Latyshev

Hello folks!

Im a long term automotive fan from Moscow (yeah, far far away). I own a lotus elise, drive it daily and race it. However, i have never engineered anything. So, i cant resist the urge and i need to scratch it!

I decided to start small, just a weekend project with total budget on everything no more then $2k, but do all bits which can be made in home enviroment myself.

Before this day i have never heard of rc ic cars (i did know there were planes, but always assumed they were bloody expensive). I was looking around the internet for small engines and to my surprise discovered a huge world of rc cars!

I stayed on the net for some hours and did some research, probably ended up with more questions then i started with, but stumbled upon this great forum. I really hope some of you will lend me a hand with all this [X(]

The project im after is a fun engineering exercise. It will be a small sportscar. Im going to weld the chassis, which will be aluminium tubular spaceframe. Suspension double wishbone all round. Body from carbon fibre (hopefully). I am trying to make it as light as possible and as well made as possible, with some serious punch to boot of course!

This is the inspiration (lotus concept circut car)




First it was going to have electrical motor, because i though that any kind of small ic engine is way out of my budget, but i was wrong! It amased me when i discovered the whole community and industry associated with rc cars [8D] I started searching all over online shops looking at different components and recommendations. Needlest to say i got lost.

Bah, enough whining, on to the questions...

1. There are two types of engines - 2 stroke and 4 stroke. Browsing this forum i got the impression that pretty much everyone uses 2 stroke engines for thier cars. Why is that? As far as i understand 4 stroke engine will provide better torque across wide range of rpms, while 2 stroke really starts to sing only at the top end? Which one should i use?

2. What kind of gearboxes do you guys use, if at all? I want to put the engine in the middle in transaxsel configuration. If there are any gearboxes, is it possible to place it behind the rear wheels? I.e. front wheels -> "driver compartment" -> engine -> rear wheels -> gearbox.

3. Are there any clutch-like mechanisms i could use of the shelf, or do i have to make my own clutch plates and actuate them through servos?

4. How about braking system? I dont get at all how rc cars brake... Id like to use traditional rotors and pads though.

And the last one - i browsed through different shops, but there seem to be only plastic pre made wheels for specific models. Since i am going to employ my own suspesion setup, how do you think it would be possible for me to connect up the wheels without manufacturing specific parts (which is going to be very hard to do)?


Sorry for a long post and thanks for help in advance!

Best regards,
Val.

SAVAGEJIM

Good luck on your endeavor. Most memebers here are not engineers and few here have little knowledge about designing space frames (space as in 3dims, not space as in Star Trek), stresses, physics and the such, so very few will understand even how to do the simplest of calculations concerning designing the parts you are attempting. Anyways, I will attempt to answer your question.

1: We use two stroke engines because the power to weight ratio. Two strokes primarily produce power everytime the piston goes up and down. The 4 stroke only puts out power half as much since it requires intake/ehaust strokes before the compression/combustion strokes. The 4 stroke has more moving parts required, this means more mass to accelerate. 2 strokes are very simple and very light. Not only that, RC cars and trucks are geard very low and therefore require the higher RPMs of 2 strokes. Very few cars are designed with higher gear ratios to accomodate a low RPM high torque engine (I frankly cannot think of one).

2: RC trucks use a very simple gear boz. Most are only 2speed. If you open one up, do not ezpect any helical cut gears, planetary gears, etc. These gear boes are very simple. The way they switch gears is by centrifical action. In between low gear and high gear is a disc with a finger that deploys when a certain RPM is reached. This finger engages the high gear. There are some 3speed gear bozes out there, and they use the very same disc and finger mechanism (way simple, but necessary to keep production cheap). Transales? I wish. That would be way cool. RCs are so simple that some of them do not even have a 2nd gear! They zip along on only one gear!

3: There are many clutch mechanisms on the market to choose from and all are made to directly bolt onto any SG shaft engine. These clutches again also work by centrifical action. Such include the Centaz style clutches used by on-road RC cars and the swining shoe arms of a clutchbell and flywheel used by RC trucks and most buggses. These are designed with the high 2 stroke RPMs in mind and the spring tensions are such that the weights cause the clutch to engage at the proper time.

4: Forget about 4 wheel brakes, they do not eizst in RC cars and trucks. Instead, they use a universal rotor and pads either in front of the transmission or behind the transmission. These are very simple devices that are mechanically engaged by the use of a simple pull of the throttle servo turning the opposite direction of W.O.T. If you are up to the challenge, feel free to design a 4 wheel disc brake system for RC cars. I'd really like to see that.

Latyshev

SavageJim, thanks a lot for reply! It is very helpful indeed.

I did some further reasearch and and it looks like there are no rear wheel drive cars?

What i am hoping to do with transmision and engine placement is on the picture: (exaust is not under gearbox, but to the side)

bozla

Four wheel drive cars a far superior than 2wd in RC. They do exist though.
[link=http://www.molzermoweryracing.com/movies/theshoe071705.wmv]Here is a video[/link] of some 5th scale F1 cars. They are 2 wheel drive and have four wheel brakes I believe. They use 22cc? four stroke engines I believe.

scottywilkes

they are not 4 stroke and aint 22cc. They use 2 stroke like weed wacker engines except they are engineered to put out higher performance. Yes some come with 4 wheel rotors for breaking and are even hydrolic. Most engines are 23, 26, 27 or 29 cc but u can get larger. Now if u are going to make a car as u say a 1/5 harm, FG or technokit chassis and layout would be a great place to start as for as designing. They have all of the features u want in your car and use normal gas pump fuel mixed with 2 stroke oil. Good luck with your engineering it sounds like a great project

Latyshev

1/5 th is a little bigish for what i want. Im looking at total length of the car to be around keyboard sized (1/10 scale i think?) and be around 1.5 kilos in weight with around 1.5 hp engine.

Although, if some components are non-existant at thatb level, then maybe i should scale up?

gt5500

RC cars use two strokes because they make loads more power then equivalent four strokes, you are right in saying that four strokes have more torque but as you will know from racing Elises the higher the power to weight ratio the lower the torque required, hence motorbikes having very high revving engines that produce high BHP's but low torque. A properly designed RC car will have a very high power to weight ratio when compared to a real car so torque is not required as much, to go fast an RC car needs to have high BHP and high revs. If you are looking at a 1/10 scale car you will only be able to use a glow engine as spark ingnition engines are much to big at will not provide the power and revs needed whilst remaining light weight. The brakes on a 1/10 scale car are very simple and operate by using a cam to force to metal or fibre backed metal plates against a metal or fibre disk. Due to the weight of a 1/10 scale car this system is very effective as it brakes on the transmision as most cars nowadays are 4WD this gives 4W braking. However on 1/8 scale 4WD buggy's you will find they have a centre diff with a brake either side of it, this is a good system as it allows you to adjust the brake bias front to rear. I think you spaceframe idea is a bit over the top unless you are building a 1/5th scale car a spaceframe will weigh much more then an off the shelf model with a simple plate chassis, with the right metal a plate chassis is very strong and light. As for wheels you are going to have to use off the shelf wheels but they are very easy to retro fit as nearly all wheels use a hex hub design, the hubs are available easily on ebay and to fit them to you axels you would just need to make your axels the right diameter and then drill a hole through it for the drive pin to fit through. Your idea for a engine and gearbox layout is a good one and its used by many cars already, one thing to note though is that if the car is 4WD the engine has to be offset to allow the propshaft to pass it, the weight of the engine is counterbalanced by the fuel tank but as the fuel runs down the weight balance will change towards the engine side. Personally I would not recommend RWD a nitro car will normally just have far too much power to try to out through the rear tyres only, most RWD nitro cars spend their whole lifes spinning about, 4WD is a much better option. Hope some of that was helpfull. Check out the Team Associated NTC3 for some ideas on engine placement that has a similar design to what you are describing.

bozla

Heres a pic of my kyosho fw05. Its a little bit dated by todays standards. This is shaft driven, but most new nitro cars are belt driven. Belt drive allows better engine placement as yo don't have to put the engine to either side of the shafts. It doesn't mean its slow though! If others can upload pics it might be useful. Your best bet is to get to a circuit and have a look at others machines for good ideas.

Also, here is a link to a guy who built an RC (with tubular spaceframe, just as you want to) from a chainsaw engine.

[link=http://www.proactivedesign.net/eric/loves/radio-control/home-made-car/default.asp]Home made RC![/link]

Doahh

I really reccomend making a brushless electric car... Less of a hastle and it would probably be easier because then you could use the back half of a pan car and not worry about a cluch tuning breaking in breaks and whatnot

CustomTamiyas4Life

My Feedback: (34)

I agree. It would be much easier to design an electric car than a nitro.

-Inverted-

I agree, however I think he is going for the scale factor.

i8tweety

My Feedback: (11)

If you're looking to manufacture most components for your project, I'd recommend building to 1/5 scale dimensions (check out RCUniverse's large scale forum for more details on 1/5 vehicles). The larger scale will make it easier to design and manufacture suspension components, shock towers, bulkheads, etc. This is because the smaller the scale, the more precise the machining needs to be.