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Old 06-03-2003, 05:02 PM   #4711
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Default Re: Re: Re: Overdrive

Quote:
Originally posted by modellor
I agree with your method. Its apparent you have some knowledge of true overdrive however your chart is still inaccurate unless you know the given rpm of the engine. If someone hands you a new engine like say motorman during a meeting and it has 4000rpm more than the engine you were running then your overdrive calculations are worth squat and you have to recalculate for each give tire diameter.
Hello modellor,

I looked over your post and thought about it a little. I think that you might possibly be confusing the overdrive ratio chart with possibly a roll out chart? Overdrive ratios are fixed and completely indipendent of engine RPM. I say this because we are talkng about a ratio of pulleys. The pulleys are of a fixed size and stay the same size irrelevant of RPM. As example taking a basic pully set up where pulley A rotates 2 turns for every rotarion of pulley B. If pulley A rotates 400 times per minute pulley B will still rotate at 50% of the speed or 200 times per minut. If you take A up to 1000 RPM, B still rotates at 1/2 the speed as it is a fixed ratio. So again with the RC cars, all of the pulleys are a fixed ratio so the only way to alter the ratio is by adjusting the tire diamter. This is what the overdrive ratio chart shows, how the ratio is altered by adjusting the only 2 variables that there are, the front and rear tires diameters.

Now I will say that if we realy wanted to get detailed thoreticaly the tire diameters should expand as the tires rotate at higher RPM's etc, but with foam tires this is negligable and may only account for 1/10-2/10's of a mm so it is not factored in. Hope this helps. Let me know if you have any aditional questions or thoughts on this.
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Old 06-03-2003, 05:11 PM   #4712
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Yep. BTW, thanks for the word about the Hudy tire box and pens the other day! Those dang boxes aren't cheap, may try "engineering" one of my own... wish me luck!

Eddiethefish: mounted up the Kawahara sway bar the other day, so nice! Thanks!
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Old 06-03-2003, 05:17 PM   #4713
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Quote:
Originally posted by kevink2
Yep. BTW, thanks for the word about the Hudy tire box and pens the other day! Those dang boxes aren't cheap, may try "engineering" one of my own... wish me luck!

Eddiethefish: mounted up the Kawahara sway bar the other day, so nice! Thanks!
Not a problem kevink2. The boxes do cost a bit. Suggestion call Serpent and ask about buying their trays, this is the cool thing. Just get a big tuperware type storage box w/ lid at Target to put it in. Saves money and still easy to sort through the stuff.
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Old 06-03-2003, 05:19 PM   #4714
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That is where you are making your mistake. True overdrive is affected by engine RPM and rollout both. Overdrive is never fixed and changes constantly as another factor changes throughout the rollout of the car (tire wear).

For example (not true figures obviously):

Engine RPM = 1000
Rear roll out is set at 1revolution per 1000rpm
Front roll out is set at 2revolutions per 1000rpm
Overdrive is calculated and set to 1.000

Now, if you increase the engine RPM x2 (for easy understanding)

The new Engine RPM = 2000
New Rear Rollout = 2 revolutions per 2000rpm
New Front Rollout = 8 revolutions per 2000rpm

When you have a rollout difference of x2 between front and rear for a fixed rpm and you double the engine rpm the front roll out actually quadruples (doubles for engine rpm double and then doubles again for the x2 rollout difference between front and rear)

The new overdrive ratio would then become 4 times less if the pulleys and tire diameters remain constant.

This is true overdrive.

Last edited by modellor; 06-03-2003 at 06:00 PM.
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Old 06-03-2003, 06:23 PM   #4715
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Quote:
Originally posted by PMT
Not a problem kevink2. The boxes do cost a bit. Suggestion call Serpent and ask about buying their trays, this is the cool thing. Just get a big tuperware type storage box w/ lid at Target to put it in. Saves money and still easy to sort through the stuff.
May I suggest placing a pack or two of Silica gel pack (desiccant) with those tires. I do this with all my electronic parts, as well.
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Old 06-03-2003, 06:31 PM   #4716
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Quote:
Originally posted by modellor
That is where you are making your mistake. True overdrive is affected by engine RPM and rollout both. Overdrive is never fixed and changes constantly as another factor changes throughout the rollout of the car (tire wear).

For example (not true figures obviously):

Engine RPM = 1000
Rear roll out is set at 1revolution per 1000rpm
Front roll out is set at 2revolutions per 1000rpm
Overdrive is calculated and set to 1.000

Now, if you increase the engine RPM x2 (for easy understanding)

The new Engine RPM = 2000
New Rear Rollout = 2 revolutions per 2000rpm
New Front Rollout = 8 revolutions per 2000rpm

When you have a rollout difference of x2 between front and rear for a fixed rpm and you double the engine rpm the front roll out actually quadruples (doubles for engine rpm double and then doubles again for the x2 rollout difference between front and rear)

The new overdrive ratio would then become 4 times greater if the pulleys and tire diameters remain constant.

This is true overdrive.
modellor, I think that there are a couple of issues causing confusion here. 1 being roll out. Roll out and a Roll Out Chart are use to make gearing choices. Roll out only factors in Rear Tire Diameter so as an example using an MRX-3:

74mm Rear Tire Dimeter

1st Gear 17T Pinion w 49T Spur = 33.6mm
2nd Gear 20T Pinion w 46T Spur = 42.1 mm

Versus

1st Gear 17T Pinion w 50T Spur = 32.9mm
2nd Gear 20T Pinion w 47T Spur = 41.2mm

As you can see this information is used to make gearing choices. From this chart you can easily see how rear tire diameters & gear choices will affect you. The mm reading listed above is the forward movement that you will get for every rotation of the pinion.

Now to touch on some of the items in your post:

"True overdrive is affected by engine RPM and rollout both."

This I belive to be incorrect. If we disregard tire wear for a minute overdrive is a fixed ratio, this ratio can only be affected by changine the teeth count on your pulleys. (again not factoring in tire wear or tire diameters). Roll out is also a fixed number again this is dictated by your gear ratio and rear tire diameter.

"Overdrive is never fixed and changes constantly as another factor changes throughout the rollout of the car (tire wear)."

That your overdrive ratio will change over the course of a run due to tire wear I will agree on. However please remember that overdrive charts are designed so that you can set your ratio at the start of a run so for the sake of the charts, the tire diameter is fixed and as such you might as well consider it a constant. However as you start to recognize what your tire wear will be you can start with a smaller split etc. usign the charts so that you guestimate what to start with so that your car comes in (has the ideal ratio) towards the middle or end of your run..


"For example (not true figures obviously):

Engine RPM = 1000
Rear roll out is set at 1revolution per 1000rpm
Front roll out is set at 2revolutions per 1000rpm
Overdrive is calculated and set to 1.000

Now, if you increase the engine RPM x2 (for easy understanding)

The new Engine RPM = 2000
New Rear Rollout = 2 revolutions per 2000rpm
New Front Rollout = 8 revolutions per 2000rpm

When you have a rollout difference of x2 between front and rear for a fixed rpm and you double the engine rpm the front roll out actually quadruples (doubles for engine rpm double and then doubles again for the x2 rollout difference between front and rear)

The new overdrive ratio would then become 4 times greater if the pulleys and tire diameters remain constant.

This is true overdrive."

This I disagree with as it is phisicaly impossible for this to be true. You have to remember that all of drive belts (both front and rear) are being driven off of one fixed point, the 2 speed shaft. For every rotation of your rear diff gear your inner lay shaft gear is rotating 2.53 times. This is fixed and does not change based on engine RPM. For every 1 rotation of your rear axle with the 16/19 pulleys your front axle is turning 1.06 times (6% overdrive). This again is fixed and does not change based on engine RPM. This is all mechanicaly fixed and would only change if your pulleys some how increased in tooth count based on engine RPM which is impossible. The only possible variable is the tire diameter (front versus rear) which since we use foam tires we can change.

If we were running rubber tires where we do not have the ability to alter tire diameter there would not be a need for a overdrive ratio chart as this would be fixed at 1.0 or neutral which is what you get when you run the stock pulleys with equal tire sizes front and rear. So no overdrive and again the drive ratio stays the same irrelivant of RPM.
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Old 06-03-2003, 07:18 PM   #4717
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Hi,

Can someone tell me that i need to get -2 degree foam tire or regular 0 degree foam tire for MTX-3?

Thanks.
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Old 06-03-2003, 08:17 PM   #4718
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Quote:
Originally posted by US_MATRIX
Hi,

Can someone tell me that i need to get -2 degree foam tire or regular 0 degree foam tire for MTX-3?

Thanks.
Both types of tyres can be used I use the mugen type tyre because it is avalible to me. But when using the regular 0 degree tyres check your track width.
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Old 06-03-2003, 08:18 PM   #4719
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For people that have used the 3 racing brake disc how have you found it? (besides being in the back of the car) Is it any good?
It seems the mugen one is a lot more pricey so would like to know if this brake disc is as good as or not..
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Old 06-03-2003, 09:01 PM   #4720
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As I will be going to run my car in the next hour, I'd like to post the pictures of my car. Should anything goes wrong with the car, I still have the pictures to show others of how it once was like.
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Old 06-03-2003, 09:04 PM   #4721
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another one.
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Old 06-03-2003, 09:07 PM   #4722
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another angle.
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Old 06-03-2003, 09:07 PM   #4723
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last one.
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Old 06-03-2003, 09:13 PM   #4724
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Quote:
Originally posted by eddiethefish
last one.
Nice looking car Eddie, be carefull with it or else you might be doing this!
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Old 06-03-2003, 09:14 PM   #4725
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Quote:
Originally posted by TBR
For people that have used the 3 racing brake disc how have you found it? (besides being in the back of the car) Is it any good?
It seems the mugen one is a lot more pricey so would like to know if this brake disc is as good as or not..
TBR the Enneti on is realy nice and not too expensive. I belive that CorseR posted a picture a ways back.
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