Using Different Rim/Tire Size to Alter Ride Height
 

This is a bit of a strange idea, and may waste a thread, but I am curious about using different sized tires for a FR vehicle.

If I put 16, or 17s on my front axle, maybe even LRR, would this have any benefit other than from being LRRs? 18 on the rear now.

I would think less air underneath, should be improved downforce, but do any more experience individuals have any ideas? I often think about it. Any lower and I think my chin spoiler will scrape, as I now must back into most curbed spots, but that is a small price to pay.

Excuse the ignorance but what is a "FR vehicle"?

Smaller wheels/tires will lower the vehicle but will also lower the effective gear ratio if you make the drive wheels smaller.

If the wheels are 1" smaller in diameter your vehicle will drop 1/2"

Smaller lighter wheels have less inertia and will accelerate quicker.

Make sure the new rims clear the brakes and the tires can handle the weight of the vehicle.

I know on my infiniti I have to run the same size all around. there are sensors that can tell the difference.

measured the gaps between the inside of the wheels and brake calipers?

You want to be careful not to use tires with less load carrying capacity than originally came on the vehicle. That's directionally towards a less safe condition - the increased risk of a tire failure.

That pretty much eliminates using a smaller diameter.

I know they do fit, I can definitely go down to 15s, and I have the stock 17s laying in the garage, I am just trying to see of there would be any added aero benefit. I don't know too much about the technicl aspects.

thank you for the advice, I try to maintain safety. I will only use appropriate tires, but will the two different sizes make a difference?

Mcrews- I thought you had a 45x, and then that would be important, but just for FR (front rear, front engine rear wheel drive), I wouldn't think it would matter. But then again, in a luxury vehicle, you probably have more tech items than most of us. I heard even your back tires help steer...she is a beautiful vehicle.

S-FR is front engine rear wheel. MR is mid engine, rear wheel, FF front front, AWD is all wheel, 4x4 is Usually FR with optional AWD, RR is 1960s Beetle and 2013 911 lol some are less common than others, my apologies.

I am stock on 17s, switched to 18s, and now thinking about what dropping the fromt end .5 inches will do. As long as the sensors are okay with it, what else may come into play?

so the really question is about lowering .......and the best way to do it.
Idid lower my front end w/ spring compressors and believe there was a small gain in mpg.
The was a popular mechanics article a while back the showed that a lowered front might reduce drag.
The draw back was that I did not align the front end and ended up cupping my tires.

My understanding (from a thread here - somewhere-) is that there is an optimal angle. I felt that with my existing slanted nose and IMPUL body kit, that i was able to achieve a desired result.

I can't think of any real safety problems with dropping the front and not the rear, but if you've got ABS or any sort of traction control then those systems might not be happy with running multiple tire sizes.

Aero-wise, the only way to find out if it helps is to thoroughly examine what you've got now and what you end up with. Dropping the front but not the rear might only increase your frontal area without giving any corresponding benefit. "Downforce" is just drag and is only useful if you corner at such high speeds that your car would fly off the road if your bodywork weren't sucking it to the road. Since you generally can't turn downforce off, that drag is working against you every moment that it isn't required to keep you from flying off the road. You may also need a belly pan and/or side skirts to actually get the effect you want.

angle templates
 

If you will make two cardboard templates,one at 16-degrees,and 10-degrees,you can use them to predict proper ground clearance.
The 16'er is for sliding in the front of the front tires.If any bodywork touches the template it will be shredded in ground strikes going up driveway ramps.
The 10'er is for the rear of the front tire and both front and back of the rear.If any part of the car touches,they are also vulnerable to ground strikes.
If you can anticipate the lower ride height with the 15" wheel/tire combo and there is no clearance issue,then your good to go.
Active suspension gives us the best of both worlds but tougher to pull off.

ThNks for all the help, guys. So now I need to find the optimal height, and see if I can match it, without going into a bad aero zone or safety zone. I guess I didn't need to ask, I should have been able to figure it out myself.

If anyone tries it, let me know, or has any results or ideas, I'd like to hear. When I try it, I will post any results. The soonest it will be is this coming spring. New 18s for the rear, and hopefully new 17s for the front.

Again, thanks for the input everybody :)

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.

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:rolleyes:

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? :confused:
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.

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.

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.

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.

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.

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.