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Old 09-03-2008, 12:51 AM   #1
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Default 4 cell 1/12th Scale Stock Parity with LiPo

We have a 1/12th Scale 4 cell Stock class that is just getting off the ground at my club.

Some guys want to come over from Touring Cars, which is great. The only problem is that they only have LiPo cells.

Obviously there is a big difference between a Stock motor (either a 27T brushed or 17.5 BL) powered by a 4 cell pack (4.8 V) and one powered by a LiPo pack (7.4 V).

We would like to be able to have these racers race with us but clearly they can't use a Stock motor with the cells they want to use.

Can anyone suggest motors (both brushed and brushless) that will perform equally (to a 4 cell powered stock) when powered by a LiPo pack?

Some suggestions for the brushed motor have been the Tamiya Sport Tuned or the Johnson Grey 540 motor. Are either of these suitable? Is there a brushless motor slower than a 17.5?
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Old 09-03-2008, 04:04 AM   #2
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Try a Novak 21.5.
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Old 09-03-2008, 04:44 AM   #3
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A simplistic approach would be to say that the 17.5 in 4 cell mode means 11000 rpm (4.8 volts x 2300kV)

Working backwards from a 7.4 volt pack means you are looking for a BL motor with a kV rating of approximately 1500 KV. The 21.5 is rated at 1800KV so it may be slightly faster when used with a Lipo but fairly close
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Old 09-03-2008, 05:06 AM   #4
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Thanks guys. I'll pass that on to Club members.
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Old 09-03-2008, 05:56 AM   #5
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Quote:
Originally Posted by cannon View Post
A simplistic approach would be to say that the 17.5 in 4 cell mode means 11000 rpm (4.8 volts x 2300kV)

Working backwards from a 7.4 volt pack means you are looking for a BL motor with a kV rating of approximately 1500 KV. The 21.5 is rated at 1800KV so it may be slightly faster when used with a Lipo but fairly close
I think it has been sais on the novak mongoose thread that a 21.5 with 7.4 volts was muchmore like a 10.5 on 4 cells.

Furthermore, only link cars can accomodate lipo, and it requires buying new batteries.(smaller)
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Old 09-03-2008, 08:31 AM   #6
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Quote:
Originally Posted by cannon View Post
A simplistic approach would be to say that the 17.5 in 4 cell mode means 11000 rpm (4.8 volts x 2300kV)

Working backwards from a 7.4 volt pack means you are looking for a BL motor with a kV rating of approximately 1500 KV. The 21.5 is rated at 1800KV so it may be slightly faster when used with a Lipo but fairly close
i would tend to disagree. based on how you did the calculations a 21.5 with 7.4v would yield 13,320 rpm and a 17.5 with 4.8v would only yield 11,040. that is almost 2000rpm less for the 4 cell which also has more weight. And then looking at the IIC the 17.5 1/12 were gearing almost a 90mm rollout. with a 21.5, which has more torque to the motor, you could probably gear almost 100mm roll. so they are nowhere near close. I would have to agree with the later statement. with a mongoose and 21.5 and lipo, is closer to a 4 cell 10.5. And yes, we have a guys testing this at our track and a 21.5 with 1cell lipo would be a close match to 17.5 4 cell
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Old 09-03-2008, 09:15 AM   #7
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I doubt that you'll be able to fit a full sized LiPo into a 12th car without a lot of work.

On the "parity" side of things it's just plain not going to happen. There's no magic bullet that's going to get the two to run the same and they probably won't even be close. On top of that if you put a full sized sedan pack in a 12th car it's going to end up beng all cobbled together and the car won't work.

For the sedan guys, it's going to be about the same cost to buy a couple of 4 cell packs as it is to buy a new motor. 2 packs is all you need for club racing.

Once the single cell LiPo 12th stuff comes out things will be better but the new stuff won't be "the same as 4 cell". I say just deal with it and run all of the cars with the same motors so that the classes don't get thinned out. If ALMS can run cars with different speeds we should be able to without any trouble.
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Old 09-03-2008, 09:22 AM   #8
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look at the vintage class rules.. they are using a combined 4 cell / lipo running and that determines what motor you can use. I am unsure of how competitive it is, but you might try their thread and ask.
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Old 09-03-2008, 01:52 PM   #9
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The voltage equivalence is only half the problem. The major reduction in weight means the cars will have less traction, and less cornering ability.

12th cars rely on the weight of an NiMH pack to gain traction, and to rotate the car in a corner. With a LiPo pack, you lose both. If you go to 3.7v (single cell) the motors get less efficient, and with a 7.4v (two cell) pack the cars will be too powerful for the traction and cornering ability lost due to the reduced weight.

People do want to run LiPo in this class, but it's not going to be as competitive as NiMH in any sense. Many have tried, but if there was an answer, someone would have succeeded - no one has. It's a lost cause until the A123 batteries are developed, and then we can run 6v with the right weight.

LiPo is just a stepping-stone to the real future of RC batteries - A123 and nanotechnology that is both inherently safe, and with a voltage more suitable to electric classes.
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Old 09-03-2008, 02:21 PM   #10
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weight reduction could be a good thing or a bad thing.

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Old 09-03-2008, 04:16 PM   #11
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The only way Lithium Ion cells like A123 will be useful will be when they actually have cells in a sub-C package. A123 has zero cells that size available (other companies do have cells of a sub-C size). A 1000mAh sub-C Li-Ion cell really isn't all that impressive when you consider that it needs to replace a 5300 mAh Orion NiMh cell. (A123 has 3.3v cells so it's 6.6v for a pack by the way)

With SMC having single cell LiPo stuff on the way, I think that LiPo will catch on. Might not be as fast as 4 cell but it's much more convenient for club racing and the price of these packs is probably going to be very reasonable. Won't be the same but should be close enough to run them together without splitting up the classes. All cars would need to meet legal weight just like we did in sedan.
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Old 09-03-2008, 05:15 PM   #12
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i would tend to disagree. based on how you did the calculations a 21.5 with 7.4v would yield 13,320 rpm and a 17.5 with 4.8v would only yield 11,040. that is almost 2000rpm less for the 4 cell which also has more weight. And then looking at the IIC the 17.5 1/12 were gearing almost a 90mm rollout. with a 21.5, which has more torque to the motor, you could probably gear almost 100mm roll. so they are nowhere near close. I would have to agree with the later statement. with a mongoose and 21.5 and lipo, is closer to a 4 cell 10.5. And yes, we have a guys testing this at our track and a 21.5 with 1cell lipo would be a close match to 17.5 4 cell

I would have thought that a 13.5 on a 1S lipo would be close to a 17.5 on 4 cell nimh. Or is the weight difference and torque difference a bigger factor than I assumed?

losi 13.5 is 2360 rpm/v and gives 8732 [email protected] (1S lipo)
losi 17.5 is 1820 rpm/v and give 8736 [email protected] (4cell nimh)

Seems like a good match to me! 13.5 has 40W more power tho, maybe that's the difference. Weight of the the car in 1/12th probably won't be that big an issue, as you'd probably have to add weight to the lipo car just to keep the f/r balance, traction, and handling in the optimum range.
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Old 09-03-2008, 05:44 PM   #13
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have the sedan guys pool their money, two six cell packs equal 3 4cell packs, you can do the math to figure out how many total packs you need, you only need two packs each driver, (although i get by just fine with ONE). in the long run it'll be cheaper and easier then finding a lipo for a 1/12 scale plus cost.
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Old 09-03-2008, 05:49 PM   #14
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I would like to see the pan cars slowed down. There may be something else on the horizon, but for now lipo seems to be a workable option.
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Old 09-03-2008, 10:13 PM   #15
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All the pro-LiPo posts are from people who have never done it. Please, go and do it and then come back and tell us it is competitive. Those who have done it have not carried on with LiPo in 12th.

Learn from other people's mistakes, you are not going to live long enough to make them all yourself.
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