[undecided]

Originally Posted by skimo

Not the guru when it comes to cars, but expert in physics (or i think).

Here is my question: 2 cars with identical weights. One has bigger wheels than the other. (again, both weights of wheels are identical). Shouldn't the one with bigger wheels give better times?

Bigger wheels = more distance per rev. So you should go faster. But, this also means you need more power.

So where is the sweet spot? Something tells me that GUESS nailed it. Slightly larger wheels in diameter and also lighter! I wonder what the run times will be on a 20' or so.

The reason is that with a bigger wheel you move a lot of the rotational mass to the outer edge. Now, if you do change wheels and the new wheels are lilghter then you will see a difference. I think the guy with the 18s probably saw a difference from the tire not the wheel.

CZ

[Guess]

Originally Posted by findtom

I have a MT 325i. Assuming I were to stop by the quarter mile track one day for the hell of it, what would I need to know to get good times (i.e. launching properly, shifting, etc...)?

This applies to the 325i MT only, other cars I have driven/owned are different.

At the Light Tree, Rev to about 2750 rpm (your optimum torque), and hold the clutch in. Dont touch the brake, as tracks are completely level so your car wont move.

On the second orange light (right before the green) drop the clutch, and at the same time press the gas to its full extent.

Shift at 6500-6600 rpm, DO NOT remove your foot from the gas, keep it 100% pressed in during shifts and all.

At shifts press clutch in as quickly as possible, put car in gear, drop clutch.

This should get you this kind of time. 14.7 stock is just about as fast as these cars can go.

[lux.sh]

Originally Posted by timzerofive

Well, when cars go bigger wheels, tire profile usually gets smaller to compensate, so the overall radius is usually the same, or changed very little. The actual factor I would think is the actual weight of the wheel and the type of tires used. The larger wheel usually means heavier in weight, and means more mass to spin, and less slower time. Also, larger the wheel, lower the tire profile, and more stiff it becomes, and in regular tires usually translates to less traction, and means you won't get as good of a launch, which means even slower time.

However, changing wheel size doesn't always equates into better or worse time, because there are so many other factors involved. For example, a larger wheel on a corvette might translate into better time if the wheel also got wider. This means more contact patches and thus the vette can launch harder, if that's its optimal power band to be launching... and there are a lot more variables.

Well said.

Just wanted to add to the person who asked that question, It really depends on what "fast" you are looking for.
Drag wise? You wouldn't want 21s because you would want smaller/lighter/wider wheels with tall, soft sidewalls available. Handling wise? You still wouldn't want over-sized big wheels for specific cars. You would want sets of wheels that is big enough to overcome the vehicle's brake clearance, preferrably wide, obviously lightweight, and shorter/stiffer sidewalls.

Simple conclusion is, weight & over-size is bad.

[timzerofive]

Originally Posted by lux.sh

Simple conclusion is, weight & over-size is bad.

Yes, and the same conclusion can drawn on all women too.

[shimshimhada]

Originally Posted by Guess

Shift at 6500-6600 rpm, DO NOT remove your foot from the gas, keep it 100% pressed in during shifts and all.

That's great advice if he has a dog-clutch gearbox...which he doesn't.

[danatk]

yeah, i was thinking, if you keep your foot on the pedal during a shift the engine is going to scream to the redline, hit the rev limiter and mess up your shift....

[glockz]

Originally Posted by danatk

yeah, i was thinking, if you keep your foot on the pedal during a shift the engine is going to scream to the redline, hit the rev limiter and mess up your shift....

That's correct if you shift like your driving Ms. Daisy. He's referring to what is known in the drag racing community as powershifting. That technique requires some practice (some people never get it right) and it can result in quicker ET's but it is hard on the drivetrain.

[timzerofive]

Actually you would want to lift slightly, not completely, but you just got to know the main purpose is to keep the rpm in the power range and to keep the momentum of the car. If you keep it floored, chances are you will over rev it and the car will bog down instead. You lift slightly to maintain rpm while shifting into the next gear then floor it.

[findtom]

Originally Posted by timzerofive

Actually you would want to lift slightly, not completely, but you just got to know the main purpose is to keep the rpm in the power range and to keep the momentum of the car. If you keep it floored, chances are you will over rev it and the car will bog down instead. You lift slightly to maintain rpm while shifting into the next gear then floor it.

I tried this a couple of times on my way home tonight. It's amazing how hard it is to break the habit of lifting my foot off the gas when I change gears.

It feels a bit abusive on the car so I don't think I'd do it again, but I can see why they call it powershifting.

[lux.sh]

Originally Posted by timzerofive

Yes, and the same conclusion can drawn on all women too.

amen to that.

[timzerofive]

Originally Posted by findtom

I tried this a couple of times on my way home tonight. It's amazing how hard it is to break the habit of lifting my foot off the gas when I change gears.

It feels a bit abusive on the car so I don't think I'd do it again, but I can see why they call it powershifting.

Ya don't do it just cuz your bored, it'll kill the clutch and tranny.

[peterpeterhaha]

when launching to get best speed, is it better to activate traction control or disable DTC/DTS whatever it is?

[FlyingFinn]

Originally Posted by Guess

Post here, and I will add you to this List

• Guess2006 BMW 325I (6 Speed) 14.786 @ 94.79 MPH

Did you get your best times DTC completely off?

[Guess]

Originally Posted by FlyingFinn

Did you get your best times DTC completely off?

yep.

[shimshimhada]

Originally Posted by glockz

That's correct if you shift like your driving Ms. Daisy. He's referring to what is known in the drag racing community as powershifting. That technique requires some practice (some people never get it right) and it can result in quicker ET's but it is hard on the drivetrain.

I'd said that the relatively small gain isn't worth the risk. Speedshifting/powershifting shouldn't be done on a stock production car not meant for that kind of abuse.

[skimo]

Originally Posted by lux.sh

Well said.

Just wanted to add to the person who asked that question, It really depends on what "fast" you are looking for.
Drag wise? You wouldn't want 21s because you would want smaller/lighter/wider wheels with tall, soft sidewalls available. Handling wise? You still wouldn't want over-sized big wheels for specific cars. You would want sets of wheels that is big enough to overcome the vehicle's brake clearance, preferrably wide, obviously lightweight, and shorter/stiffer sidewalls.

Simple conclusion is, weight & over-size is bad.

Probably starting to drift off topic, but I am curious as to why drag racers (the sling shot type cars) have MASSIVE wheels ? Why not wide-small ones?

to calrify, by "fast" i mean drag wise.

[Ed_330i]

because they put down so much power, and so fast that they need a huge tire to "catch"

or else they would just spin the tires endlessly.

[Guess]

Here are my timeslips for the 14.7 runs. These are back to back runs, I am car number 8003. Sorry for the crumples in the paper.

[xdrive]

Wow. That's fast for a 325i stock. Anyone out there clocked their 330i stock?

[visor]

14.8 for a 325i! That's great!

But damn! Who's car 7972 with 12.9 @ 107mph in the 1/4 mile?!

[SCA]

Originally Posted by visor

But damn! Who's car 7972 with 12.9 @ 107mph in the 1/4 mile?!

Seriously...

[Tierfreund]

Originally Posted by skimo

Not the guru when it comes to cars, but expert in physics (or i think).

Here is my question: 2 cars with identical weights. One has bigger wheels than the other. (again, both weights of wheels are identical). Shouldn't the one with bigger wheels give better times?

Bigger wheels = more distance per rev. So you should go faster. But, this also means you need more power.

So where is the sweet spot? Something tells me that GUESS nailed it. Slightly larger wheels in diameter and also lighter! I wonder what the run times will be on a 20' or so.

Well that´s more like elementary school physics... :

Of course the bigger wheels have to come with tires with a lower sidewall. All wheel/tire combinations that are allowed on the E90 will have more or less the same diameter. The bigger diameter of bigger wheels allwas has to be offest with lower tires. Otherwise the car would be running really awkward (tires rubbing, speedo showing wrong etc.)

And when you go into grad school physics, you´ll find out the a car with bigger wheel diameter will normally accelerate less fast than one with smaller wheel because more of the wheels mass is more closer to the outer edge of the wheel. Even if the whole wheel/tire combo has the same overall weight as the smaller combo (which it often isn´t. Unless you spend lot´s of $$, the bigger wheels and tires usually are a lot heavier), the rotating mass will be bigger and therefore harder to accelerate (drawing more engine power just to get rolling). So on a dragstrip usually the smaller wheel with a higher tire will be faster.

Only that often the smaller wheels and their higher sidewall tires have a harder rubber compound and are not exactly perfmormance tires. Then, depending on the car, the engine might have enough torque to smoke the tires, braking traction and therefore loosing acceleration.

So the best dragstrip time for a particular vehicle is usually acchieved with the smallest diameter wheels size with fitting higher sidewall tires that manages to successfully bring the engine power on the ground.

Wanna loose your car in a pink slip drag race: Put some nice bling 22inch spinners on it.....