Big Wheels vs Computer????????

roberto · #1 02-14-2008, 07:39 PM

I have recently picked up a X5 with 22" wheels yeeehaa --- now how will I know if the computer knows this change has happened as the bigger wheels will increase the Distance per revolution - therefore the trip computer will be in-accurate!!!

Helpppppppp!!!!!!

toohotdoc · #2 02-14-2008, 08:23 PM

Well what tire size do you have, its all about what tire size you have.

vegasX5 · #3 02-14-2008, 08:37 PM

It shouldn't change unless your overall rotating mass has increased (which may be likely considering how much larger the rims are). You obviously need a much thinner sidewall to compensate for the increased diameter of the wheels. What size tires are on the wheels?

lemans4.8 · #4 02-14-2008, 08:45 PM

increasing the wheel size will not affect the computer, but the sidewall size will! But it's a very minor difference!

BMWLOVER · #5 02-14-2008, 10:38 PM

so if i wanted to go from 19" wheells to 20". would it make a diference. and what do you mean by side wall?

FSETH · #6 02-14-2008, 11:09 PM

These posts are right on. You need to determine the overall height of the wheel with the tire on it and compare that to stock. I don't think this is something that a computer will pick up on. You may have to get your speedo adjusted if their is a big difference between the height of the 22's with tires and the original wheel/tire package

Sidewall height is the height of the tire off of the rim. Basically the part of the tire you see when looking at a car from the side that has all the writing on it.

blueagle · #7 02-14-2008, 11:45 PM

roberto,

Show us some pics of your X5 with 22 " rims

lemans4.8 · #8 02-15-2008, 12:14 AM

ok guys, just talked to my friend who is a tech at bmw.... apparently, the computer pics up the "info" from somehwere near the hub area (forgot the actual name) but it's close to the tpms in the E53 so going from 17's to 24's would not make any difference to the computer.... just the quality of the ride and height!!

FSETH · #9 02-15-2008, 12:25 AM

Quote:

Originally Posted by lemans4.8

ok guys, just talked to my friend who is a tech at bmw.... apparently, the computer pics up the "info" from somehwere near the hub area (forgot the actual name) but it's close to the tpms in the E53 so going from 17's to 24's would not make any difference to the computer.... just the quality of the ride and height!!

Wow! I would have never guessed that.

Actually, the more I think about it how can that be true? The overall size of the wheel with tire is going to affect the speed. The computer would have to compensate for overall wheel & tire size. How can that be possible? How can it be done without the computer knowing the overall size of the wheel/tire?

TurnAround · #10 02-15-2008, 01:07 AM

Quote:

Originally Posted by FSETH

The overall size of the wheel with tire is going to affect the speed. The computer would have to compensate for overall wheel & tire size. How can that be possible? How can it be done without the computer knowing the overall size of the wheel/tire?

This is a fun little problem isn't it. Cool.

Here's what I think it is; The wheel sensor works just like a bicycle speedo computer works. You can mount the sensor to the bike frame anywhere up and down the radius of the wheel. You then mount a small magnet on the the wheel's spoke and line it up with the sensor on the frame so the magnet passes by it. It is irrelevant where you mount the sensor along the radius of the circle because it takes the same amount of time for the magnet to fly by regardless of where it is.

Close in to the hub, it flys by slowly. Far out, it flys by very fast. But the TIME it took to fly by is exactly the same. To calculate speed from this you need an equation that also feeds off of the wheel diameter. Once you have that, you have the speed at all times... with "magnet fly by TIME" being the only variable in the equation.

So the answer is, the wheel sensor and magnet on our cars isn't the focus of the equation. Its the hard coded wheel size number that BMW burned into the firmware chip. If you physically change the wheel's outside diameter to a larger size, and have no way to tell the computer this new number, the car speedo will still report 55mph, but the wheel diameter moves the car along physically faster.

As to the original poster's wheel size question, everyone nailed it. When you up your wheel size, you buy a tire with a shorter side wall to compensate, and you end up with the same overall outside rolling diameter, plus or minus a 1/4" with metric sized tires.

If you deliberately do not follow the tire calculator's suggestion on a shorter side wall size and manage to find a tire with the same sidewall size, then yeah.. yer overall rolling diameter will be a lot bigger.

Hence, the muscle car look of the 60's for the rear wheels / tires.