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Old 03-30-2007, 07:19 PM   #16
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Originally Posted by twiggy
IMO, a flywheel would have the most effect on idle rather than acceleration, top speed, responsiveness etc.
there are always other changes you can make to the car which would counter-act these effects (if any) given by a heavier/lighter flywheel.
Your correct , a 1g lighter weight flywheel wont do much thats why weve all kinda moved onto the thought basis that the drivetrain as a whole is the major factor that can be adjusted.

So getting lightweight / hollow shafts and braking systems and such will take on the same effect as the flywheel since its all connected. Albeit the flywheel will be truly connected to the motor , where as the drivetrain wont effect the motor until clutch engagement.

At that time the entire system comes into effect and any case the car doesnt move without engagement. So the drivetrain should play out to these theories of weight on the drivetrain/flywheel. Other than idle. Where either way there isnt much to in terms of lightening that.
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Old 03-30-2007, 07:20 PM   #17
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Originally Posted by rmargiotta
It's just like a dirt bike... Heavy flywheel will make it idle better, and may take a little away from acceleration (but this would probably be negligible). Lighter flywheel throttle inputs more 'snappy'.
Yup , which a heavier flywheel will give you more grab out of the hole in the loose/low traction stuff.
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Old 03-30-2007, 08:21 PM   #18
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I found that a lighter flywheel also comes down off of RPM much faster and helps the car with turn in. As long as the motor is in its upper RPM range the clutch is engauged when you let off of the gas the motor will help slow the car(like a dirrect drive unit in circle track cars 1:1). A heavier flywheel will help slow the RPM drop when coming off of throttle and help the car roll through the turn. I found I use this as part of my set-up on differant tracks with sweeper-v-18degree hairpin turns.

Really good stuff here guys!!!!!!!
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Old 03-30-2007, 08:29 PM   #19
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Originally Posted by Artificial-I
Yup , which a heavier flywheel will give you more grab out of the hole in the loose/low traction stuff.

I found I can also use the clutch set-up to handle this issue. I use heavier flywheel weights and/or a softer clucth spring or less clutch bell clearance and/or a softer clutch pad to control how the car comes out of the corner.

I see that useing a differant weight flywheel is more or less a throttle response issue. Which can be very useful in a car set-up. Maybe something like useing our EXP setting on our radio for the throttle.

Great stuff guys.
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Old 03-30-2007, 08:48 PM   #20
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You posted that your flywheel is 1g lighter and your hollow shafts save much more weight, but what about the location of the removed weight. While the hollow shafts do save quite a bit of weight, I'm not convinced that it really gives you much better spool up. With the flywheel, only 1g loss, but the weight is removed far from the center of the crank which reduces the inertial mass by quite a bit. With the hollow shafts, you are removing weight right at the center. I'm not sure it makes any difference in how quickly the drivetrain can spool up.
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Old 03-30-2007, 10:39 PM   #21
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A heavier flywheel does not give more torque, give you more "grab" out of the hole or anything like that. It will not give more grunt, rip, punch. It lessens acceleration. A heavier flywheel will make the engine use more energy to accelerate and it will then decelerate at a slower rate. A lighter flywheel will have the opposite effect. That is why a heavier flywheel might make the car accelerate more smoothly on a low bite surface. More the engines energy is used to turn the drivetrain. Therefore less energy is available at the wheels. If you have low traction, you should be adjusting your clutch.

If all other factors stayed constant, running a lighter flywheel should be better as you will use less of the engines power to move the flywheel. Not everything stays constant however. A lighter flywheel will probably dissapate less heat. That could be a problem for the clutch or clutch bearings. Also you need the weight of the flywheel to keep your engine running at low RPM. A lighter flywheel will make the car a bit more jerky on acceleration and deceleration. A heavier flywheel will be smoother. It will accelerate slower as well as decelerate more slowly.
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Old 03-31-2007, 01:46 AM   #22
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I run an RC10GT in dirt oval, i used to run the stock lightweight flywheel at first and didnt think i had a need for the heavy one. Got bored one day figured what the hell i would give the heavy one a shot and just like mentioned above, the heavy weight fly kept the clutches engaged longer heading off into a turn when i got off the throttle, this in turn helped slow the car down at the end of the straights. I no longer have to tap the brake heading into the turns if i hit my normal marks i can even drive it in a little deeper now if needed and grab a little brake to pass somebody easier.

Now as for speed differences, i havent noticed any, some say the heavier flywheel will increase the top rpm of the motor a small amount, which in thery increases the top end of the vehicle. On the other hand i have heard the lightweight one does the same exact thing because of the lighter weight. I am not sure which thery is more correct without a very accurate way of measuring the engine RPM. But like i said i have noticed zero difference in speed. I have dropped about 2 tenths a lap on average with the heavy weight but the main advantage is that the car is easier to drive on a highspeed bluegroove dirt oval track.

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Old 03-31-2007, 03:25 AM   #23
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Unfortunately, rotational physics is a bit beyond my grasp and I could not find the correct formulas that apply to our discussion here. It seems that in order to resolve the issues of torque and rotating mass of the flywheel and drive train, you need to understand moments of inertia, angular acceleration and how all of this pertains to discs, cylinders, and hollow cylinders of varying mass and radii. Throw in the appropriate formula for torque and now we're getting somewhere.

Understanding moments of inertia is just a starting point....

This is really just a physics problem. We need to find someone that can explain all of this in layman's terms, give us the correct formulas so that we can plug the numbers into the equations and then come to a consensus as to what all of this really means.

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Old 03-31-2007, 03:51 AM   #24
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I have modified my standard mtx-4 flywheel. Now its 3g lighter and I feel that the engines responser better and quicker. I will never miss it.
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Old 03-31-2007, 06:08 AM   #25
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so which is better ??? so confuss .....

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Old 03-31-2007, 06:37 AM   #26
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the one thing that has not been mentioned is that the track would pay a part in this question if you had a large track with long straights yes you would reach peak revs faster with lighten flywheel but you don't want to be at peak revs for long or will blow engine so gearing comes into question wich could slow acceleration. if it is short and techical by all means a lighten flywheel would be best, but you could gear a standard flywheel for similar acceleration.

from my experience over the last 12yrs+ with road and race cars it would come down to persons choice there is no right or wrong answer to this question: Yes lighten flywheel- faster acceleration, faster deceleration
Yes standard flywheel- slower acceleration, slower deceleration

but with standard flywheel the extra weight would help you keep some engine momentum for acceleration.

the only way to answer this, is for people to try both on the same track an see which works best for them, if you are faster with lighten stick with it, if you the same or slower with lighten you know the choice is harder which way to go.

sorry long winded but my 2 cents worth
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Old 03-31-2007, 07:23 AM   #27
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here is my response i posted on mtx4 thread, i still think it is a valid point;


dude, i think artificial is wrong. a lightweight transmission on a car is beneficial especially when the engiens we use rely on rpm to produce power. Your example of a motorcross bike is wrong to apply to a rc car. why? because a flywheel on a motorcross bike is on the rear wheel. you are getting more traction not due to the heavier flywheel, but due to the added weight on the rear wheel which acts as a lever pushing the tyre down more (ie imagine accelerating a bike as 5 guys push down on the rear).

guys, a lightweight drivetrain is the best modification you can do to a car. Remember that these engines rev upt o 41K + so therefore the quicker you get it up to it's peak rev range (whatever it may be...20,000 to 30,000, or whatever it is) then the more punchy the car will be and the better power it will have out of the corners.

a lightweight drive train does not make the car less grippy, this is a falacy, that originates from a bad example (ie motocross). a car that is close to minimum weight will wear out the tyres less (to an extent) use less fuel (to an extent) place less strain on the engine (to an extent) and will be more punchy out of corners.

drivetrain mass i remember someone saying here ages ago, is equivalent to x2 static weight (or something like that) basically what i am saying is that rotating mass is x2 more important and worthwhile eliminating than static weight.

artificial your comment of pivot balls being better to lighten goes against what you said about motocross bikes and your flywheel example. ie, if your flywheel motorcross example was correct, then the heavier pivot balls would also seem a better option if we continue your analogy. ie more weight then on the ends of an arm, according to the motocross flywheel principle would be better as it places more downwards force on the wheels, hence more grip right? BUT OF COURSE THIS IS WRONG! reduction of unsprung weight is important, and so is rotating mass.

experiment by drilling holes in a flywheel (ACURATELY!) and you will eventually find the maximum you can drill it out to without affecting idle. this is the lightest the flywheel can go without ruining the idle...done problem solved, and fallacy of a heavier flywheel is good analogy solved!

remember these are not drag cars, or motocross bikes, two things that require and rely on weight over rear wheels to lay down power effectively.
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Old 03-31-2007, 07:41 AM   #28
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Originally Posted by Scott Fisher
I don't really think a heavier flywheel will give more torque.
torque is directly proportional to the moment of inertia of the rotating body. Moment of inertia is depends on mass. more mass > more moment of inertia > more torque

Now notice the lower left equation on the image i attached. The solution to this simple differential equation is on the 2nd pic attached. Notice that the angular velocity on the left side also depends on the moment of inertia. More moment of inertia>> lesser maximum angular velocity
lesser moment of inertia>> greater maximun angular velocity

in short, heavier flywheel will give you more torque and a lighter one will give you more RPM..
Attached Thumbnails
Heaver vs. Lighter Flywheel-torque1.jpg   Heaver vs. Lighter Flywheel-torque2.jpg  

Last edited by fritzD; 03-31-2007 at 07:55 AM.
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Old 03-31-2007, 08:13 AM   #29
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These little buzzards have virtually no torque, so the lighter flywheel is generally used to lessen rotating mass- which improves acceleration out of the corner..... a$$uming the track can take it

NASCAR and similar "roundy round" (as I call 'em) cars are already moving and need the acceleration (RPM) out of a corner. That's why they use crankshafts that weight 45#, titanium valves and pushrods, two ring- lightweight pistons, aluminum flywheels and featherweight titanium rods.

Dragracing cars need torque to get off the line and you won't find a 45# crank in a drag car- more like a 55# crank (I'm talking a SBC engine for similarity). Drag engines generally use heavier, beefier engine components- steel rods, stainless steel valves, hardened steel pushrods, etc....

We are more like the "roundy round" cars.... already moving. We use lightweight stuff to improve acceleration (RPM). We don't look for torque out of these engines, although HP is a function of torque..... the dyno lines of HP/torque ALWAYS cross at 5252 RPM. At 5252 RPM we are probably "thinking" about engaging the clutch. With identical real life cars, the one with the heavier flywheel (the testing variable) would leave the starting line from a dead stop quicker and would accelerate out of a corner slower. It's simple physics.

The bottom line is that we would use the lighter flywheel to let the engine spool up faster out of the corner. ANY rotating mass that is lightened will improve accleration (RPM)- shafts, pulley hubs, axles, rims, etc....
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Old 03-31-2007, 08:54 AM   #30
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Just to add to the mix...I can remember reading an article a very long time ago which said a heavier fly wheel can be of advantage when racing on tracks that have large uphill sections. The theory was that the rotational mass would allow the car to retain higher rpm longer in this particular instance. Just more food for thought.
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