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high speed on low kv motor and nimh 8.4v please help!

high speed on low kv motor and nimh 8.4v please help!

Old 08-13-2020, 02:34 AM
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Post high speed on low kv motor and nimh 8.4v please help!

hi guys, I'm new to this forum and this is my first thread please go easy on me.

So my university wants us to make a electric rc car which can do 100kmph but due to the budget and rule of using only a nimh battery i am not sure how to go with this
so far i have decided to go with a low kv motor 3300kv or 4000kv with a 60 amp esc and do a bigger pinion gear and smaller spur gear.
its a grass field where we will be racing, not yet decided to do 1/10 or 1/8. we have to design the chassis our self as well.
any advice on mad gear ratios as its just a 50 meter straight track. and have to do 5 laps.
I'm new to this field and any help or advice would mean a lot

thanks a lot guys!
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Old 08-13-2020, 03:01 AM
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so 62 mph in the US speed .. on a grass track.. My hats off to you if you can do this .its going to be a real challenge to do this...bummer you wont be able to post pics or a video .. send some 1 a link to a vid of this challenge when its done.i looked back at this reading 50 meter track and had to do 5 laps..then I read its a straight track so down and back...
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Old 08-13-2020, 03:21 AM
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impossible
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Old 08-13-2020, 05:32 AM
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How tall is the pinion to achieve those speeds?? How large is the motor, physical size?
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Old 08-14-2020, 05:41 AM
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Just keep slapping on additional NiMh cells until you reach the max voltage that the ESC is rated to handle, if possible I would get this 60A ESC here which is rated to handle up to 15S-NiMh:
https://hobbyking.com/en_us/hobbykin...en_us_products

Only problem is that ESC is out of stock, so I would recommend this ESC here which is higher quality:
https://www.hobbywingdirect.com/prod...nt=20187925828

I don't see any reason why you can't hit your target speed with a 3660-3500KV motor in a 1/10 SCT platform that shares the same size of a 1/8 buggy, the SCT platform will be lighter to make it easier to reach top speed, you'll need to modify the battery tray to fit the larger battery but totally doable with this platform here:
https://www.ebay.com/itm/HOBAO-Hyper...QAAOSw0wRaCSQx

and this motor here:
https://www.banggood.com/Surpass-Hob...r_warehouse=CN

Here's a decent range of pinions to start out with:
https://www.banggood.com/5PCS-ZD-Rac...r_warehouse=CN

More info here:
What gearing for upgraded electronics?


Here is the same platform with a 13T pinion and smaller 540 sized motor running on 2S Lipo which is almost half the voltage which clocked 41mph on my GPS, so there's no reason why you can't hit 60mph+ with higher voltage/gearing:

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Old 08-14-2020, 06:30 AM
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In a Field of thicket of grass at 62 mph?? Or we talking like golfing greens or turf..
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Old 08-14-2020, 06:58 AM
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the op cant change certain parameters. Like 8.4v nimh. or the grass track...and esc. he just cant add cell to make that speed possible..he is stuck to 6 cells nimh..so 15 cells on 4000kv .
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Old 08-14-2020, 06:59 AM
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Tenergy Propel series 1.2v 5000mah nimh subc
30A+ discharge dump. Basically 30c. $5$ each

or

Tenergy Platinum series is 50amp discharge. 7.50 each.

Tenergy tool grade series 2200mah, about 20 amp discharge 1.90 each.

These bad boys have the punch needed. There may be higher rated discharge cells out there.

Tenergy has a new website: power.tenergy.com

their old one is still up yet.
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Old 08-16-2020, 06:51 AM
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Originally Posted by the rc guy View Post
the op cant change certain parameters. Like 8.4v nimh. or the grass track...and esc. he just cant add cell to make that speed possible..he is stuck to 6 cells nimh..so 15 cells on 4000kv .
I didn't see where he was limited to 6 cell Nimh....or am I having a duh moment... I like this idea though. Build Challenges can be VERY fun.
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Old 08-16-2020, 08:03 AM
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read the title of his post.. here -- nimh 8.4v please.. that be 6 cells in my book
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Old 08-16-2020, 08:53 AM
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Originally Posted by the rc guy View Post
read the title of his post.. here -- nimh 8.4v please.. that be 6 cells in my book
Packs are rated by nominal voltage. The nominal voltage of a NiMH cell is 1.2V so an 8.4V NiMh pack is 7 cells.
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Old 08-21-2020, 03:47 PM
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Obviously you're gonna have to make your chassis light as possible, I would go with a top fuel dragster type design, probably borrowing from the Rj speed legends style vertical rails rather than a more traditional plate. Do a straight axle versus independent suspension which will keep costs and weight down at the cost of handling, and maybe look into some monster truck tires for the rear with no inserts (something with a weaker carcass and not so stiff) so that they can swell as the rpm's increase and effectively giving you extra gear ratio.... Make your chassis long for extra stability, use an older stadium truck front suspension for the front to try and keep some sort of controllability in it since your gonna be doing this on grass. use a front spring that is just heavy enough to keep the front end from crashing the ground when you hit the brakes and tune your shock oil to control weight transfer...
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Old 08-21-2020, 04:53 PM
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Without doing the whole experiment. I'd start with what's needed to get 100km/hr. 4000kv @ 8.4v should net 33600 rpm. Get the circumference of the tire (i'd advocate 1/10 for weight) then calculate what rpm the tire will need to spin to get 100km per hr. add some fudge factor but the difference between the rpm of the tire and the rpm of the motor will give you the ideal drive ratio. how you get there with pinions and spurs will be up to you. I'd look at a spur attached to the rear wheel in a direct drive type setup. Weight will be key to avoid cooking either the ESC or the motor but consider your aero so the thing doesn't get airborne.
Should be fun but I'd start with the math/physics before settling on the final solution. I doubt you learn the 6 Ps at Uni.... Proper planning prevents piss poor performance.....
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Old 08-23-2020, 03:41 PM
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Originally Posted by the rc guy View Post
the op cant change certain parameters. Like 8.4v nimh. or the grass track...and esc. he just cant add cell to make that speed possible..he is stuck to 6 cells nimh..so 15 cells on 4000kv .
Yup....duh moment. lol
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Old 08-24-2020, 10:24 PM
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I think given the 8.4v limit you should scale down to 1/18 or 1/16th scale. I would build something based on the mini t or the wltoys 14401. With an 8000kv motor geared just right you can get close to 100kph.
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