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

[JCL]

Originally Posted by TurnAround

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.

What you are referring to as 'fly by time' is not quite right. The time does vary with wheel speed, as the sensor is moving faster. However, the sensor is simply emitting a pulse, and the computer is counting pulses. Integrated over time, ie counting pulses in a given time, you get pulses/second. You are correct that there is a calculation in the computer to convert pulses (which are a constant per one wheel revolution) to a distance, as all the optional tires have the same rolling circumference. Yes, this is the same as a bicycle computer in concept.

[lemans4.8]

It works by the number of revolutions the hub makes! DOesn't have to do with tire size! (according to BMW tech friend) A lot of busses now come with a counter on the outside hub of the wheel. It measures how many miles are on that set (2 tires per side axal, so total 4 tires per axal) based on how many revolutions the device makes!!

[vegasX5]

Originally Posted by lemans4.8

It works by the number of revolutions the hub makes! DOesn't have to do with tire size! (according to BMW tech friend)

If it counts the number of revolutions of the hub, then think about an extreme example. If it were possible to put 12" rims with low profile tires on the X5, wouldn't the lower rotating diameter cause a higher number of revolutions at a given speed? At that same given speed, a 26" rim with large tires would cause a lower number of revolutions, right?

Now, of course the difference between an 18" wheel with higher profile tires and a 20-22" rim with low profile tires isn't nearly that extreme, but the principle is the same. Even if it's a minute difference, the rolling diameter does change when you change your wheel/tire setup.

I increased my overall rolling diameter by a little more than an inch (1.25") when I put on the RR wheels, and I noticed a lot of difference in terms of handling, ride, acceleration, deceleration, fuel economy, stabilty, traction, basically everything. Part of the equation for me however is the increase of unsprung weight with the new wheels and slightly different offset, but it changed everything with my X. I imagine that it affected the computer/odo/speedo. For example - there's a speed sign right by my house that has a radar built into the unit to tell your speed. The speed limit is 45mph so it is high enough to expose a little inaccuracy in a speedometer. When I had the 18" factory wheels on, I would pass by showing 46-47mph on my cluster in the X. With the 19" wheels, it shows spot on 45mph on the cluster and on the radar. Now I know this entirely unscientific, but it does reinforce my opinion enough for me to believe it. Until I see a schematic of the device that shows exactly how the mph and/or mileage is calculated on the X, I'll continue to think that increasing your wheel/tire size DOES affect the computer and it DOES NOT automatically adjust to compensate.

[FSETH]

Originally Posted by vegasX5

If it counts the number of revolutions of the hub, then think about an extreme example. If it were possible to put 12" rims with low profile tires on the X5, wouldn't the lower rotating diameter cause a higher number of revolutions at a given speed? At that same given speed, a 26" rim with large tires would cause a lower number of revolutions, right?

Now, of course the difference between an 18" wheel with higher profile tires and a 20-22" rim with low profile tires isn't nearly that extreme, but the principle is the same. Even if it's a minute difference, the rolling diameter does change when you change your wheel/tire setup.

I increased my overall rolling diameter by a little more than an inch (1.25") when I put on the RR wheels, and I noticed a lot of difference in terms of handling, ride, acceleration, deceleration, fuel economy, stabilty, traction, basically everything. Part of the equation for me however is the increase of unsprung weight with the new wheels and slightly different offset, but it changed everything with my X. I imagine that it affected the computer/odo/speedo. For example - there's a speed sign right by my house that has a radar built into the unit to tell your speed. The speed limit is 45mph so it is high enough to expose a little inaccuracy in a speedometer. When I had the 18" factory wheels on, I would pass by showing 46-47mph on my cluster in the X. With the 19" wheels, it shows spot on 45mph on the cluster and on the radar. Now I know this entirely unscientific, but it does reinforce my opinion enough for me to believe it. Until I see a schematic of the device that shows exactly how the mph and/or mileage is calculated on the X, I'll continue to think that increasing your wheel/tire size DOES affect the computer and it DOES NOT automatically adjust to compensate.

I have to agree with you vegas. If you had 2 wheels with hub speeds of 50 mph, tire 1 is a 20" rim with a tire sidewall measurement of 35 and tire 2 is a 17" rim with a tire sidewall measurement of 35 the overall diameter of the 20" rim and tire is larger and would cover more ground, making the car a little faster than the car with the 17" rim.

I am starting to feel like I am taking the SAT's all over again.

[lemans4.8]

Originally Posted by vegasX5

If it counts the number of revolutions of the hub, then think about an extreme example. If it were possible to put 12" rims with low profile tires on the X5, wouldn't the lower rotating diameter cause a higher number of revolutions at a given speed? At that same given speed, a 26" rim with large tires would cause a lower number of revolutions, right?

Now, of course the difference between an 18" wheel with higher profile tires and a 20-22" rim with low profile tires isn't nearly that extreme, but the principle is the same. Even if it's a minute difference, the rolling diameter does change when you change your wheel/tire setup.

I increased my overall rolling diameter by a little more than an inch (1.25") when I put on the RR wheels, and I noticed a lot of difference in terms of handling, ride, acceleration, deceleration, fuel economy, stabilty, traction, basically everything. Part of the equation for me however is the increase of unsprung weight with the new wheels and slightly different offset, but it changed everything with my X. I imagine that it affected the computer/odo/speedo. For example - there's a speed sign right by my house that has a radar built into the unit to tell your speed. The speed limit is 45mph so it is high enough to expose a little inaccuracy in a speedometer. When I had the 18" factory wheels on, I would pass by showing 46-47mph on my cluster in the X. With the 19" wheels, it shows spot on 45mph on the cluster and on the radar. Now I know this entirely unscientific, but it does reinforce my opinion enough for me to believe it. Until I see a schematic of the device that shows exactly how the mph and/or mileage is calculated on the X, I'll continue to think that increasing your wheel/tire size DOES affect the computer and it DOES NOT automatically adjust to compensate.

I'm not doubting you, but it is nearly impossible to get the same radar speed and speedometer speed (assuming you have a North American car!) Established by the USDoT, speedometers are required to have a ~5% mph inaccuracy. So if the speedometer said you were going 80mph, you were really doing 76mph. A lot of the people over on M3board have set up their nav to accurately measure their land speed using GPS! And sadly, i've proven this in 3 different cars..... I was pulled over on three different ocassions where I had cruise control set to 75 and a cop clocked me going 71 (in a 55) on a flat, even surface! But i'm not doubting you, i'm just stating the facts!!

On a different note, the size of the hub DOES NOT change! You don't replace the hub when you replace the wheel. An X5 will have the same size hub with 13 inch rims to 30+ inch rims. The hubs for all E53 X5's are the same size. The speed at which the hubs (and brakes) spin is determined by the rotational speed of the axle. Just because you got a bigger wheel doesn't mean that your axle is spinning at a different speed.

However, changing wheel size DOES change ride comfort and handling/traction. Acceleration and breaking are determined by several factors: Wheel and tire weight and the type/quality of the tires.

[FSETH]

On a different note, the size of the hub DOES NOT change! You don't replace the hub when you replace the wheel. An X5 will have the same size hub with 13 inch rims to 30+ inch rims. The hubs for all E53 X5's are the same size. The speed at which the hubs (and brakes) spin is determined by the rotational speed of the axle. Just because you got a bigger wheel doesn't mean that your axle is spinning at a different speed.

You are correct. The hub/axle speed will be the same as the speedo regardless of wheels sizes. That is the issue here. Vehicle speed is not necessarily the same as hub/axle speed once the overall diameter of the original wheels and tires are changed. Vehicle speed depends on the overall size of the wheels. Think about it for a minute, hub spped at 50 mph with a 10" wheel would not be the same vehicle speed as a 22" wheel. I don't see how a computer can accurately measure and compensate for different overall wheel and tire sizes unless it knew the exact overall diameter of the wheel and tire which would have to be programmed into the computer or sensors would have to be installed in the outermost part of the tire (the tread) to accurately determine that.

[JCL]

lemans4.8:

Maybe it is just me, but it seems that the issue is being confused instead of clarified. You started by relating rotating mass to this issue, when in fact that has nothing to do with it. Now you are talking about hub speed. The problem is that hub speed has nothing to do with it. You seem to be referring to the speed of the surface of the hub, which is completely irrelevant. What you need to think about is the rotating frequency. One rotation of the axle equals one rotation of the hub, equals one rotation of the wheel. They all turn at the same frequency.

The counter in the hub measures pulses, which relate to rotations. This is the info the sensor is picking up, nothing more. There is a constant factor for tire circumference. Leave aside all the wheel size and sidewall height issues; the only thing that matters is rolling circumference. One rotation (one pulse) multiplied by the constant factor for rolling circumference, equals vehicle speed.

Because of this, going from 17s to 24s does matter (since you note in your example that the height changes), as they won't have the same circumference. There is nothing that picks up tire size or wheel size, just a constant factor in the computer. It isn't complicated, it is in fact exactly the same as a bicycle computer (except the bicycle computer can be programmed for different wheels when you set it up).

Others are saying this, but it doesn't appear to be resolving the confusion. Maybe you could clarify what is not making sense.

[roberto]

Hi Guys - thanks for all the response - just got back to the forum after being away for a couple of days

I will take a pic of the wheels and yes I understand that the overall Wheel Diameter may be the same

(its night time here in Scotland just now) I'll post a pic of the 22" wheels with the lower profile tyres tomorrow ie reduced side-wall to the BMW standard therefore the overall circumference may well be the same - which , as someone has pointed out to me may mean the Trip computer will not need to be adjusted......

Can someone tell me what the standard delivery wheel size and tyre sizes are??? ---Thanks Robt

[TurnAround]

Originally Posted by JCL

lemans4.8:
The counter in the hub measures pulses, which relate to rotations. This is the info the sensor is picking up, nothing more. There is a constant factor for tire circumference. Leave aside all the wheel size and sidewall height issues; the only thing that matters is rolling circumference. One rotation (one pulse) multiplied by the constant factor for rolling circumference, equals vehicle speed.

Because of this, going from 17s to 24s does matter (since you note in your example that the height changes), as they won't have the same circumference. There is nothing that picks up tire size or wheel size, just a constant factor in the computer.

Bingo. That's what I think we're all saying; The computer is only aware of the one constant tire circumference that it was programmed with at the factory. Unlike a bicycle computer, which is designed to work with lots of different wheel sizes (or they couldn't sell it), a car's computer doesn't offer that as a user programmable parameter.

Ok, that's known. The car says 55 mph. But, you've got, let's say, much larger circumference tires instead of stock programming. The speed keeps saying 55 mph, but you're really traveling much faster because the larger tire covers more distance for the same frequency of hub ring pulses.

So in other words, we're all right.. nothing is changing about the computer or the sensor, but the real physical world has changed.. the dude has larger or smaller tire circumference, so their actual / real speed has now changed (faster or slower) it's just being reported incorrectly at the dashboard.

But the main thing for me was to work out in my head how the hub ring might work with the pulses and the calcs. This all makes sense to me now. In fact, the pulse system could be on the drive shaft and not a wheel and still produce the same feedback to the speedo calcs.

I did also just drink a beer, so this could be entirely wrong.

[TurnAround]

Originally Posted by roberto

Can someone tell me what the standard delivery wheel size and tyre sizes are??? ---Thanks Robt

It's different for each wheel size. 17, 18, 19, 20, 21, 22

Someone with your wheel size could report back the tire size for those wheels. The other way is to literally measure the diameter (top to bottom measurement) of the tire.

Here's a brief explanation of the tire size calc.

All tires are now metric. It didn't always used to be that way. US and Brit tires were in inches only. About 25 years ago the everyone standardized on a metric system.

I'm about to order a set of 19" wheels for my X5. That means I have to use the tire size recommended by the tire calc program on TireRack.com, which is basically the same tire calc program from any tire reseller. The end goal is the same --> For a given rim size, hit the same outside tire diameter as what the manufacturer of the car calls for. If you deviate, you deviate at your own desire. The tire resellers always quote tire sizes based on what your vehicle needs to hit as an OEM tire diameter. If you increase your rim size, they'll quote you a new tire size with a smaller tire sidewall size so that the overall diameter remains the same.

Now here's the thing with the system they've developed. It has to work with every car manufacturer's speedometer and with every wheel combination. They can't make size combinations in tenths of millimeters because it is just too cost ineffective. So instead, they average where they can. As a result, for any given car, front or rear wheel, you have a "close enough" calculation. THis... is why for example, if you look at the two tire sidewalls of my BMW Z3 Coupe, that the rear tire has a sidewall that is nearly 1/2" smaller than the front tire. Both rims are 17", but the tires have different diameters. This is due to the averaging effect and BMW just having to accept what is available in metric combinations.

So for my new X5 19" wheels I'll be getting these two tire sizes:
Front: 255/50 R19
Rear: 285/45 R19

The 19 refers to the 19" wheel. Both tires will fit over these rims. The first number (255 or 285) refers to the width of the wheel in millimeters. Why is the rear wider? Because the front rims are 9" wide, and the rear wheels are 10" wide. 255 and 285 are the correct width in metric to match the inch width of the wheels.

The number after that represents the tire sidewall height, but, it is expressed as a percentage of the width in mm. So for the front, the tire sidewall height is 50% of the 255mm width. For the rear, the sidewall height is 45% of 285mm width. Why is the rear aspect ratio smaller? Because the tire is wider. If they used 50% like the front, the rear sidewall height would be too tall. --> As the tire gets wider, the aspect ratio percentage gets smaller.

But tire manufacturers don't make tires like 255/47.5 R19. They do the aspect ratio percentages ONLY in 5 % changes. If you sit and do the math all day, you'll see that cars like BMW's with staggered wheel widths front and rear frequently end up with two ever so slightly different tire diameters.

More than you ever wanted to know! But.. once you know it, you'll totally understand tire sizing!! :-)