Using Different Rim/Tire Size to Alter Ride Height

[baldlobo]

so long as the sidewall of the tire is the same front and rear it'll drop it 1/2" if you change rims

take a 15" rim with 205/60(sidewall 123)r15 tire vs 205/50(sidewall 102.5)r15.

the 205/50r15 tire will drop it close to the same as changing the rim.

watch load and speed ratings if doing this.

ps. it might change weight transfer a touch too.

[MTrenk]

Something else to consider is the weight reduction, and also the reduction of rotational inertia. Generally, the rim will have more of an effect on rotational inertia than the tire.
Inertia resists movement, so rotational inertia will resist rotation. Less R.I. will mean it's easier to accelerate, so you are reducing the load on the engine. Therefore, you can accelerate to the same speed with less effort: more efficiency!

• The further away mass is from the center of rotation, the more inertia.
• The more mass, the more inertia.

Unless you are rockin custom Carbon Fiber rims, your rim is going to be the main factor for inertia. Using the aforementioned principles, best thing you can do is get a smaller, lighter rim. Then, going a step further, you can get a smaller tire. This will lower your car, pushing more air over the top, reducing drag created from the turbulence on the underside of the car (unless you have installed an effective underbody.) AND, you'll be reducing the overall weight of the car. It's a win-win-win!

[mcrews]

Originally Posted by MTrenk

Something else to consider is the weight reduction, and also the reduction of rotational inertia. Generally, the rim will have more of an effect on rotational inertia than the tire.
Inertia resists movement, so rotational inertia will resist rotation. Less R.I. will mean it's easier to accelerate, so you are reducing the load on the engine. Therefore, you can accelerate to the same speed with less effort: more efficiency!

• The further away mass is from the center of rotation, the more inertia.
• The more mass, the more inertia.

Unless you are rockin custom Carbon Fiber rims, your rim is going to be the main factor for inertia. Using the aforementioned principles, best thing you can do is get a smaller, lighter rim. Then, going a step further, you can get a smaller tire. This will lower your car, pushing more air over the top, reducing drag created from the turbulence on the underside of the car (unless you have installed an effective underbody.) AND, you'll be reducing the overall weight of the car. It's a win-win-win!

not hardly

while therories are nice.........

1. smaller tire will have lower weight capabilities. (see search)
2. smaller tires reduce the final gear ratio, resulting in higher rpms resulting in lower mpg. (see search)
3. smaller tires reduce drive comfort
4. you assume no underbelly pan.....that is one of the major projects we do. (see search) the drag reduction from a pan is the single greatest reduction in drag. Why skip it?
5. finally, lowered cars bang into things (see search) sowe me a lowered car that has been drivin 40k miles and I'll so you a pile of crap.

[UltArc]

Originally Posted by MTrenk

Something else to consider is the weight reduction, and also the reduction of rotational inertia. Generally, the rim will have more of an effect on rotational inertia than the tire.
Inertia resists movement, so rotational inertia will resist rotation. Less R.I. will mean it's easier to accelerate, so you are reducing the load on the engine. Therefore, you can accelerate to the same speed with less effort: more efficiency!

• The further away mass is from the center of rotation, the more inertia.
• The more mass, the more inertia.

Unless you are rockin custom Carbon Fiber rims, your rim is going to be the main factor for inertia. Using the aforementioned principles, best thing you can do is get a smaller, lighter rim. Then, going a step further, you can get a smaller tire. This will lower your car, pushing more air over the top, reducing drag created from the turbulence on the underside of the car (unless you have installed an effective underbody.) AND, you'll be reducing the overall weight of the car. It's a win-win-win!

IF I was racing for good money, this wasn't my only car, and I had a more disposable income, I would have already done all of this, or be waiting for a chance to get it done.

Trust me, none of those are so. I don't know how important a lot of those factors are considering others. If one were to do everything you said with all equipment and work free, only paying for materials, I imagine the FE gain would be lower, and the cost higher, than doing a simple belly pan.

On that note, welcome to the board, your input is appreciated! I only shrug it off because that is not cost effective for the serious guys here, let alone me. There are surely other members who may have use for such knowledge.

Originally Posted by mcrews

not hardly

while therories are nice.........

1. smaller tire will have lower weight capabilities. (see search)
2. smaller tires reduce the final gear ratio, resulting in higher rpms resulting in lower mpg. (see search)
3. smaller tires reduce drive comfort
4. you assume no underbelly pan.....that is one of the major projects we do. (see search) the drag reduction from a pan is the single greatest reduction in drag. Why skip it?
5. finally, lowered cars bang into things (see search) sowe me a lowered car that has been drivin 40k miles and I'll so you a pile of crap.

For me, I am dropping .5 inch up front, to stock height. So the height right now is minimal, BUT over the next few years I could see going as drastic as 16/20, but Who knows, I shop by deals, not by wants.

I crave an underpan. I have a 6 year all inclusive warranty, so I am thinking after this winter when I get underneath to clean her real well, I will figure out how easy it is to do, and decide from there. If I do it, I want to learn more about diffusers, to actually work on the bottom. Such as the underbody of the Ford GT. surely there are more, I am just not familiar with others.

[MTrenk]

I didn't mean to imply "skipping" the underbody, I was just hoping to give some insight to the problem stated at the beginning of the thread in the most general terms possible, while giving some more helpful information if anyone was curious about certain aspects of the physics of just the rotating stuff.

Everything you mentioned was correct mcrews, however, we cannot say which is more or less beneficial without running numbers. If i had to guess, if you were running steel hubs, and switched to wire thin aluminium rims, you'd be able to feel the difference. The cost will be very large if you're buying new, or you could go to the junkyard and pick out about anything other than a steel hub and save several pounds, probably for a decent price too. After all, it doesn't really matter what it looks like if you cover it up with a nice aero wheel cover!

Some more evidence that the underbody is the MOST important part of any aero project: research in racing shows that ground effects can begin to produce usable downforce at as low as 20 mph, vs. 50-60 mph for a rear wing!!! This means that for around town cars, having an underbody is completely necessary for getting MAX mpg. In order to achieve decent downforce, the race cars must be very low to the ground. However, since we are not looking to produce downforce (drag), but aerodynamic efficiency, lowering the car is not necessary, but wouldn't hurt.

Hopefully I'm not preaching to the choir about this stuff. I've just spent my whole life learning about cars and want to help everyone achieve exactly what they want out of their cars. As much as I love racing cars, I can't help but be completed enthralled by this website I stumbled upon this past week. I love seeing a positive community committed to learning what they can about their cars, and wrenchin their way to more efficiency and happy wallets.

[UltArc]

Originally Posted by MTrenk

I didn't mean to imply "skipping" the underbody, I was just hoping to give some insight to the problem stated at the beginning of the thread in the most general terms possible, while giving some more helpful information if anyone was curious about certain aspects of the physics of just the rotating stuff.

Everything you mentioned was correct mcrews, however, we cannot say which is more or less beneficial without running numbers. If i had to guess, if you were running steel hubs, and switched to wire thin aluminium rims, you'd be able to feel the difference. The cost will be very large if you're buying new, or you could go to the junkyard and pick out about anything other than a steel hub and save several pounds, probably for a decent price too. After all, it doesn't really matter what it looks like if you cover it up with a nice aero wheel cover!

Some more evidence that the underbody is the MOST important part of any aero project: research in racing shows that ground effects can begin to produce usable downforce at as low as 20 mph, vs. 50-60 mph for a rear wing!!! This means that for around town cars, having an underbody is completely necessary for getting MAX mpg. In order to achieve decent downforce, the race cars must be very low to the ground. However, since we are not looking to produce downforce (drag), but aerodynamic efficiency, lowering the car is not necessary, but wouldn't hurt.

Hopefully I'm not preaching to the choir about this stuff. I've just spent my whole life learning about cars and want to help everyone achieve exactly what they want out of their cars. As much as I love racing cars, I can't help but be completed enthralled by this website I stumbled upon this past week. I love seeing a positive community committed to learning what they can about their cars, and wrenchin their way to more efficiency and happy wallets.

This is actually backwards for us. From what you're saying, downforce-drag/friction increases at lower speeds, sooner than a wing. We don't want downforce. Really, we should be adding wings to gain some flight, less rubber on the ground, less friction. Better than pumping up the tires.

I haven't researched my rims, but I doubt the stock Mustang GT rims are that bad in weight, or the stock 17s from the base which is what I would switch to.

True, a lighter fly wheel, drive shaft, rims would have great results. But not for the cost.

[Varn]

Since this post is in the aerodynamics section, I would say that you you should go to narrow to get less aero drag. On my vw I went with 155/80 and got narrower more aero tires than the stock 175/70s.
My narrow tires are about a half inch taller than the ones I replaced.

[MTrenk]

I guess I should have been more clear. A rear wing will not become effective until around 50-60 miles per hour, unless you have designed the wing to stall at lower speeds. However with an underbody, you can stall at speeds around 20 miles per hour, and the effects only increase with speed. At 70 miles per hour, the rear wing (unless designed as such) will still not have as much effect on the aerodynamics of the car.

The point is that this information can be used for multiple design strategies, including economy. It's not backwards info for you guys, it's info you can use. 1000's of man hours go into designing underbodies for both racecars and extreme eco cars. I guess what I'm trying to say, is with a proper underbody for an eco car, you can see results at low speeds that seem rather ridiculous. I hope that clears things up.