Scale 1/8 buggy

[nativepaul]

Hi Peeps, I'm thinking of making a scale(ish) 1/8th scale brushless buggy.

Does anyone make scale rally or offroad bodies to fit 1/8th buggys, (Wheels inside the arches not truck style)

I will quite possibly use a Hot Bodies Lightning 2 pro, as they are sold near me at a reasonable price without the engine, saving me the hastle of trying to sell it is this suitable for a brushless conversion? any mroe problems than the average buggy?

I have an feagio 8xl and 12 cells from a boat, but my boat is only a little oveer 2kg and I hear buggys are well over 3kg, will the 8xl be enough for the heavyer buggy?

I heard a CC mamba max will handle 12 cells if you use a reciever pack or external bec, and its the cheapest 100A controller that Ive seen, and by a fair margin, so I'd like to use one if it isnt gonna melt. so is it enough for 1/8 buggies or do they mean 12x 1b1500s instead of 6x ib4200s on a little motor in a 1/10th?

What about brakes. motor brakeing heat up controllers, so will the mamba max only handle that kinda system only if I use the mechanical brakes, or is it up to the task of stoping it as well as making it go? I'd like to strip as much out of it as I can to save weight.

Ive been told that if you dont use a slipper cluch in a brushless 1/10th you will eat gears, do I need one for a 1/8th? or are the metal gears man enough for the task compared to the cheap plastic ones found in tamiyas, if I need one who sells em? I guess a tamiya one would smoke instantly with 12 cells and a 1/8th to push? the 8xl has a 5mm shaft.

It seems like all there is to it is than making a motor mount, maybe removing the brakes, maybe removing the spur and puting a slipper and new shaft on it, adding a mounting tray for a battery, mounting the esc, and extending the body posts?
Is there more to it than that that ive not seen?

I know its lots of questions but if you can answer any of them i would be gratefull. Paul

[Sylvester]

Welcome to the forums!

Not sure on the body. The lightning 2 pro looks like a great candidate for BL!

The 8xl will be more than enough to power the buggy. A Mamba maxx is perfect on 12 cells with a UBEC. I would reccomend a fan on it. The MM has great brakes, so no problems there.

Id try to use a Plastic spur gear for the center diff, and a 5mm bore pinion off mike.

[nativepaul]

Glad the stuff I decribed will work well, at those budget prices hopefully it wont be too long before I can afford one. i was half expecting to hear that all that stuff wasnt suitable and I'd have to spend a lot more than that to make it work reliably.

Any idea how to gear the 8xl in a lightning 2 pro? ie what size spur and pinion, should I wait till I get the buggy then count the teeth and get direct replacements? or will I need to use a different size spur and pinion for a higher/lower than stock ratio in my set up?

[Sylvester]

What size is the spur gear? You will have to gear pretty high since its a 8xl on 12 cells.

[nativepaul]

If I go for a plastic spur like you sugest, I think the kyosho nylon ones fit (can anyone confirm this), RC-Monster sell them in 44 or 46 tooth versions, which would be better? if that fits mod 1 pinions he sells those form 10 tooth to 20 tooth excluding 17 and 19

thats quite a selection of ratios available if that stuff fits togeather on a lightning 2 pro

I have no idea what kinda ratio i am after tho, prahaps you can help me with a sugestion?

[nativepaul]

Can anyone help me with gearing? I downloaded Brian G's speed calculator but I dont have a copy of excel to run it on.

The 8XL is 2084kv, with 12 cells at 1.1v per cell thats 27,500 rpm

I would like to get a top speed of 40 mph on the 4.58" diamiter buggy tyres.

The plastic spurs are only available in 44 and 46 tooth but theres pinions from 10-20, so theres alot of potential ratios

I will have to buy both the spur and the pinion anyway, so I have no bias to use any particular one.

[nativepaul]

I just read that the 46 tooth pinion wont fit in the lightning without chasis mods, so I will have to use a 44 tooth spur.

[BrianG]

Originally Posted by nativepaul

Can anyone help me with gearing? I downloaded Brian G's speed calculator but I dont have a copy of excel to run it on....

You don't download it, you just enter the values right in the webpage...

The link is at the bottom of my sig...

Anyway, a 16T pinion will get you there based on the information you specified. BTW: the 8XL has a loaded kv of 1875. The value you posted is unloaded.

[nativepaul]

Sorry Brian, I must have been thinking of someone elses version of the software, and an inferior incomplete one at that? yours is easy to use and doesnt require any preinstalled software to function. Thank you for taking the time.

It looks like you used 1.2V/Cell for your calculations, that seems high to me, but I havent run round cells since 1700s, is that a realistic voltage under load?

[Serum]

You need to set the voltage per cell with Brians calculator, don't know what made you think it took 1.2V per cel?


A more realistic voltage is 1V (this is under a 60-70A load)

[nativepaul]

Thanks Serum, at 1v per cell I need a 19t pinion for 40mph, but as noone makes 19t pinions, should I go down to 18t ot up to 20t for the best comprimise between acceleration and speed?

[Serum]

Yeah, I would go for an 18, it will most likely give you the speed you want.

I ran my buggy on 14 GP3300's/7XL, it was calculated to go 60 km/h and it did 71 km/h

[nativepaul]

Ok I think I see what is happening here. The buggys speed is limited by the gearing and the motor is unloading, as it unloads its kv goes up and the cells put out more volts resulting in higher rpm and speed.

I guess thats why Brian used 1.2v/cell in his example, (loaded kv and unloaded V aproximating a medium load at a gear restricted top speed)?

In Serum's example, is his motor starting to unload at 60kph continue unloading untill at 71kph in no longer has any power to accelerate.

If he has room to do 71kph would he be better off, putting on a biger pinion so he doesnt start unloading untill 71kph to improve his 60-70 acceleration? or is it best to compromise that high speed acceleration to get to 60 qiucker?

Too much speed and you sacrifice acceleration and can get high temps.
Too much acceleration is wheelspin and a drop in speed for nothing.
Too much temps and everythine else becomes unimpotrant.

There must be an ideal compromise between speed, acceleration, and temps, 40mph was a noobs wild guess. Where do you with your experiance think it should be?

I know nothing of the limits of an 1/8 4wd buggy. What is the speed of a buggy that is totaly limited to traction, (and im guessing all buggys will have a similar finite limit) with a given torque (XL size motor, geared to suit). For example, if I was to gear a buggy for 10 mph, full throttle would give me wheelspin untill 10mph.
Would it do the same geared for 20? 30? 40?

Can anyone with experiance with an XL and mamba max (useing electronic brakes) tell me what loaded speed I can get, before I risk cooking something?

I think im learning a lot, but it's just a noobs crackpot theorys untill someone confirms or disputes them, or I prove or disprove them.
Paul

[Serum]

it took about 20-30 meters to hit fill speed.

[BrianG]

You're welcome paul. There was effort made to make my version as easy to use as possible. :)

As to the battery voltage rating: I do understand that the battery voltage will drop under heavy load. But that load happens under heavy acceleration (anytime the motor is forced to change speed in a short amount of time). Once you get near the top speed, the rate of acceleration will be less, therefore less load and therefore less voltage loss, so you get closer to the nominal voltage. What I do is make a worst case and best case scenario. I figure everything using low battery voltage (~1v/cell) and then again at a more ideal battery voltage (~1.3v/cell). The real figure is usually somewhere in the middle.

Generally, you match your motor and battery voltage so that you don't exceed ~35k rpm. If using a low KV motor, you can use higher voltage. If you are using a faster motor, you use less voltage. Then, you try to gear approriately for the speed you want ... within reason. Usually, you get less losses using higher voltage and a low kv motor since current will be less. But there are usually limits to how many batteries you can physically fit, not to mention the capacility of the ESC.

If you are going to use this buggy for racing on an offroad track, then 35mph seems to be the common desired top speed. If on-road racing, you can go faster more easily since there is more traction control and the terrain is easier to accelerate on.