racing tranny 2

[wangp1283]

It's often said that the moment of inertia of the transmission is very important to a light, racing car. So important that shaving a pound off the transmission rotating part is going to do more good than shaving a pound off the vehicle body. Can someone explain? Is it really significant? I think there is also a formula that relates all this, and it has a lot to do with the gear ratios.

Another thing I've noticed is that a lot of racing cars and motorcycles have gear ratios that are very closely spaced. For example, a lot of those Japanese road bikes may have a 5 speed transmission with a overall ratio span less than 3 or 2.5. (if you divide the tallest gear by the shortest, they are usually less than 3). Why is this?

Does anyone know the gear span (and ratios if possible) of a typical F1 car or Rally car?

Thanks

 

[Greenlight]

Metalguy said "My AA/FC had a very heavy triple disk Crowerglide. Throttle response in neutral on a blown fuel engine is *immediate*, even with a heavy flywheel/clutch.

Now, where does your formula account for engine RPM rate of increase?"

Whenever I am trying to grasp a concept I try to look at the problem at the extremes (often exaggerated extremes).

Rev the engine without a flywheel attached from idle to 8,000 (or whatever rpm is your max).

Next, attach a 200" diameter, 5" thick, solid steel flywheel to the engine and rev it from idle to 8,000 rpm.

Now you can easily get the picture.

Remember, inertia is to angular coordinates as mass is to linear coordinates.

Try lifting a mass of 1kg quickly (say 0.1 sec) from a height of one meter to a height of two meters. Now try lifting 5kg just as quickly. It takes more force to do it.

Now try rotating a 1kg, .4m dia., aluminum disc from zero rpm to 5000 rpm in 0.1 sec and repeat with a 5kg, .4m dia., steel disc. The heavier disc will require more torque.

It's easy to say the "heavy" Crowerglide rev'ed "instantly", but if you measured the time in milliseconds you would easily see the lightweight clutch (lower inertia) would rev a quicker "instantly". The heavy Crowerglide was likely not so heavy when you compare it to the crankshaft, balancer, pulleys, frictional losses of the pistons, etc. which also had to rev "instantly".

The example I presented of the 9 and 10 lb flywheel and the equivalent weights should give you an idea of how much quicker your car, boat, airplane, etc. will accelerate. If you car weighs 5000 lbs., then the 24 lbs. of equiv. wt. won't make much difference. But if you car weighs 960 lbs. (as does my dragster, total wt. including the driver) the 23 lbs. of equiv. wt. in first gear makes about .02 sec. in the first 60 ft.

I hope this helps you envision the affect of lower inertia.

Regards,

Greenlight

 

[kenre]

Another way to look at it is in the operation of an Inertial Dyno. All the engine is accelerating is the flywheel.
I can vary the accel rates from idle to max needed rpm from 2 secs to 6 or 7 seconds just by adding a heavier flywheel.

Ken

 

[Metalguy]

Greenlight

What I have a problem with is that engines usually don't accelerate at constant rates. Once past the torque peak the rate slows down, assuming a constant load. In real life that load is constantly increasing, so how can your figures show a simple weight equiv. no.? My example with the Bonneville car was to show just how slow the high gear accel. can be. Flywheel effect is lower as the rpm increase slows down. This is elementary.

Take your super light dragster. What RPMs did you use for launch (0 speed) and for 60' speed? How many seconds?

Lighter flywheel, etc. are almost always good, but the effect diminishes with each higher gear (as has been well noted), and also with other factors, so a single number doesn't do it.

 

[NormPeterson]

Once you start to worry about the applicability of an approximate correlation factor you need to go back and rewrite your acceleration formulae in terms of F=M*a plus several cases of T=I*alpha where relations do exist between a and the various alphas.

Norm

 

[carnage1]

It's often said that the moment of inertia of the transmission is very important to a light, racing car. So important that shaving a pound off the transmission rotating part is going to do more good than shaving a pound off the vehicle body.

An important piece of this question is where the weight is being reduced. Rotating mass in general is more "important" pound for pound than body weight. However the rotating mass of any transmission internals that are not mechanicly part of the output shaft are the most important. The less mass in the clutch disk, input shaft and intermediate shaft the faster you can shift and the smaller your synchros can be (if you have them). The faster you shift the more time you are puitting power to the ground.

As an added bonus weight out of this part of the transmission is still taken off of the total rotating mass and the body mass.

 

[gt1guy]

Would a 10lb, 8in. dia. flywheel accelerate faster than a
10lb 14in dia. flywheel given the same input?

Kevin

 

[NormPeterson]

Rotational moment of inertia is an integral involving mass*radius^2, so in most cases the answer would be "yes". But you could contrive a combination with severely skewed radial mass distributions in which the opposite would be true.

Norm

 

[bentwings1]

I'm with metal guy being ex AA/FC driver. Even back in the mid 70's you would never get the pedal to the floor before the engine blew believe me. The throttle response of a blown fuel motor is rediculous. Look at it another way. Assuming the blown alcohol motor motor produces 3000 hp at wide open throttle and torque and hp cross at 5250 rpm even at 2500 rpm idle going to WOT in .1 sec (slow) the crank rods pistons flywheel and clutch only weigh 200 pounds and you probably have at least 2000 hp and resulting torque all trying to rotate a measley 200 pounds up to speed. It doesn't make one iota difference here 5 pounds 10 pounds. What counts is how strong is everthing. It's said that the crankshaft on today's top fuel motors can twist up to 20 degrees. I'd have to see it myself but a lot of onboard research has been done on this stuff.
I don't remember what the Crowerglide weighed but a complete 3 disc was one you didn't want to pack around very far. Probably 35 pounds with aluminum flywheel. We went from an aluminum flywheel to a 1" solid steel flywheel and went quicker and faster no other changes.
In really old days it wasn't uncommon to have 60-70 pound flywheels on a 283 Chev. This is a small hp motor and very high reving. They would buzz them to 9500 on the line and drop the clutch. The inertia of the flywheel plus the still added hp would launch the car like a shotput. It was how fast you could row the gears that mattered. The motor hardly changed speed at all. As noted close ratio transmissions ruled the roost with this set up.

 

[BiPolarMoment]

Can't argue with that that... but bear in mind that it is an N^2 relationship, so once you are out of first gear then engine inertia matters less and less. Also, it is far easier to pull a couple of kg out of the body than 0.5 kg (or whatever) out of an engine.

True, but it's really easy to shave a pound off your wheels--at least from stock. A stock car often has wheels approaching or exceeding 20 lbs, most which is at the circumference. Aftermarket aluminum alloy wheels (or magnesium) can usually be less than 10 lbs (heavily dependent on size). This can affect performance in easily noticeable ways--and not just acceleration but also handling as the unsprung mass is reduced.

 

[peterhill]

It reduces power needed to accelerate the drive line. It has most effect in 1st, less in 2nd gear ratio, not a lot in 3rd and nearly no effect in top or overdrive. It has no effect on constant speed running as drivetrain is not accelerating. It has most effect on fast running parts like the engine flywheel and less on slow parts like wheels.

If you have enough power to spin the wheels in 1st and 2nd any effective increase in power will just spin the wheels faster and may cause wheel spin in 3rd as well. BMW F1 1500cc M13 turbo had enough power to spin the wheels in 4th.

If you don't have enough power to spin the wheels or wheelie in 1st, then a reduction in flywheel mass may help achieve this. The driveablity issues this brings may actually reduce 0-60mph times, impact 1/4 mile times and will give poor engine feel in normal road use, with very rapid drop in engine rpm between gear changes and lack of inertia resulting in a tendency to stall easily.

Conclusion.
It has little use and would be a waste of time and money removing mass from rotating parts on FWD vehicles that can wheelspin anyway. Just makes more expensive tyre smoke, quicker.
It has no useful effect on modern powerful tunned RWD cars that can wheelspin in 1st and 2nd (or higher gears). Just makes yet more expensive tyre smoke.
It's only effective for AWD/RWD cars that can't wheelspin in 1st with rules, regulations or laws that prevent the addition of proper power adders like turbocharger, supercharger or nitrous oxide, all of which work just as well in top gear as 1st. In fact they work so well in 1st that they often reduce the power added in 1st by reduced boost or staged nitrous to avoid making expensive tyre smoke. So that really means racing of vintage or classic vehicles from a time when it was a part of the tuners normal methods. (It's time for "professional" tuners to wake up, get smart, learn a bit of physics and move on)