Im no expert, but the xB can only fot what a 17'?
i want to save up and put like some crazy 25's on it hahahahaa! :twisted: :twisted:

 

25's are not a practical size. the xB can fit a nice 18" or 19" rim. if can hold a 20", but that is not practical also.

 

personally I think anything over 18 inches is way to much rolling mass for this type of car....your really screwing up braking with those bigger rims

 

Please explain if you are for real. What is the difference, if any between a 215/45r17 and a 215/35r19. You stated above that anything over 18" has too much rolling mass. Doesn't rolling mass have to do with overall height and weight? Don't wafflle and disappear from this forum and pretend you don't follow up on your own posts'. Thank you for your reply!! Dubbox

 

Rolling mass deals with anything pertaining to the wheels tires and breaks....as you go to a larger rim it which tends to be heavier it takes alot more breaking power to stop the automobile that is using that large rim....and also accelerating it takes power to get your car rolling which this car doesnt have to get adequate acceleration off the line....so thats why talking to someone that deals with large rims they will always recommend upgrading to large rotors to counteract the crappier breaking distance

 

1. spelling

breaks=brakes
it which=it
breaking=braking

2. salespeople

if you are buying big rims, an easy add on sale would be big brakes especially, if the person being sold is ignorant. They do fill the wheels better, but mainly makes the salesperson more money.

3. engineering

rolling mass is something important on BMX bikes and motorcycles where the wheel weight represents a significant part of the overall vehicle weight. Not a negligible factor on a 100hp 2400lb vehicle. Also take into consideration stock steel wheels weigh more than larger aftermarket alloys

4. proof

something you did not provide to back up your opinions.

I decided to refresh my Physics and came up with the following conclusion: from the point of view of acceleration, an increase of X in wheel or tire weight is no worse than an increase of 2X in passenger weight. Not 6x, not 8x, just 2x worst case. This is why.

At any given speed/gear combination there is maximum torque T available at the wheels. The torque does two things: (1) opposes the rolling resistance and the aerodynamic drag and (2) accelerates the car. The equation that describes the equilibrium is:

T = I*u + M*r*a + D*r

*: denotes multiplication
T: torque at the wheels
I: total moment of inertia of the rotating parts
u: angular accelaration of the rotating parts
M: total mass of the vehicle
r: external radius of the tires
a: linear acceleration
D: total drag and rolling resistance

The first term, I*u, is the torque used in making the wheels rotate faster. The second, M*r*a, is the torque used to accelerate everything (wheels included) in the direction of movement. The third is the torque used to cancel the drag and rolling resistance.

Another way of writing the equation above is:

T/r = I*u/r + M*a + D

Now the terms are forces. The left side is the force available at the contact patch.

If the tires are not slipping, the angular acceleration and the linear acceleration are related in the following way:

a = u*r

Replacing in the equation and moving things around, we get

T/r - D = (I/r^2 + M)*a

^: denotes exponentiation (r^2 means "r squared")

We can say that the left side is the force available for acceleration. Such force accelerates an "non-rotating equivalent mass" E,

E = I/r^2 + M

Now suppose that we increase the weight of the wheels and tires by an amount X. Both the total mass and the moment of inertia will increase; let's call the new mass M' and the new moment of inertia I'. Obviously,

M' = M + X

What about I'? Well, the moment of inertia of a "punctual mass" [m] (a mass concentrated in a point) at a distance [r] from the axis of rotation is [m*r^2]. That is, the moment of inertia depends critically on the distance between the mass and the axis of rotation.

In a real wheel + tire combination the mass is distributed in different amounts at different distances from the center. In order to compute the total moment of inertia we would need to know the mass distribution and use integral calculus. We can do a simpler thing though. We can make the pessimistic assumption that all of the mass increment is located on the periphery of the tire, that is, at a distance [r] from the center. This assumption is pessimistic because in a real wheel some of the weight will be located closer that [r] and will contribute less to the total momentum (it is not too pessimistic though: most of the weight is located pretty far from the center, if not at the periphery). So now we can compute I',

I' = I + X*r^2

The new "equivalent mass" is,

E' = I'/r^2 + M' = I/r^2 + M + 2*X

In other words, from the acceleration point of view, the equivalent non-rotating mass increment corresponding to an increment X in rotating mass is - at worst - 2X.

NOTE: After doing some approximations and assumptions about weight distribution in a typical wheel + tire combo, I believe that 1.7X is a better approximation. A 10lb/wheel weight increase would not hurt acceleration worse than carrying RinTinTin.

Comments anyone?

 

holy crap ... ... my brain glazed over about halfway through that.

 

Sorry,

Did not mean to glaze over any brains..... Just hate to see ignorance spread like wildfire. I was in rare form last night after being up 48 hours straight working :oops: .

 

Although your grasp of mathematics is VERY impressive, my non scientific butt dyno disagrees with you. I am not straight out calling bull____ on you or looking for a fight. I dont sell upgraded brake packages so I don't have alterior motives. I just disagree!!

 

damn, feel like i just took a math course. i follow it though, i get what he's saying about it and it makes sense.

 

I thought trd was for toyota racing division ...

apparently it refers to Torque divided by Radius minus Drag!

good post lavabox ...

 

thanks for getting my back tbblizzard. been out of school a long time now, but math never changes.

 

i have seen first hand a dyno testing out this rotational mass exercise.

late model ford mustang 4.6l, did base line runs.

went +2 on the wheels and BBK redid the dyno runs

overall it lost 2.1 hp and 2.3 lb/ft

now in my book thats not a lot to lose sleep over.

 

I'll tread lightly through this thread... but my understanding is that so long as the weight/mass does not increase, the larger wheel will not degrade performance in a measurable way. So if the stock steel wheels are swapped for larger yet lighter wheels the end result should be negligable. Or you could get some 13" alloys and make that car rock with all the extra power!

 

Ok then, If it has been proven that acceleration is'nt considerably affected, how about braking distances??

 

Lavabox, great analysis! Glad to see that there are folks here who actually think about how things work!

I'd say that the effect of the wheel mass is closer to 1X than 2X. since the moment of inertia of 1 kg of mass about the wheel axis is exactly the same as the moment of intertial of 1KG of body mass about the instantaneous pivot of the tire/ground interface. The radii are the same, so the moments of inertia are the same as well.

Unless you markedly change the rolling radius of the wheel, there will be little effect on acceleration or braking. However, going to wheels that would require body modifications implies that the rolling radius will be changed. If somebody's bucks-up enough to handle the extensive body mods, a brake and final drive change won't distress that person too much.

 

Oh yeah. First off if you want to disagree with someone's post, then disagree and post up some facts to back your statement up. There is no reason to make fun of his spelling or anything else for that matter. Normally I would watch and laugh, but he happens to be joining my club, so I decided to step in and put my 2 cents in.

First, you posted "spelling" and pointed out all of his mistakes, why? I don't know, maybe you're insecure or something. BUT "it which" is NOT a spelling mistake, it is a grammar mistake, he didn't spell either word, he just used them wrong. So before you go into trying to sound like a Rhodes Scholar with everyone, check yourself.

Second, almost his entire post was pertaining to stopping power when going to a bigger rim. Which is true, when you step up to bigger rims and tires you aren't required to upgrade your brakes, BUT it is a very smart thing to do. He posted 1 sentence in his entire post about acceleration and that caused you to "explain" it with a post that was longer than the Bible. Also, it looked like a cut and paste that you threw together.

Third, sales people will make a suggestion on brakes when someone buys larger rims and tires. Sure they get more money, but as I said before it is a valid point. They don't pull this ____ out of the air, maybe if you went in somewhere and bought new rims and tires and they suggested adding a new seat cover or an air freshener, then you would have a point. When going with bigger rims and tires you almost always gain added weight, added weight spinning around is harder to stop with the same brakes on a car. Upgrading brakes with larger/heavier rims and tires is a universal rule, everyone knows this. When you upgrade your brakes it shortens stopping distance, when you add weight to a vehicle or to the wheels it lengthens stopping distance. By upgrading you are balancing out the equation.

Fourth, I'm glad to see you have a groupie, maybe you 2 can talk math.

Peace,

-Z

 

I've seen an xB with 20' rims on it and as cool as it looked, I really would'nt see anyone actually driving it on the road. But it looked cool non-the-less.

 

E=mc^2

There!

 

A stock wheel weights about 17.5 ibs, now you can compair the weight to your aftermarket wheels... When I was switching from wheel to wheel, and i can definately tell a difference in acceleration from a stock wheel to a cast wheel to a forged wheel. I also know big brakes on a scion kills the acceleration. Talk to anyone with a big brake set up on their scion, i can assure you that they will tell you there is a difference in acceleration but does help in braking.