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CorollaMaster 04-27-2008 03:51 PM

Wheels - Large or Small Diameter? Width?
 
Hey all,

I'm new to this, but I was wondering what is better:

Rims
- Larger diameters or smaller diameters?
- Larger or smaller tire widths?

My first guess is that a larger rim with smaller tires and smaller width would be best for rotational inertia of the wheels. But then again are you losing more energy trying to get the larger diameter accelerating in the first place? :confused:

I was also wondering if anyone knew of a company that makes good light weight rims that might make a difference? or is it better to go with the run of the mill black and some flat hub caps? :cool:

I'm also considering wheel skirts. My friend works in autobody and is willing to help me out so that they look somewhat professional. One step at a time I guess. Thanks in advance for the help!

toomuch 04-27-2008 04:31 PM

By better, I assume you mean more efficient. The smaller the width of the contact patch (tire), the more efficient.
http://en.wikipedia.org/wiki/Rolling_friction
http://en.wikipedia.org/wiki/Low-rol...sistance_tires
http://www.treehugger.com/files/2005...olling_res.php

When I had 175 MM contact patches at each corner, I had better average fuel economy than my current 185 MM width tires. (I do a lot of highway driving)

My technical knowledge regarding about diameter not impressive, but big tires take more energy to get moving. Smaller tires take less. Think of those little go kart tires. Easy to get them moving, but you are more limited in speed and are more dependent upon changing gears.

Forged rims are very lightweight, but cost a lot of money. If you are lucky, maybe you can find some small, discontinued set for $100 per rim. Usually the aerodynamic profile of these rims are the complete opposite of aero hubcap covers. These rims will be much stronger than steel rims. Good brands that i know of are BBS & SSR.
http://www.tirerack.com/wheels/Wheel...All&sort=Brand
http://www.tirerack.com/wheels/Wheel...All&sort=Brand
Use google for more. (not entirely sure those links will work)
Hope that helped :)

guitarterry 04-27-2008 04:37 PM

Some general rules?
Contact patch is derermined by air preasure A 22" tire at 32 psi has the same contact patch as a 12" tire at 32 psi
Reciprical weight weighs 10 more. i know this sounds funny. but it takes alot more to stop or start a tire than to say just lift it. Sprung weight is also more than non sprung weight. but that difference is for handling as opposed to mpg
So I guess tires have four variables, weight, aero, rolling resistance, and final drive ratio. those little 12" tires make weight and aero go down. rolling resistance is a smaller variable. then a person can use gears to make the final drive ratio back up. The best tire would be an extremely skinny and big around tire, like a bike tire. unfortunately as the diameter gets bigger, the width and weight gets bigger, and those two overly outweigh the diameter advantages

CorollaMaster 04-27-2008 04:53 PM

Thanks for your replies!

So it looks like trying to lower the contact patch is the best way to go with the tires.
I've also been doing some googling after toomuch's post and it seems like finding low RR tires is the way to do that. Another question that popped into my mind on the same topic is:

If I were to keep the same overall diameter of my current wheel base, which would be more efficient - larger diameter or smaller diameter lightweight rims?

I'll obviously try to keep the width as small as possible with low RR tires and correct pressure, but I'm trying to figure out what size rims would be the most beneficial efficiency-wise?

My hunch is that if you have something that has a good rotational inertia then it would be bad for starting and stopping but really good for steady highway driving.

Guitarterry - what did you mean by the reciprocal weight?

guitarterry 04-27-2008 05:12 PM

rotational weight. again the width of the tire and also the diameter of the the do not affect the contact patch. weight and air presure do. for an example, if you had a 3000 lb car and 30 psi in your tires the contact patch will be the same size no matter what tire u use 22" down to 12". Actually its an old racers trick. You can weigh your car by going out and using paper and a ruler. here is a link
http://www.rockcastle.org/activities.../weighcar.html

toomuch 04-27-2008 05:12 PM

skinnier tires certainly have a disadvantage, less traction! Traction is also related to the tire compund and tread pattern, but the less of it you have on the road, the less traction you are going to have. So I wouldn't go too skinny, its better to not loose control of your car.
I definitely don't know everything or close to it about this subject, so keep on your quest!
As for your diameter question, I don't know. You could get the weight measures for a certain tire and wheel setup and see which weighs the least. Let us know, I am curious. I have done a little of that myself, but I forget what the result was.
Not sure about your hunch other, maybe someone else is.

Good luck!:D

toomuch 04-27-2008 05:13 PM

Quote:

Originally Posted by guitarterry (Post 21848)
You can weigh your car by going out and using paper and a ruler. here is a link
http://www.rockcastle.org/activities.../weighcar.html

Thanks!

guitarterry 04-27-2008 05:30 PM

see toomuch, its a common mistake to think tire size affects tire patch. and some of your statements are wrong. Its a common mistake. But hard to fix once people get it in there head.

toomuch 04-27-2008 05:42 PM

GuitarTerry, thank you for keeping the forums straight. You are right, I do not fully understand all of the workings of the tire and wheel. I understand that the tire pressure affects the form of the tire, and is what holds up the vehicle. But shouldn't a Corvette tire, with a 245MM width have a larger contact area than my 185MM wide tire corolla within reasonable pressure amounts? (& maybe i am still wrong, & still am not getting something!) could it just be that the patch is wider and not as long?
My experience with tire width and pressure is that at 175mm i could get 39mpg, and now with the 185mm s i get 35 mpg. There must be some other factor. Pumping up the PSI on the 185s from 33 to 35 has netted me about 1-2 mpg on average. the 175s were kept between 33 and 34.

CorollaMaster:
Maybe you might want to check this out:
http://auto.howstuffworks.com/tire.htm
I love that site, but I haven't read that article lately. Don't have time to now either. Cya

guitarterry 04-27-2008 06:12 PM

the contact patch of the two tires u mentioned would be the same on the same car, with the same psi. how the tire reacts is where the difference is. eg, the 245 tire would be more stable because it is wider. There are lots of other factors. but Thats for a different thread. Now for your mpg statement. did you factor the different diameters of the tire into your mpg figures. if you dont adjust the figures the 175mm at 39mpg is probably the same as the 185mm at 35mpg

guitarterry 04-27-2008 06:42 PM

frank ????? The size is the same, even if you can measure it corectly of not. Its seems you want to change the discusion about the size of the patch, to your ability to measure it. Secondly I run my tires at 45 psi. when i said the best tires was a skinny tire, i was refering to FE only. Which is what this site is about. You might be right, but it doesnt affect the conversation. The conctact patch is the same, whether you can measure it accurately. the best tire for FE is a skinny tire. even if you have to air it to 50 psi.

CorollaMaster 04-27-2008 07:44 PM

Quote:

Actually its an old racers trick. You can weigh your car by going out and using paper and a ruler. here is a link
http://www.rockcastle.org/activities.../weighcar.html
Thanks for the trick! :)

However, I wasn't actually trying to lower my contact area by changing the diameter of the wheel. I was trying to figure out which is better for gas in terms of rotational inertia: ie, which of the following options will the engine end up doing the least amount of work - having a wheel with a smaller diameter (so that more of the weight is in the center of the wheel) or having a wheel with a larger diameter (so most of the weight is towards the outskirts of the rim).

I had to pull out my book from highschool :rolleyes:, but after brushing up on my physics a little bit, I think I figured it out.

While you want light wheels it matters where the weight is focused. If its towards the center of the wheel or spread out for that matter, its going to take less energy to accelerate because of the rotational inertia. However, with that said they will also tend deccelerate faster since they won't tend towards staying in motion. So if you plan on coasting or doing a lot of highway driving its not the best option.

Now if you could get light wheels that have a larger diameter where most of the wheel weight is focused towards the outer rim, then the wheel should stay rolling longer (kinda like it'd have more momentum). So this **should** increase mpg for highway driving and for coasting situations.

So I guess my conclusion at this point in the thread anyway would be that I want to find skinnier lighter rims that are larger than my current ones with most of the weight focused towards the outer wheel and then see what I can find for RR tires. :cool: this should potentially increase my mpg.

Thanks for all your responses so far! If you find anything more please let me know! I'll keep you posted on what I end up doing! :D Please keep on posting if you have more ideas!

toomuch 04-27-2008 08:24 PM

thank you for contributing to your own question. as corny as it sounds, it seems like we are all learning from each other. Damn that stings, I can't believe i said that. anyway, maybe you can find a set of used forged rims on ebay or craigslist. I know I'd take a pair if they were cheap enough! make sure they will fit your vehicle. there are a lot of variables to consider, like offset. Check out tire racks take on this:
http://www.tirerack.com/wheels/tech/...urrentpage=106
all of their tech articles are great
"To properly fit on a vehicle the wheel must have the proper bolt pattern, centerbore, offset, width and load capacity." -tire tack
might want to check this out too
http://www.tirerack.com/wheels/tech/...jsp?techid=108

toomuch 04-27-2008 08:50 PM

Forged lug nuts will help lower the weight at the center.

LostCause 04-27-2008 09:18 PM

Here's an important question: where does rolling resistance come from?

I think the contact patch in and of itself is irrelevant. An incompressible, perfectly smooth wheel rotating on a frictionless bearing rolling over an incompressible, perfectly smooth surface should see no friction (as long as it is not slipping)...whether that wheel is 1 foot wide or 1 mile wide.

As far as the tire is concerned, rolling resistance comes from flexing of the wheel. While the tread compresses, the majority of the flexing comes from the sidewall. Higher pressure doesn't work primarily by decreasing the contact patch, it works by decreasing sidewall flexure. Unlike balloons, radial tires have metal belts running under the tread, limiting their shape anyways.

Now, why would a skinny tire be more beneficial then a wider tire? Less flexing and surface area for slipping, but specifically how?

I think increasing wheel weight to aid in cruising is a bad idea. It's akin to putting extra weight in your car to tackle hills. Sure you will glide longer, but you'll take more energy to get up to speed in the first place. Unfortunately, the 2nd Law of Thermodynamics rules...you'll always get less energy out of a glide then you put into the acceleration. When everything is appropriately sized, light weight maximizes efficiciency.

Increasing wheel diameter may also be helpful by lowering engine rpms. Generally a useful feature, but must be appropriately sized.

Intuitively, I'd choose a wheel first based on weight, then diameter, then width.

- LostCause

CorollaMaster 04-27-2008 09:57 PM

toomuch - thanks again for the post, that was a good article and a neat site!

LostCause - I think the sidewall flex would be another reason to pick a larger diameter rim and slimmer tires. There is less sidewall to flex, and as a result probably a lower contact surface as well. The main benefit though is the displacement of all the weight to the outside of the wheel, that has to be another factor of rolling resistance.

I also wanted to respond to your comment about the contact patch. The contact patch is relevant because its the amount of area that the car is putting energy into to make it move. It's like trying to run in the summer with snow shoes versus running with just sneakers. The sneakers have a smaller surface area and allow you to focus the energy into moving you forward on a smaller area. With the snow shoes there is a larger surface area and the force you apply is displaced accordingly so it takes more energy to move as fast as you would in sneakers.

I guess the problem is that we are not talking just about rolling, we're mainly talking about how much gas is used going from the engine to the tires, and if you have a smaller contact patch you will end up using less.

LostCause 04-27-2008 10:09 PM

Quote:

Originally Posted by CorollaMaster (Post 21909)
LostCause - I think the sidewall flex would be another reason to pick a larger diameter rim and slimmer tires. There is less sidewall to flex, and as a result probably a lower contact surface as well. The main benefit though is the displacement of all the weight to the outside of the wheel.

I also wanted to respond to your comment about the contact patch. The contact patch is relevant because its the amount of area that the car is putting energy into to make it move...

Larger diameter rim = more weight. A slimmer tire also doesn't necessarily mean less sidewall flex, as it could just be concentrated into a smaller area.

The contact patch is irrelevant in an ideal world. If nothing ever deformed/slipped, it wouldn't make a difference if you were pushing a car with a tennis shoe or the head of a pin. Therefore, in the real world, it is not the contact patch that is the issue, it is something else. The contact patch just manifests an issue concentrated somewhere else.

I'm not saying to disregard the contact patch, but to understand that rolling resistance does not lie solely in its geometric shape.

- LostCause

CorollaMaster 04-27-2008 10:23 PM

Quote:

A slimmer tire also doesn't necessarily mean less sidewall flex, as it could just be concentrated into a smaller area.
Wouldn't that mean that less of the sidewall is flexing if its in a smaller area?:confused:

In addition, the amount of surface area that the power is being put into by the engine definitely effects the gas mileage. While the contact patch isn't solely responsible for rolling resistance, which I agree has a lot of different factors that add to it from what I've been reading (rotational inertia, design pattern of tread, skinniness of the tire, etc), it is definitely still very important to factor in. Unfortunately as you said, its not a perfect world with tires that don't deform. :(

guitarterry 04-27-2008 11:14 PM

hey frank, if your contact patch is circlualar at high psi then u would figure area with pie r squared. using that formula 2.5 *2.5*3.14 gives a total of 1000. again i am not saying that this method is accurate. but the theory of contact patches being the same for the same psi and weight is. Just for the record. The weight method is not super accurate. There is lots of variables, (air gauge, measuring methods, even tread patterns, etc can effect the results) .

guitarterry 04-30-2008 09:07 PM

I think we are both on the same page. Just we are on different subject. My point was that tire size does not affect contact size. Alot of people wrongly think that. Now the side note to what is what you said. That bigger tires CAN run at a lower psi and still be stable. Then (at the lower PSI) it would give a higher contact patch. Its kind of a circle. Kind of like octane. Higher octane does not increase performance or economy,BUT if you can increase the timing that will increase the perfromance. Therefore octane allows better performance. I have also been reading about this flywheel subject. My two cents, weight will hurt economy, no way around it. BUT a engine designed with a heavy flywheel might get great gas milage. Eg. a 440 big block in a 69 charger with a super light flywheel will probably get worse gas mileage than xfi metro with a super heavy flywheel. does that prove that a super heavy flywheel is fuel efficent NO. It is just that the design need a heavy flywheel for some reason. (maybe drivability)

JohnnyGrey 04-30-2008 10:27 PM

-A certain amount of energy goes into deforming the rubber when it hits the road. The square footage of rubber that meets the road per rotation is a function of tire width and does not change with contact patch. In other words, a 195mm tire has 5.4% more area that must make and break contact with the road surface than a 185mm tire.

-In poor conditions, a fat tire means you displace more water and snow.

-A larger/fatter tire may mean more rotating mass, which can be a small factor in city mpg.

-A thinner tire will help shave those last few square inches off your frontal area.

-Fatter tires tend to have lower max psi ratings. Yes, a 225mm and 195mm tire will have similar contact patches at the same PSI, but chances are, the 195mm tire will allow for more pressure (and therefore a smaller contact patch) than the 225mm tire. I'm no tire engineer, but it would seem easier for a tire to preserve its composure under pressure when the distance between sidewalls is minimized.

LostCause 04-30-2008 11:34 PM

Quote:

Originally Posted by JohnnyGrey (Post 22534)
-A certain amount of energy goes into deforming the rubber when it hits the road. The square footage of rubber that meets the road per rotation is a function of tire width and does not change with contact patch. In other words, a 195mm tire has 5.4% more area that must make and break contact with the road surface than a 185mm tire.

Good points, but I have a question over the first statement.

Generally, a wide tire and a skinny tire have a contact patch of equal area at a given psi, right? If the skinny tire has a square contact patch, the wide tire would have a thin rectangular one.

Wouldn't the area of tire deformity be equal on both? A wider tire has more area than a skinny one, but less of it is in contact with the ground at a particular moment. A wider tire will also cause a smaller area of the sidewall to flex at a time, which intuitively seems more desirable then decreasing tread flexing.

I've only tentatively gone over this in my mind, so sorry if I've made a blatant mistake...:o

- LostCause

JohnnyGrey 05-01-2008 12:45 PM

Quote:

Wouldn't the area of tire deformity be equal on both? A wider tire has more area than a skinny one, but less of it is in contact with the ground at a particular moment.
For any given moment, yes. Per mile however, the fatter tire has a larger square footage making and breaking contact with the road. That square footage has to deform over every imperfection in the road. Visualize a roll of toilet paper unraveling. The fatter the roll, the more square footage it's going to cover per mile of length.

LostCause 05-01-2008 06:05 PM

Nicely put. This has become a very informative thread. I think you have answered why square contact patches are better than skinny ones.

I believe this thread has been definitively answered: The lowest rolling resistance can be achieved by the skinniest tire that allows a high enough psi to maintain a square contact patch.

Now that the issue of excessive tire width has been determined, I'd like to look at the opposite end of the issue. When do tires become too skinny?

I've heard that the lowest rolling resistance that is routinely achieved are steel wheels on steel rails (trains). If that is the case, wouldn't the ultimate conventional road tire be one that is under high pressure and sized just large enough to prevent road damage/deflection from localized pressure? I realize tradeoffs are being made between efficiency and practicality, but practicality aside...

Lastly, how does silicon lower a tire's rolling resistance? I'm pretty sure the main additive in LRR tires is silicon, but what benefit does it provide. I'll research on my own, but if anyone knows the answer already...:)

- LostCause

LostCause 05-07-2008 02:13 AM

Just out of curiosity spurred on by this thread and cbergeron's new set of tires, I've been searching the net for the skinniest set of wheels and tires around.

DOT approved (legal):
Firestone F560 125/?/15

http://store.coker.com/images/D/fire...tire-large.jpg

Classic tire from www.cokertire.com

Width ~4.92"
Max PSI unknown (~35?)

Temporary (illegal)
Continental 105/95 r17

http://www.ityre.com/i/tyre/20708.jpg

Width ~4.13"
Max PSI unknown
__________________________________________________ __

As far as I've found, the skinniest tire (in mm) per rim size is:

Legal
13" - 155
14" - 117 (?)
15" - 135
16" - ?
17" - ?

Illegal
13" - ?
14" - 117 (?)
15" - 115
16" - 115
17" - 105

If anyone is aware of smaller sizes, feel free to post.

- LostCause

LostCause 05-08-2008 12:44 AM

Quote:

Originally Posted by Frank Lee (Post 24017)
I'd have to run ridiculously high psi to achieve a square patch, or achieve some radical weight loss on the car, or both. It just may be that the stock size is already the optimal one.

Thanks for the Coker idea. I'll take a look. Edit: (Original thread updated)

1.) Could someone verify how you calculate the psi needed for a square contact patch?
I'm assuming it's just:

vehicle weight / (4 * area of square contact patch)
__________________________________________________ _______________

Hypothetically, a CRX HF that weights 1700lbs running a tire psi of 70 (reasonable for a "temporary" tire) would only need tires 2.46" wide (62mm).

My Thunderbird, which weighs ~3569lb, running 55psi would only need a tire 4.03" wide (102mm).

Tire width would need to increase with load (driver, gas, oil, etc.) and traction would be severely reduced, but does this sound reasonable? Those tires would be extremely skinny. :D
2.) If skinny enough tires don't exist, what is better: a square contact patch or max psi?
A higher psi causes more rubber to deflect than an ideal tire, but it is still an equal amount to the square contact patch of a lower psi. I would say higher psi simply because the amount of rubber being deflected will always equal, but the tire will be stiffer and therefore incur less friction.

I think this is another relevation: In the absence of an ideal tire width, maximum psi offers lower rolling resistance than a square contact patch.
Assuming the conclusions being made are correct, this thread has been extremely useful to me. I'm surprised...and happy. :)

- LostCause

LostCause 05-08-2008 01:14 AM

Quote:

Originally Posted by Frank Lee (Post 24166)
What I have found is, there is quite a difference from the hypothetical calculations and the real world experiment results. All my calculations showed that a 155 series tire would have a square patch at 50 psi on the front; but then my experiment showed that airing up my 185 series tire to 50 psi STILL wasn't enough to achieve a square patch.

"Could someone verify how you calculate the psi needed for a square contact patch?"

I'd go by axle weight rather than total weight/4. Most cars, being FWD, carry about 2/3 of their mass on the front axle unless really loaded. Then once you have a sq in value for the front and rear patches, you need to know how wide the tread is THAT IS CONTACTING THE GROUND.

Good point about the weight distribution. I hadn't though of that. I wonder how much of the tire actually lifts from the sides, but I figured it would be small. I didn't notice any appreciable change when I pumped up my tires, but you are right, contact patch is smaller then tire width and should be noted.

I'm wondering if the psi issue has something to do with the tire being a flexible toroid rather than an impermeable cylinder. The only thing I can come up with is that a tire is flexible, so it is subject to differential stress. Like a rolling pin pushing on dough, some of the force is being redistributed to the sides rather then straight down.

An impermeable cylinder would allow the entire air pressure to support the axle while the toroid must share that pressure with the axle and the further compression of the rest of the air in the tire (plus wheel bulging).

I might be getting a little too deep and lost into all of this, so feel free to set me straight. :) If anyone has been able to achieve a square contact patch, would they mind posting vehicle weight, tire width, and psi? Now that I think of it...I should probably perform my own test. :o

- LostCause

guitarterry 05-13-2008 08:59 PM

First off, I dont know why we are trying to get a square patch, an article in a tire ad? i measured my mountain bike tire. front was 3 by 1 and back was 4 by 1, at 50 psi, i know those square measurements but i weigh 250 and it come out to 350. The point is though i know that my mountain tires are not fe compared to road tires. The tires should still be skinnier although the patch is way longer than it is square. I then measured my car front 4.75 by 4.24 averaged and figured round (by franks sugestion) give me a weight of 1177 back was 4 by 3.75 both with 50 psi and a weight of 706. Total of 2354 lbs and my car weighed 2300 on the scales. By the way to get an accurate area use carbon paper, jack car up and put under, let down. Use that paper to figure area and make sure that ur presure gauge is accurate. Still I would think that a square contact patch is not a goal, like a bicycle longer and skinnier will be better.

kevlar 05-13-2008 09:28 PM

WOW! deep stuff!
All I can say at this point is that my Yota truck will soon need a new set of skins. I intend to go one size skinnier and see what happens. All of this will be documented as a success/fail project. Damn the books!

LostCause 05-14-2008 03:43 AM

I think the reason for a square contact patch was well developed earlier in this thread. Using an ideal tire width, a square contact patch minimizes overall wheel deflection (stationary and moving).

The rationale bicycle riders use against excessively high tire pressures is that slipping begins to occur. I don't think the issue has anything to do with patch geometry. If slipping and road surface deformation can be limited, higher pressures should theoretically be ideal.

The circular contact patch issue seems logical, but I'd assume it would look more like a rounded off square than a circle. Without a more advanced mathematical technique or a super-specialized scale, I have no idea what the pressure distribution is like. For that reason, I think a list showing "ideal" and "actual" car widths to achieve a square contact patch should be made. If we get an idea of how far off the ideal is from reality, it'll be easier to say what width tire is ideal for a given car.

To that end, I'm going to measure the contact patch geometry of a few cars I have access to and I'll post the difference between ideal width (taking into account weight distribution) and the actual needed width (if I don't far exceed max PSI). Also, I'll post contact patch width vs tire width.

FrankLee already posted his data:

1993 Ford Tempo - 2520lbs Loaded (Front - 1590lb Rear - 930lb)

Ideal CP: Front - 119mm Rear - 106mm
Actual CP: Front - 127mm Rear - 127mm
Actual/Ideal: Front - 107% Rear - 120%

Actual tire width: 185mm
Actual CP width: 127mm
CPW/TW: 69%

While it's not perfect, it should give a general idea...

- LostCause

EngineeredCivic 05-14-2008 09:33 AM

Be aware guys that when you use a narrower tire (175/70/14 vs a 185/70/14) you are also changing the diameter which will throw off your odometer which will effect your MPG calculation. Make sure if you go narrow you compsnate with a taller profile to keep the same diameter.


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