Braking distance increases with size of rim ie 26" vs.

[jz04r1]

I'm trying to convince my friend that it's true but I haven't been able to find any articles through google. Can anyone help me or prove me wrong??? I think Car and Driver had a article about it but I don't have that mag anymore. Thanks in advance :D

[Vansquish]

I can prove it from a theoretical standpoint, but not with actual test results. Basically, as you increase the size of the rim, the moment of inertia of the wheel grows. This moment of inertia is the wheel's resistance to movement. In other words, if you have a very large wheel, the moment of inertia will be large and it will be hard to get it rolling in the first place, then, when you want to stop, it will have more angular momentum and will be more resistant to stopping than a smaller wheel.

What all this means is that the larger the wheels on your car, the larger their moment of inertia, and the harder it will be to get them rolling, or to stop them. So...if you fit large wheels to your car, you're going to increase your stopping distances, sometimes by a significant margin (esp. in the case of fitting 26" wheels to something that might've only had 17"'s to begin with).

[gigdy]

Go freshman physucks.

[Vansquish]

lol...definetely stuff from freshman year and HS....at this point we're talking quantum mechanics, tensor calculus and string theory.

[graywolf624]

Inertia depends on object but always involves: mass * squared(radius) In case of a wheel shaped object: 1/2 m squared(r)
Torque= inertia * angular accelleration
torque=1/2 mass * squared(radius) * angular acceleration

Thus larger radius and maybe even addition larger mass of those larger wheels = increased inertia which means for the same torque less angular acceleration occurs.

For a similar reason smaller and larger tire/wheel ratios have a large effect on gearing and hense also top speed, acceleration, ect.

[jz04r1]

cool thanks for the help

[Vansquish]

OK...we have a little bit of confusion still going here. Graywolf, the moment of inertia is more or less as you have described it except for your specific case. In fact, a wheel is a combination of the characteristics of a hollow cylinder, and a series of rods rotated about their midpoints, or if you prefer, something partway between a hollow cylinder and a solid disk. In any case, I=(1/2)mr^2 is not an accurate depiction of what happens with a car's wheel, as that is the solid cylinder case.

If one really wanted to calculate the moment of inertia of a car's wheel, it would be a sum of the different parts, i.e. for a simple 6 spoke wheel with solid spokes and a solid rim, the moment of inertia would be something like:

I = I(spokes) + I(rim) = 6*(1/3)ML^2 + mr^2 where L is the length of a single spoke (from the hub out to the rim of the wheel), M is the mass of a single spoke, r is the radius of the cylindrical part of the wheel, and m is the mass of the cylindrical bit. Then of course you have to add the tire onto that, and that comes to another term of the form, mr^2 but with different masses and different radii. Of course this is still quite inaccurate, as any wheel you might find on a car is a much more complicated shape than this simple depiction. If you really wanted to calculate the moment of inertia from a theoretical standpoint, you'd have to consider the hub, the bearings, the driveshaft, the reciprocating masses of the engine, the brake disk and so on and so forth.

As for torque being simply inertia times the angular accel, that's also not entirely accurate. The sum of all torques present is the product of the angular acceleration and the moment of inertia. Thus, for the case of a wheel on the road, you have to account for the torque applied by the coefficient of static friction, as well as that of the brakes, driveshafts, reciprocating masses of the engine etc...etc...etc...

[graywolf624]

Im well aware. but I think your going over the audiences head if you get more technical then I did. When presenting this particular concept to a class in freshmen physics or likewise.. you need to factor out the extra shit to simplify. Esepeciallly in the case that not all wheels have spokes.. some literally are a solid cylinder with holes cut in the middle of moderate size. Obviously making the equation even more complicated..
Same with the friction and other forces involved.

In other words.. keep it simple when your explaining it. Otherwise I could write you a 50 page paper.. including the suspension influence from the new ic height. Theres a reason engineers use modeling software.

Assume a basic cylinder wheel of solid mass distribution and all other things being equal. Thats the way it was actually worded on my physics 1 final in college. Otherwise I have to write out math after finding out the specific makeup of his wheels, tires, suspension design, ect.

[Vansquish]

Im well aware. but I think your going over the audiences head if you get more technical then I did. When presenting this particular concept to a class in freshmen physics or likewise.. you need to factor out the extra shit to simplify. Esepeciallly in the case that not all wheels have spokes.. some literally are a solid cylinder with holes cut in the middle of moderate size. Obviously making the equation even more complicated..
Same with the friction and other forces involved.

In other words.. keep it simple when your explaining it. Otherwise I could write you a 50 page paper.. including the suspension influence from the new ic height. Theres a reason engineers use modeling software.

Assume a basic cylinder wheel of solid mass distribution and all other things being equal. Thats the way it was actually worded on my physics 1 final in college. Otherwise I have to write out math after finding out the specific makeup of his wheels, tires, suspension design, ect.

If you look at my original post on this page I think you'll find I did explain it in the simplest terms possible, using no equations and no attempt at a mathematical explanation. The slightly more in-depth discussion was directed more at you and the other people here on JW with a higher understanding of the basic physics involved with answering the question posed. Since the explanation you gave was somewhat flawed even at the level at which you wrote it seemed logical to make things right.

[graywolf624]

didnt read it .. sorry..

But yeah.

[5vz-fe]

Braking distance increase (also leads to higher brake wear)
Acceleration decrease (also leads to less gas mileage)
ride comfort decreases
cornering ability decreases

That's all I can think of.

[DeMoN]

As far as I know, the bigger the rim, the bigger the wheel = more grip... but thats the end of story. Worst accelleration, decelleration etc.

[antonioledesma]

at last... you were taking this to a "technical" level for an untrained mind :wink:

now, back in the real world, just checking out some raw numbers without a calculator and no more than your explanations (had those classes years ago, so I don't remember nothing :? )... the braking distance that will increase is almost meaningless; doesn't it?
those are the kind of calculations that you consider zero or simply don't care.

this seems like my car's gas mileage of I consider I use a goatee :mrgreen: "oh shit, I lost $1 in this year"

[SFDMALEX]

As far as I know, the bigger the rim, the bigger the wheel = more grip... but thats the end of story. Worst accelleration, decelleration etc.

The wider the rim the more grip, not bigger as in radius/diameter.

[T-Bird]

OK we actually tested this in my brakes class over the summer and there is a significant difference in braking distance between a 16" rim and a 19"/20" combo without upgraded brakes afterward he did add larger brakes though. And yes width makes grip but height can also but not nearly aswell as width.

[graywolf624]

As far as I know, the bigger the rim, the bigger the wheel = more grip... but thats the end of story.
Not really true. Wider wheels = larger foot print. Larger diameter wheels do not equal bigger footprint. Instead you get a change in the shape of the footprint. (perhaps a slight increase in foot print.. but thats mainly a minor side effect. It isnt proportional as in a width increase, but it does exist since a tire is not a rigid structure. Read my article on tires posted here some time ago for more info.
Thats why in drag cars (non top fuel) the rule is generally small diameter and as wide as possible.


)... the braking distance that will increase is almost meaningless; doesn't it?
those are the kind of calculations that you consider zero or simply don't care.
They are more significant then you think.
You can use this tool http://www.csgnetwork.com/multirpmcalc.html
to calculate the difference youll see in rpm at a given mph.

There are only 3 real reasons to go with larger diameter wheels..of varying degrees of importance.
1) clearance for larger brakes.
2) looks
3) higher top speed all other factors equal.

[antonioledesma]

^^ thanks a lot for the link!! needed one some months ago, but not now :wink:

and yes, they are more significant than I thought, of course I would not consider them, but you, vansquish and zrfk are the know-all for cars and can consider this factors

[5vz-fe]

3) higher top speed all other factors equal.

I don't think u will have a higer top speed......(conservation of energy)

[sentra_dude]

3) higher top speed all other factors equal.

I don't think u will have a higer top speed......(conservation of energy)

lol...its called having a larger circumference, in effect changing your gearing. Why do you think you have to re-calibrate your speedo when you change wheel size? For each revolution of a larger wheel you go farther...

[Vansquish]

3) higher top speed all other factors equal.

I don't think u will have a higer top speed......(conservation of energy)

lol...its called having a larger circumference, in effect changing your gearing. Why do you think you have to re-calibrate your speedo when you change wheel size? For each revolution of a larger wheel you go farther...

Right...but you're neglecting the higher moment of inertia of the wheel and the fact that it actually takes more energy to get it rolling. All in all unless you manage to increase the diameter of the wheel without increasing the moment of inertia of the wheel (i.e. by using a lighter-weight material or redistributing the masses of the wheel so that they lie closer to the hub) then you will in fact lower the top speed of the car. That it takes more energy to move an object with a larger moment of inertia is I think what 5vz-fe was trying to say. Indeed this is the case.

On a slightly different note, one of the other things you have to include when doing top-speed calculations is the fact that there is a fair amount of tire-growth depending on how fast you're moving. In the Mclaren F1, it accounted for something like 5-10mph total in their calculations (simulations showed it would probably max out at 225-230mph, but we know it actually can go faster)

[5vz-fe]

Thanks for typing all that out for me Vanquish.

On a side note, as long as u manage to pull down the same (wheel) horsepower with the larger wheel on a dyno, it will provide u with the SAME top speed. (As Vanquish, higher inertia is a mean of power loss thus it will show up on dyno as part of ur drivetrain loss). Of course, all this is under the assumption that the car is not having a redline limited topspeed.

[23790554]

wouldn;'t bigger wheels be more stable at higher speed?

bigger wheels is also more sexy

[Iwan]

bigger wheels is also more sexy

don't know if i can agree to this if you look at those freaks here (http://www.thewolfweb.com/message_topic.aspx?topic=252943&page=1).

i think 19/20 inch for 'normal' cars is the maximum - but i remember
back to the early 90's. in those days, 17 inch was the 'maximum'.....


cheers,
iwan