel from spinning.

See what I'm getting at?

The larger diameter the wheel, the more torque is needed to spin it in order to overcome friction and push the car forward.

Smaller wheels will essentially make your gear ratios shorter also. This means that it will take less revolutions of the wheels to get to redline, which means you are shifting (into your optimal torque band) sooner than the guy with bigger wheels. In addition it takes less force for you to move the wheels because they are smaller. If we had a 50,000 RPM redline and were running on perfectly paved streets, and if we had 15 gears, we might do well with 3 inch tires.


:D

[chubbychu]

okay i think i got it thanks!

[mental]

well see its like this bigger wheels or smaller wheels with bigger tires cause more rotational mass... thus making it harder for your motor to turn the wheels thus putting less power to the wheels and aswell the wheels are bigger so they prolly wiegh quiet a bit making them even more difficult to turn... thats what i know... also smaller tires make you go faster cuz you put more power to the ground... on a tire you have this... number... this is an example but here goes...
205/55/16... so it means(for those who dont understand) 205 is the width, 55 is the aspect ratio, and 16 is the wheel diameter... so if you have like 205/55/16 the tires have a certain amont of bulge if you lower the aspect ratio you put more rubber to the asphalt... make sense... the more the aspect ratio comes down the closer you get to the actual width of the tire... so if you have 205/55 you may wish and if depends on how stupid it will look to but you may wish to get a lower profile tire like a 205/45 or 205/50... cool got it, hope i helped... ohh and that teacher might be really cool but teachers are pretty much morons i proved that many a times at trinity high school when i was there...

[YamatoTwinkie]

Another reason: having large wheels/tires is like trying to accelerate off the line from a higher gear- it makes it much more difficult to pick up speed initially. I'd suggest reading how lever arms, pulleys, and gears work. Specifically, how they provide torque.

[C1V1C]

Let's say it with plain language :)

Bigger Wheel : Heavier, larger rotational inertia, one turn of axle - further distance.Since it is heavier and larger in diameter, it needs more power to turn in.

Smaller Wheel : Lighter may be, smaller rotational inertia, one turn - shorter distance, less power needed to turn the axle.

Difference in tire profile might not be noticeable with all these changes, I assume 40 to 45 profile with same width. However speedo will show differently in term of speed.

Hope this helps ..

[Peking]

Originally posted by C1V1C
Let's say it with plain language :)

Bigger Wheel : Heavier, larger rotational inertia, one turn of axle - further distance.Since it is heavier and larger in diameter, it needs more power to turn in.

Smaller Wheel : Lighter may be, smaller rotational inertia, one turn - shorter distance, less power needed to turn the axle.

Difference in tire profile might not be noticeable with all these changes, I assume 40 to 45 profile with same width. However speedo will show differently in term of speed.

Hope this helps ..

That's what I think :)

[02NITEHAWKSi EP]

You guys covered just about everything. One other point. If we did have a 50,000 rpm redline, which would ROCK!!. . . The wheels better be in perfect balance. The wheels and tires need to be able to handle the stresses of the centrifugal forces exerted on them and the stresses provided by friction, impact and heat buildup.

Way back in the day of the speed records being 40-60 mph this was the case, when guys shattered them the tires of the day could not handle the stresses and just disintegrated. We don

[ImPo Skyline]

its been said, so ill just say it again.

Smaller WHEEL will make your car quicker 0-60.

Larger WHEEL will make your car faster top end.

a 16" WHEEL with 55 series rubber is the optimal performance size.

[Dunrick]

Originally posted by ImPo Skyline
its been said, so ill just say it again.

Smaller tire will make your car quicker 0-60.

Larger tire will make your car faster top end.

a 16" tire with 55 series rubber is the optimal performance size.

how does a larger tire help top end? In what way?

[ImPo Skyline]

Originally posted by Dunrick
how does a larger tire help top end? In what way?
opps! i mean rim total wheel size.

[Siman]

less contact patch with road= less friction= greater top speed/top speed acceleration:D

[Dunrick]

Originally posted by Siman
less contact patch with road= less friction= greater top speed/top speed acceleration:D


word. breaking it down like that is all this needs.

[Graywolf]

Also, a taller tire, means more rotational mass, which means more inertia, which means its easy to keep it moving faster once its already moving fast, just like a heavy flywheel.

[SmoothOperator]

This is how it was explained to me, and it made perfect sense...

Picture a figure skater....spinning in the same spot. When the skater sticks her arms out, the spin slows down. When she brings her arms in, the begins spinning faster.

These same, simple rules of physics apply to your wheel size. Move the rotational weight out, and you'll be slower. Bring it in, and your wheel spin speeds up.

hope that helps

[BSEVEER]

A wheel with a larger than stock diameter will give the car a "taller" final gear ratio and visa versa.

[glw]

Originally posted by SmoothOperator
This is how it was explained to me, and it made perfect sense...

Picture a figure skater....spinning in the same spot. When the skater sticks her arms out, the spin slows down. When she brings her arms in, the begins spinning faster.

These same, simple rules of physics apply to your wheel size. Move the rotational weight out, and you'll be slower. Bring it in, and your wheel spin speeds up.

hope that helps

yeah, but if she only stuck her index finger out on both hands, what would happen??? arms are big and heavy and stretch out several feet. the only differences on the wheels and tires are a couple pounds and inches...

[SmoothOperator]

yeah, but if she only stuck her index finger out on both hands, what would happen???
Those "pounds and inches" make a difference. And I was using the "skaters arms" as a reference to help him understand.
I'll play along regardless.....If she just stuck her index finger out....she WOULD slow down. Just not as much as if she put her arms out. Happy now?

[glw]

Originally posted by SmoothOperator
Those "pounds and inches" make a difference. And I was using the "skaters arms" as a reference to help him understand.
I'll play along regardless.....If she just stuck her index finger out....she WOULD slow down. Just not as much as if she put her arms out. Happy now? quite happy (but she wouldn't slow down much more). i just think a lot of people talk size and forget about it's really the weight thats the problem. some people will go out and buy heavy cast 15" wheels when a set of light forged 16" or 17" wheels would actually be much better performing. there are other things to consider as well. tire tread width, tread pattern, compound, tire weight, sidewall height and construction are some of them. buying a good wheel and putting crappy tires on them is probably worse than buying taller wheels and great tires...

i'm not arguing that a taller heavier forged wheel is better than a shorter lighter forged wheel - but i might argue that a real life comparison of some 12lb forged 17" volk ce28n's with some 16lb cast 15" wheels would prove that weight and tire play just as large (or larger) part in the decision...

oh... and the weight of the tire is about the same between the various sizes (15"-17") and it is actually further out than the wheel. i'm sure this adds to (or subtracts from) the equation as well.

a good choice also depends on what it is you are doing - drags, road racing, autox, street-bling, etc...

"it's not all about size..." :eek: :confused: :angel:

[HokieSi]

Originally posted by SmoothOperator
This is how it was explained to me, and it made perfect sense...

Picture a figure skater....spinning in the same spot. When the skater sticks her arms out, the spin slows down. When she brings her arms in, the begins spinning faster.

These same, simple rules of physics apply to your wheel size. Move the rotational weight out, and you'll be slower. Bring it in, and your wheel spin speeds up.

hope that helps

This is correct. Here are some other things to think about.

Assumption: The drivetrain is constant.
Variables: The wheel diameter, weight and distribution of weight

Thats its partially true for tire size. It is best to have the weight of an object near the center of rotation. (ie if you roll two cylinders down a ramp, one with a hollow center and one with a solid center both having the same mass and diameter the solid one would get to the bottom faster.

We can generalize this by saying:
Low profile/skinny tires help reduce the outer rotational mass.

The biggest factors in spinning a wheel are weight, diameter and distribution of weight (less outer rotational mass, skinnier tires help).

Now if you have a wheel that is great for acceleration it probably sucks friction-wise. Thats where the trade-offs begin.

Things to remember
1. Reduce weight and diameter of wheels
2. Reduce the outer rotational mass (get skinny / low profile tires).
3. You need friction (grip) to accelerate
4. There will be a trade off between your chosen small wheel/tire combo and friction needed to accelerate.

[Si-Sissy-PA]

The *Real Magic* is in how if you don't calibrate your speedometer when you have smaller outside diameter wheels/tires, your dashboard tells you you're going faster :) and this is almost as magical as the extra 5hp a Mugen sticker gives you!

[ImPo Skyline]

well, unless you'd like to add some 10" daytons to your civic i'd say that a larger tire than your stock 15" with 60's would make your speedometer read slower. for instance, i have 18's with 40's im on the freeway next to my friend with a stock civic dx. His speedo will read that were going 70 (we are going 70) but mine because of my wheel size will read 65.

[Dunrick]

18's w/ 40 sidewall wont be 5 mph off....


I could be wrong, but it should be more like 1-2 mph 2 being the most.

[HokieSi]

Its pretty easy to figure out how much the tire will be off. All you need to know is the difference of the tire diameter. That will give you the percentage it will be off.

S = (Old Size - New Size) / (Old Size)
S is the Percentage difference increase or decrease.

If you want to know what your speedo will be reading at this new speed do this.

T = Amt the speedo is off
T = S*(Mph)

If you go out and measure it, there will be a very small error because the tire diameter is effected by rotation and psi.

So those numbers are approximatly how much you will be off.

If your total tire diameter is 24 inches then you would have to increase the overall diameter to about 1.7 inches to be reading 65 at 70 mph.

[kuhlka]

eh, according to your equations...

(15 - 17) / 15 = -0.13333

-0.1333 x 65 = -8.6666mph

hmmm. That isn't a small change when it comes to getting pulled over for speeding. That would mean with 17" rims I'd show I'm going 56 when I'm actually going 65. I know and understand all this already because my father and I put some 18" wheels on a mustang that previously had 16" and the speedo was waaaay off.

[Tenacious G]

Originally posted by kuhlka
eh, according to your equations...

(15 - 17) / 15 = -0.13333

-0.1333 x 65 = -8.6666mph

hmmm. That isn't a small change when it comes to getting pulled over for speeding. That would mean with 17" rims I'd show I'm going 56 when I'm actually going 65. I know and understand all this already because my father and I put some 18" wheels on a mustang that previously had 16" and the speedo was waaaay off.

it's not just wheel size you have to factor in. the diameter also will be affected by the tire. the total diameter you should be looking at is with the tire, not just the wheel.

[ImPo Skyline]

Originally posted by Dunrick
18's w/ 40 sidewall wont be 5 mph off....


I could be wrong, but it should be more like 1-2 mph 2 being the most. I was just trying to prove my point, but ya had to bust my balls. Your right its not 5mph off, it is about 3.5 at most. Sence its an analog readout for the speedometer in our civics and not digital, the opinion of the user will very.

[Dunrick]

This thread was already answered, and ontop of that, all the tire/wheel diameter questions can be answered if somebody is willing to drop the link to the wheel/tire diameter java thing. I dont remember it, but if you search, you can find it.

[kuhlka]

Yeah Tenacious, I was assuming the same basic tire. Even then, if you add the tire to both the wheel numbers I put into the equation, there is still a pretty big difference there.

Being someone who has gotten tickets for this kind of thing in the past, make sure you get that speedo set up right before driving around on different tires before you see the good old red and blue flashies in your rear view.

Also, to answer the initial question in this post; Anyone wanting to know this kind of information should take a basic physics class if they're still in high school or college. You'll learn quite a bit about how the 'physical' world works, especially if you stay after class and ask your professor questions about specific things like this question.

[chubbychu]

Originally posted by Dunrick
This thread was already answered, and ontop of that, all the tire/wheel diameter questions can be answered if somebody is willing to drop the link to the wheel/tire diameter java thing. I dont remember it, but if you search, you can find it.

http://www.miata.net/garage/tirecalc.html

[pedzola]

Heres the one I use:

http://www.1010tires.com/tiresizecalculator.asp

[Tenacious G]

Originally posted by kuhlka
Yeah Tenacious, I was assuming the same basic tire. Even then, if you add the tire to both the wheel numbers I put into the equation, there is still a pretty big difference there.

yeah, if you kept a 60-series tire while upgrading from a 15" wheel to a 17" wheel, you'll see a huge discrepency in speedometer readings. but in most instances, a lower profile tire is used when upgrading to a larger diameter wheel to help offset the diameter difference.

but you're right, you will see variation of a couple mph on the speedometer when messing with tire/wheel diameters. the sites listed above offer helpful info.

[HokieSi]

Originally posted by Tenacious G
yeah, if you kept a 60-series tire while upgrading from a 15" wheel to a 17" wheel, you'll see a huge discrepency in speedometer readings. but in most instances, a lower profile tire is used when upgrading to a larger diameter wheel to help offset the diameter difference.

but you're right, you will see variation of a couple mph on the speedometer when messing with tire/wheel diameters. the sites listed above offer helpful info.


The faster you go the more variation you will see also.

[BarracksSi]

I can't believe I have to post this...

http://www.miata.net/garage/tirecalc.html

If you put 205/40 tires on 18" rims, you're only going to be fast by 1%, which is 0.65 mph at 65 (thus, an indicated 65 mph would be a true 65.65).

In other words, you can bling-bling as much as you want without worrying about speedometer errors IF you get the right tire size.

In drag racing, friction -- on the drive wheels -- is your FRIEND. The tires have to push against the road to move the car. How else are you going to go further than three feet?

Bigass drag slicks have huge sidewalls for lower rotational inertia and for flexibility. When a car's throwing a thousand horsepower to the ground, it's going to squish a flexy tire even more, giving a bigger contact patch -- which means more friction, which means that more power is transferred to going forwards.

But, that's going to slow down the engine if it were going on a long high-speed lakebed run (not to mention the aerodynamic penalty). Change the wheels to some skinny ones, make the whole thing ultra-aerodynamic, and the flying mile will go by in a hurry.

Track cars (or, cars that turn during a race) have large rims to allow room for bigass brakes. Sure, accelerating is great, but worthless if you have to use your brakes earlier than the guy next to you (who's got 14" carbon discs) and he beats you into a turn.

Want faster drag times? Get the biggest tires you can fit into your wheel wells on the smallest rims that fit over your brakes. Want faster track times? Get brakes as big as you can afford, then put on the smallest wheels that still fit. Want a fast salt flats time? Strap on a rocket motor.

Of course, all those scenarios figure that you've tuned the motor to within an inch of exploding. If you don't have a lot of power, maybe you can't push a big racing slick or large-diameter rims. That's where the experimentation comes in handy.

[BarracksSi]

Originally posted by BarracksSi
I can't believe I have to post this...



I also can't believe how frustrated I sounded! lol... I need some sleep...