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Old 01-16-2003, 02:24 PM   #61
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there is a pretty simple formula for calculating overdrive ratios.



Anyone care for it be posted or are we taking this further than people really want to go?
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Old 01-16-2003, 02:33 PM   #62
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Yeah. Feel free.

Is it the one I sent to you for the Yokomo.
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Old 01-16-2003, 03:15 PM   #63
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Overdrive is not as complex as you are trying to make it.
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Old 01-16-2003, 03:23 PM   #64
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True. Overdrive is not complex at all.

Its all rather simple mathematics.

I was just saying that a lot of different variables are taken into account when calculating it. There are simple charts available and simple formulae to work out the ratios but they dont give an exact figure. But the figure they give is not far off correct and that is why people prefer to use it.

I prefer using the simple version myself. Doesn't put the brain under so much pressure

Need to keep it clear to concentrate on my major problem. She is always nagging about me racing so much
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Old 01-16-2003, 04:49 PM   #65
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Quote:
Originally posted by modellor
I understand what you are saying but look at the wheels as the third order. Take into consideration the volume of the wheels (by which I mean different diameters equal different mass).

At a given rpm the rear wheels will rotate at a certain speed related to the gearing given. If the front wheels are smaller they have to rotate faster to keep up. Now, if you apply overdrive the front wheels go faster again at the percentage set.

Now, if you increase the rpm the rear wheels will increase speed to match the rpm of the engine. But because the front wheels are smaller and have less volume they will rotate at a greater speed again. The percentage of overdrive may stay the same but the front wheels will rotate faster at a greater percentage in comparison to the rear wheels.
assuming a car was set up with zero over drive (the front and rear drive ratio the same, the front tires and rear tires diameters the same), and we all agrees that the rear tires will wear at a faster rate than the front tires, right ? so as time goes by, and according to modellor's explanation, the rear will be in over drive mode because the rear tires will be smaller in diameter ! but i donít think that is what I had seem on the track.

not trying to make things complicated, just trying to understand the connection between rpm of the engine and over/under drive ratio, and now the mass of the wheel is in the equation tooÖÖÖ

inertia of the wheels may or may not affect the fianl drive of the axial, but even if it does, i donít think the effect is big enough for it to become a dominant factor.

data
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Old 01-16-2003, 04:52 PM   #66
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Okay, so OD is front pulling faster, either due to pulley or fronts larger than rears.

Now, another question - y'all say that UD (fronts pulling slower, due to pully or fronts smaller than rears) is easier to drive and has more steering. This seems wierd to me. I would think it would be squirrely as all getout - if the rears are driving more than the fronts, unless there's a one-way in the front, wouldn't the rear be trying to step out all the time? Or was that an assumption?
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Old 01-16-2003, 06:15 PM   #67
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Alright modellor, I looked over you post a couple of time. At first I thought that you were smoking some of that wacky tabacky from Holland! I think that I'm starting to see what you are getting at, from your post:


>>>I understand what you are saying but look at the wheels as the third order. Take into consideration the volume of the wheels (by which I mean different diameters equal different mass).<<<<

I agree, smaller wheels are lighter and larger wheels are heavier. So I'm asuming that you are now looking at rotating mass???

>>>At a given rpm the rear wheels will rotate at a certain speed related to the gearing given. If the front wheels are smaller they have to rotate faster to keep up. Now, if you apply overdrive the front wheels go faster again at the percentage set.<<<<

Correct explanation, this is overdrive.

>>>>Now, if you increase the rpm the rear wheels will increase speed to match the rpm of the engine.<<<<

Correct.

>>>But because the front wheels are smaller and have less volume they will rotate at a greater speed again. The percentage of overdrive may stay the same but the front wheels will rotate faster at a greater percentage in comparison to the rear wheels.<<<

Not correct. What I think that you are trying to say is that becuase the front wheels are smaller they will weigh less which means that their rotating mass is less than the rear wheels. Becuase of this, theoreticaly the front wheels should accelerate faster than the (heavier) rear wheels when an equal force is applied to them. However since our cars have front one ways which free wheel when the front wheels are going faster than the one way pulley (like when braking, hence no front brakes) under acceleration (or I should say when on power) it is phisicaly impossible for the front wheels to rotate more than x percent faster than the rear wheels. X being dictated by the overdrive ratio (overdrive percentage) which is fixed because the front one way would release and their would be no drive aplied to the wheels. Which means that the percentage of overdrive will stay the same (I'm excluding tire wear) and that the speed of rotation of the front wheels will be dictated by the overdrive ratio and will remain constant while accelerating or under power. This ratio will only change when decelerating or braking as then in essence the front wheels are free wheeling.
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Old 01-16-2003, 06:33 PM   #68
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are you guys hurting yourselfs?? overthinking the overdriving...

if you overdrive the geearing for the front and then run a smaller tire( as moddelers examp, your goal is to have them rotating to pull equally, very simillar to a one to one rarto and same size tires right...

try not to make to much work of it,
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Old 01-16-2003, 06:35 PM   #69
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Sorry about the length of the last post but just trying to be as precise as possible.

Data, it is important to remember that just because the front & rear tires are the same size it does not meen that your drive ratio is 1. The thing to remember is that realy, drive ratio is dictated by the pulleys and the tire diameters are used to adjust this.

Now you posted: so as time goes by, and according to modellor's explanation, the rear will be in over drive mode because the rear tires will be smaller in diameter ! but i donít think that is what I had seem on the track.

This is entirely possible. Keep in mind that we are talking small amounts of tire wear making diferences in overdrive. Again with an MRX-3, Front 67mm Rear74, OD Ratio 1.003. Front 67 Rear 74.5 OD Ratio .996 By this example by just going up .5mm diameter in the rear we have gone from on overdrive scenario (front pulling more than the rear pushing) to an underdrive scenario (Rear pushing more than the front is pulling).

Now Boomer in regards to your question. OD ratio is important for changing the handeling of your car by altering the drive characteristic. Think of it as a 4 wheel drive rally car. On short, technical tracks with lots of turns you would want to run more front OD as the front wheels will help pull the rest of the car through the turns. On longer, fast, sweeping tracks you would want to run more rear drive as it will allow you to acheive a higher top speed (larger rear wheels) and the car would be less twitchy/more stable. This is of course simplified but it is what you would feel by changing the drive ratio. Don't let all of this freak you out, once you see an OD Ratio chart it will be a lot easier to understand.
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Old 01-16-2003, 09:58 PM   #70
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TKT... where are you buying your tires from? Mugen stock tires are at least 10mm smaller.... GRP foams are 65mm stock height
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Old 01-16-2003, 10:08 PM   #71
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Quote:
Originally posted by TKT
Now Boomer in regards to your question. OD ratio is important for changing the handeling of your car by altering the drive characteristic. Think of it as a 4 wheel drive rally car. On short, technical tracks with lots of turns you would want to run more front OD as the front wheels will help pull the rest of the car through the turns. On longer, fast, sweeping tracks you would want to run more rear drive as it will allow you to acheive a higher top speed (larger rear wheels) and the car would be less twitchy/more stable. This is of course simplified but it is what you would feel by changing the drive ratio. Don't let all of this freak you out, once you see an OD Ratio chart it will be a lot easier to understand.
No, I understand OD. I use it - I actually found it by accident. My rear wheels got smaller and I didn't notice it at first, but my handling got WAY better. . .changed to a new set and my handling was worse. . .investigated and found out about OD.

I'm just asking about how UD is more stable. . .with the rears pushing faster than the fronts, I would think that the rear would be constantly wanting to step out.

UNLESS we're assuming a front one-way, which I'm not running so that may be where my misunderstanding is coming from.
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Old 01-16-2003, 10:13 PM   #72
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Quote:
Originally posted by sparksy
TKT... where are you buying your tires from? Mugen stock tires are at least 10mm smaller.... GRP foams are 65mm stock height
Don't know about GRPs (too expensive), but Ulfra's come standard with 7 mm split. It might even be 67/74. It's been a while....
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Old 01-16-2003, 10:38 PM   #73
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Quote:
Originally posted by TKT
Data, it is important to remember that just because the front & rear tires are the same size it does not meen that your drive ratio is 1. The thing to remember is that realy, drive ratio is dictated by the pulleys and the tire diameters are used to adjust this. )
well, i did say the drive ratio being the same front and rear.......

Quote:
Originally posted by TKT
[B]Now you posted: so as time goes by, and according to modellor's explanation, the rear will be in over drive mode because the rear tires will be smaller in diameter ! but i donít think that is what I had seem on the track.

This is entirely possible. Keep in mind that we are talking small amounts of tire wear making diferences in overdrive. [B]
hehe, not if you are running at revelation. the rear tires on my car will be all gone in half a day at revelation. probably the way i drive...... *sigh*

and yes, i am cheap, i won't throw away the tires until i can see the white of the rim through the foam.
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Old 01-16-2003, 11:28 PM   #74
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Guys, if you need a web enabled site to do overdrive calculations, this is a good website.

http://home.zonnet.nl/realrolfje/pro...t/overdriv.htm
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Old 01-17-2003, 03:58 AM   #75
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Quote:
Originally posted by TKT
Not correct. What I think that you are trying to say is that becuase the front wheels are smaller they will weigh less which means that their rotating mass is less than the rear wheels. Becuase of this, theoreticaly the front wheels should accelerate faster than the (heavier) rear wheels when an equal force is applied to them. However since our cars have front one ways which free wheel when the front wheels are going faster than the one way pulley (like when braking, hence no front brakes) under acceleration (or I should say when on power) it is phisicaly impossible for the front wheels to rotate more than x percent faster than the rear wheels. X being dictated by the overdrive ratio (overdrive percentage) which is fixed because the front one way would release and their would be no drive aplied to the wheels. Which means that the percentage of overdrive will stay the same (I'm excluding tire wear) and that the speed of rotation of the front wheels will be dictated by the overdrive ratio and will remain constant while accelerating or under power. This ratio will only change when decelerating or braking as then in essence the front wheels are free wheeling.
Right. So by increasing the RPM this in effect alters the overdrive because you can run a smaller percentage to obtain the same effect.
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