Question For Engineer Types...

[The Chief]

Did someone call for an engineer? Actually I am not an engineer, I have engineering degrees (BS/MS) that that does not make one an engineer.

Archimedes, so the story goes, was not around then, but almost!. Archimedes said, give me a lever long enough and I can move the world, or some translation from Greek. Therein lies the answer to the question. Nope, same amount of power, conservation of energy or some such rule. Takes less torque but you gotta travel farther.

 

[Beans]

Making fun of me are we?

On a not very related note, since in many aircraft you have two different tire pressures (one for the mains, one for the nosewheels), which one is more important when trying to determine your hydroplaning speed? It would seem to me that as long as you have rudder authority, the mains would be the most important thing, but I could be wrong....

/useless knowledge questions

Work both numbers. Whichever one is worst is the one to worry about.

 

[TTUalumn07]

Making fun of me are we?

It's from Spaceballs....i know some of you have to have seen this movie.

 

[D_Rob]

Making fun of me are we?

On a not very related note, since in many aircraft you have two different tire pressures (one for the mains, one for the nosewheels), which one is more important when trying to determine your hydroplaning speed? It would seem to me that as long as you have rudder authority, the mains would be the most important thing, but I could be wrong....

/useless knowledge questions

Stop ruining America! (Aka bringing up discus items from your flight today)

I think a lot of it depends on the specific plane. Since in the 34 the nose wheel dosent really do much other than keep the nose off the ground, I would worry more about the main mounts.

 

[PropAddict]

So I have been having a debate with a friend at work about how tire size would affect mileage/speedometer readings. It is my assertion that if you went from smaller to larger tires (with a larger outside circumference) you would get a speedometer reading that is lower than actual groundspeed, as the speedometer is based on the rotation of the axle, not the groundspeed...and that your mileage would actually decrease because it takes more power to push the vehicle on those tires as they have a greater surface area and would create more resistance/friction.

Am I right, or am I talking more out of my ass than usual?

Thanks,
Pickle

(please explain it in English to me, I was an Animal Science major in school)

The speedo error is dead on. You are supposed to change the settings, either in the wheelboxes (in old cars) or the computer (new cars) to account for changes in tire size.

A bigger contact patch ("tire footprint") will sap more energy away, lowering the gas milage. I haven't done the math on it, so I can't say how much precisely, but I suspect it's small. Now, factor in that that larger tire has a lot more rubber to spin up, there's some more inefficiency. I've always suspected there are aerodynamic losses as well (look at how much space a Ferrari has between its undercarriage and the road; now look at how much that Wrangler has).

The empirical data: 2005 Wrangler Rubicon 31.5'' Wrangler MTRs to 34'' Swampers gave me a loss of ~2mpg in mixed city/hwy driving. It also made the ride unbearably loud above 60 mph, with the top down. . .

 

[MIDNJAC]

Stop ruining America! (Aka bringing up discus items from your flight today)

I think a lot of it depends on the specific plane. Since in the 34 the nose wheel dosent really do much other than keep the nose off the ground, I would worry more about the main mounts.

That's pretty much what I said, and then we both kind of just scratched our heads and moved on.

 

[nugget61]

On newer Japanese imports, the tire size will affect the speedo but not the odometer. They actually have a pretty slick system that ensures that the odometer is spot on (which the company can be sued for if its off), however there is decidedly more slack in the speedometer readings.
To answer, yes, if you up the tire size your speedo will read high. (Going 14s to 15s makes my speedo read about 1mph higher)

 

[Beans]

It should be the other way around. At the same speed: Bigger wheel circumference => slower driveshaft => slower revs on the speedo cable => slower indicated speed

 

[nugget61]

It should be the other way around. At the same speed: Bigger wheel circumference => slower driveshaft => slower revs on the speedo cable => slower indicated speed

Think of it this way: Your car is spinning your axle and thus the tire at 1 revolution per minute. Lets say thats 10mph. Then you put a bigger tire on. You're still spinning the axle and thus the tire at 1rpm, but now the bigger tire covers more ground in the same amount of time. So since you go more distance and time remains the same, your speed goes up. Since it measures speed on the axle (at least mine does), the axle is still doing the 10mph. However, the tire will be moving faster, say 11mph. Not much, but there.

 

[HokiePilot]

Think of it this way: Your car is spinning your axle and thus the tire at 1 revolution per minute. Lets say thats 10mph. Then you put a bigger tire on. You're still spinning the axle and thus the tire at 1rpm, but now the bigger tire covers more ground in the same amount of time. So since you go more distance and time remains the same, your speed goes up. Since it measures speed on the axle (at least mine does), the axle is still doing the 10mph. However, the tire will be moving faster, say 11mph. Not much, but there.

Yeah, your speedo will read slower than you are actually going.

 

[IKE]

(Going 14s to 15s makes my speedo read about 1mph higher)

It's the outer diameter that matters. For example, if you go from a 195/65R15 to a 225/35R17, you actually have a smaller outer diameter, even though the rim is 2" bigger (the math is left for an exercise...) I did this on my car and found the indicated speed to be about 7% higher than actual speed.

The indicated speed is directly proportional to the angular velocity of the drive axle.

w = v / (pi · d) , where:

w is the angular velocity of the drive axle,
v is the vehicle's actual speed on the ground, and
d is the outer diameter ( rim + tire )

So, the OP is right: increase outer diameter and you decrease indicated speed for a given actual ground speed (they are inversely proportional).

As to the second question: the effect on actual mileage probably depends less on the outer diameter, and more on:
a) the combined weight of the new rims/tires compared to the old set (rotational weight carries a high energy cost - this is why big brake rotors are usually not a good mod), and
b) the efficiency of the engine at different rpm.

 

[porw0004]

This thread is really starting to remind me of the whole "airplane on a treadmill" conundrum. Some people get it.. and some people are just stupid.

 

[Omni]

This thread is really starting to remind me of the whole "airplane on a treadmill" conundrum. Some people get it.. and some people are just stupid.

It's only a conundrum because the specifs of the problem are not stated. People interpret the loose description different ways and therefore come to different conclusions. Not right and wrong, just different.

 

[xj220]

Another thing to think about when going with a larger wheel/tire package is that you'll be increasing rotational inertia which also zaps power. Now it all depends on the wheel/tires (ie going from stock steelies to aluminum race wheels) but assuming all else is same, the bigger wheels will hamper you. I remember reading an article a magazine did where they tried to make a stock Nissan Maxima do 14s in the quarter mile without doing anything to the engine. In addition to stripping it down, they actually went with the stock wheels vice aftermarket big wheels since the car would be faster. I'll see if I can find it.

 

[xj220]

Here's the article. Pretty good read overall: http://www.daleholley.com/nissan.htm