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Old 02-06-2003, 03:30 AM   #16
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Efficiency - From what I've studied over the years, Axiom is correct. DW has some good points but I think he's confusing separate issues. Efficiency being discussed in this scenario is the relationship of power being offered by the motor and the car's ability to take that supplied power and lay it down on the track. The car in this case does not care how efficient the motor is, its given X amount of power, now the question becomes, how much of the provided power can it apply to forward motion on the track. So the ability to lay down more power means that the chassis is more efficiently using the power given to it. But like dw said, efficiency (of the motor) does not equate to being the fastest. An easy example of this is driving your regular car. If you go 0-60mph in 8 seconds, and then do the same thing in 12 seconds, you will use more fuel doing it in 8 seconds but you will also get there faster.

Oneways -

Getting back on topic, diffs many times waste energy supplied by the motor. So when you apply throttle hoping for more pull or power laid down on the track, the diff may waste some of this desired energy.

Agreeing with what several have said, here's why a one-way can be more efficient while turning, especially off throttle on corner entry. And I'll give you a simple test to do to prove it to yourselves. When your car turns on the track, the front wheels follow a different turn radius than the rear wheels do. But since both the front and rear diff center pulleys are forced to turn at the same speed, something has to give. Without the one-way, its the traction of one or more of the car's tires that must give or the energy is lost in the diff action. But with a one-way, the front wheels are free to rotate at speeds faster than the rear wheels are and independantly of each other. If you take this to extremes, as long as your rear tires have sufficient traction in a turn, most all of the speed provided by the motor will be applied to the slower turning end of the car (which is the rear in this case) and the front is free to rotate even that much faster than the motor speed.

Here's a simple test to display the battle that goes on in a 4wd TC's drive train with standard diffs in the car. Take the pinion out of your car so that the drivetrain can spin freely. You don't have to turn your car on to do this. Put your car on a smooth surface like a glass counter, big tweak board, or something smooth and clean like that. Now turn the front wheels all the way to one side (left or right). Now press down on the middle of the car to apply force down on the wheels. Now slowly move your car forward while maintaining this downward pressure. In most cases, it will take less than about 12" to feel your car greatly resisting moving any further forward. This is because the drive train is fighting itself and the smooth surface doesn't want to let the tires slip. Doing this same test with a oneway will not give this same result.

There's alot more that I could get into about one-ways effects onpower but I don't want to hijack the thread...haha. It can be pretty complicated but really relies so much upon the setup and traction of each and every tire of the car. Would love to hear feedback and I'll add more later on if you want. Great topic.
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Old 02-06-2003, 04:02 AM   #17
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Quote:
Here's a simple test to display the battle that goes on in a 4wd TC's drive train with standard diffs in the car. Take the pinion out of your car so that the drivetrain can spin freely. You don't have to turn your car on to do this. Put your car on a smooth surface like a glass counter, big tweak board, or something smooth and clean like that. Now turn the front wheels all the way to one side (left or right). Now press down on the middle of the car to apply force down on the wheels. Now slowly move your car forward while maintaining this downward pressure. In most cases, it will take less than about 12" to feel your car greatly resisting moving any further forward. This is because the drive train is fighting itself and the smooth surface doesn't want to let the tires slip. Doing this same test with a oneway will not give this same result.
How about with front and rear diffs fitted and a centre one way pulley?
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Old 02-06-2003, 08:28 AM   #18
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I didn't want to have to go into this much detail but I realise people aren't clear on where I am coming from.

I'm going to ignore corner entry efficiency because I think it is largely irrelevant as it just decreases braking effect and doesn't effect battery power consumption.

Consider corner exit with a car with 2 diffs with gentle throttle application. The front wants to travel further (and hence faster) than the rear. With a differential at the front only the inside front wheel (the most lightly loaded wheel) needs to break traction. The front diff slows down and the difference in speeds between the two diffs is compensated for by an decrease in the speed of the inside front which is acting as a brake. At low speeds the power wasted by this inside wheel are high because not much weight is transferred and so the tyre produces a lot of traction. As cornering speed increases the weight on the inside front wheel reduces as well as the friction between tyre and track so the power wasted by this wheel is less at higher speeds. If lots of power is applied very suddenly then the inside rear tyre will also break traction. The car will virtually stop accelerating and lots of power will be wasted.

Now compare that to a car fitted with a 1 way diff on corner exit. For gentle throttle applications the front wheels freewheel. No wheels have to break traction so efficiency is high. The car is 2wd effectively.
However in order for either front wheel to pull out of a corner the inside rear wheel must break traction. As the speeds of the diffs must be the same. The 1 way therefore wastes more power than a diff would under this situation as the traction of the inside rear tyre is higher than that of the inside front because there is more weight on it.
Additionally in order for the outside front to pull the inside front must also slip causing further power losses.

The 1 way will provide more traction for really violent throttle applications though. With a diff the two inside wheels accelerate to sillyspeeds whilst the outer ones can become virtually still. With a 1 way 3 wheels have to accelerate instead of 2, and as the outside front can be more heavily loaded ( in terms of weight) than the inside rear at the middle of hairpins where this commonly occurs this makes a big difference.

So in summary the 1 way is more efficient on corner exit for gentle throttle applications and really aggressive ones but a diff is more efficient for medium thottle applications.

This doesn't take into account the fact that your motor and transmission are less efficient under high loadings (and at very high rpm too with a belt drive). With a 1 way you have to accelerate hard out of the corners and keep your speed higher simply to make sure you compensate for the earlier braking neccesary. Accelerating harder is guaranteed to use more battery power so I still remain doubtful about the efficiency benefits of 1 ways on most tracks.
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Old 02-06-2003, 02:29 PM   #19
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Thanks for your replies. Now it's getting really interesting.

However, I think I have to re-phrase the subject: Will the use of one-ways increase top-speed? And especially, the use of both a one-way pulley and a one-way diff?

With the explanations above, the following sounds a little like simplified marketing bla-bla.

From www.hpiracing.com: Front One-Way Differential (Pro 3)
Extra turn-in, higher drivetrain efficiency and more top speeds make this the option of choice for experienced racers.
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Old 02-06-2003, 08:55 PM   #20
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Quote:
Originally posted by Dave Kurschner
There's alot more that I could get into about one-ways effects onpower but I don't want to hijack the thread...haha.
Hijack it
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Old 02-07-2003, 01:56 AM   #21
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Mort - If you do this test with just a center one-way, the front belt would be allowed to rotate faster than the rear belt so this binding won't really be there. A center one-way has a different effect than a front one-way does. Since as I said, the front belt (front diff) can spin at speeds equal to or faster than the rear, this relieves the resistance of the drive train by letting it spin faster. However, you still have the effects of a front differential. The front diff provides some resistance to the front wheels being able to just free wheel so the only resistance you should feel is dependant upon your diff settings.

Cole - In my opinion, there is very little to be gained in terms of straight line speed (top speed) using any combination of one-ways. In order for them to benefit you or affect your overall speed, the front and rear tires would have to be different diameters. One can theorize that as you are accelerating, the rear wheels compress more due to the weight transfer effectively making the rear tires slightly smaller in rollout diameter. If that's the case, then the front wheels become slightly larger in comparison for a short amount of time. Since the front is larger, more of the drive trainís power can be utilized through the front diff to pull the car more because a front one-way acts as a solid axle under power in a straight line. But this is again going to an extreme level of detail that's not really worth thinking about for longer than it took you to read the last 2 sentenses..hehe. If anything, the added weight of one-way mechanisms provides more inertia to the drive train which can have its benefits for straight speed.

But races are won and lost in the corners. Such a small fraction of lap time differences between racers actually has to do with straight away speed. The use of one-ways and the possible gains of efficiency or speed with one-ways are primarily meant for aiding cornering capabilities.

ISO Octane - Stickem up! I'll start out with a little about on-power effects of one-ways. Allot of people compare the on-power performance of a one-way to the performance of a spool (solid front axel). Although they can be similar, that's not entirely true. In most cases, you should get more middle and exit corner pull with a car equipped with one-ways much like a spool. Because oneways are either on or they are off, there's no in-between. But the difference is that with a spool, the front wheels are forced to spin at the same speed as the drive train, no matter what. And think about this, if both front tires are glued to the track with a spool, and you turn, since the front outside wheel turns fastest, and the inside wheel has to either equal this speed (which means it would scrub) or stick to the track at a slower speed. If the inside wheel sticks to the track at its lower speed, that means that the spool now wants to turn faster than the current drive train speed and can actually transfer energy back into the drive train forcing it to spin faster. This is part of why you may hear clicking sounds in a car like the Losi, while cornering, when running a spool. (You are forcing the belt to try and skip) That one may take a while to think through .
But back to one-ways, I'm going to kind of replace a long explanation with this. The front inside wheel (during a turn) will turn the speed of the drive train since its not allowed to turn slower like a diffed wheel, therefore under full power the outside wheel (given it has enough grip) will still freewheel faster than the drive train. In the real world, itís somewhere in-between due to the normal side drift of the front wheels that takes place. So you may actually get pull from that outside wheels with enough drift in the car. So in most cases, a spool will give the feeling of more "pull" out of the corners than a car with one-ways will. Enought babbling for now, I'll set the hostages free.

Last edited by Dave Kurschner; 02-07-2003 at 02:02 AM.
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Old 02-07-2003, 03:07 AM   #22
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Currently I see one advantage of using both the one-way diff and the pulley: During off-throttle going into the corner (off-throttle, not braking), you'll get less braking from the drivetrain, thus the car wont slide as easy, as with a one-way diff only.

I'm going to try it soon. First I build my new car in a full time 4wd version. Then set it up. Afterwards I mount the one-ways and re-adjust the setup, if nessecary.
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Old 02-07-2003, 05:53 AM   #23
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Quote:
Originally posted by Dave Kurschner
Cole - In my opinion, there is very little to be gained in terms of straight line speed (top speed) using any combination of one-ways. In order for them to benefit you or affect your overall speed, the front and rear tires would have to be different diameters. One can theorize that as you are accelerating, the rear wheels compress more due to the weight transfer effectively making the rear tires slightly smaller in rollout diameter. If that's the case, then the front wheels become slightly larger in comparison for a short amount of time. Since the front is larger, more of the drive trainís power can be utilized through the front diff to pull the car more because a front one-way acts as a solid axle under power in a straight line. But this is again going to an extreme level of detail that's not really worth thinking about for longer than it took you to read the last 2 sentenses..hehe. If anything, the added weight of one-way mechanisms provides more inertia to the drive train which can have its benefits for straight speed.
Diff weight; My one-way diff weighs 20 grams, while my ball diff weighs 24 grams. However, I'm waiting for graphite outdrives. I'm a little excited to what difference those graphite diff out drives will make. Even thogh I might reduce the weight with around 10 grams, then the weight are very close to the center of the diff. I like to try it out.

I have the the option of using a one-way pulley with one teeht less. This way the front will free wheel when on-power and the car will behave like an rear-wheeler most of the time. This should be an advantage on a straightline. Turning in, it should be the same as a 4wd with a normal one-way. But coming out of the turns might be difficult. If throttle are applied to much and too early, the car will slide a little, and the front wheels will go in action and pull the car back on the decided course. It will work, but cost som time. I think it'll be too hard to handle, at least for me.

As you said, it's in the corners races are lost and won.
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Old 02-07-2003, 02:31 PM   #24
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Cole, what you're referring to is called overdrive. When the car has a smaller pulley in front than rear, the front will spin faster than the rear at all times. This will make the car pull harder out of corners but will also give it a lot of front bite. The overdrive setup is good for very high bite tracks and mod foam where you generally need lots of front bite. However, most people do not run the overdrive setup because it messes up the internal ratio and its just not a popular thing that most people do.
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Old 02-08-2003, 12:21 AM   #25
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Thanks Axiom.
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Old 02-10-2003, 12:22 AM   #26
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Quote:
Originally posted by Axiom5B
Cole, what you're referring to is called overdrive. When the car has a smaller pulley in front than rear, the front will spin faster than the rear at all times. This will make the car pull harder out of corners but will also give it a lot of front bite. The overdrive setup is good for very high bite tracks and mod foam where you generally need lots of front bite. However, most people do not run the overdrive setup because it messes up the internal ratio and its just not a popular thing that most people do.
Hmmm, did some re-thinking.

If the front pulley is 1 tooth smaller than the rear pulley, then when the lay shaft turned once, the rear belt will have moved one tooth more than the front belt. Thus the rear will spin faster.
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Old 02-10-2003, 01:43 AM   #27
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ColeTrickle : I think you may be wrong here.
Look at it this way.
Imagine you have a single belt car like the XXX-S.
For the sake of simplicity, let assume the single drive belt has 200 teeth. We use a marker and mark one teeth to track it.
Let the front pulley be on over drive and have a 49 teeth pulley.
Let the rear pulley be on underdrive and have a 50 teeth pulley.

On completely one revolution of the single drive belt, the front pulley will have turned 200/49= 4.08 revolutions.
The rear pulley would have turned 200/50= 4.00 revolutions.

The front differential is therefore being overdriven by 2 percent.

You need to rethink ur arguement such that it now takes "one more teeth of movement to spin the rear diff one complete revolution".
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Old 02-10-2003, 04:27 AM   #28
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I'm dont think that a single belt example is comparable;

I'm thinking of the center pulleys mounted on the center layshaft in a two belt car. Not the diff pulleys.
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Old 02-10-2003, 05:43 AM   #29
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ColeTricke :Hmm if you are talking abt undersizing the layshaft pulley that powers the front diff, then yes I believe the rear axle will be overdriven. Just how in blazes you want to drive a car like that I dun know. I think the more conventional thing to do will be to oversize the said front pulley on the layshaft to overdrive the front pulley.
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