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Upcoming 1/4 scale trophy truck build

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Upcoming 1/4 scale trophy truck build

Old 02-16-2011, 10:34 AM
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IAmSupernova
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Default Upcoming 1/4 scale trophy truck build

I'm planning to very soon start building a 1/4 scale trophy truck, and I wanted to bring the idea here first to get any input that someone might be willing to give me, or let me know if there's an inherent design flaw I've over looked. I know the best way about thing is to keep things as simple as possible, but to keep costs down (as is, it's going to cost atleast $1,000) I've had to be pretty creative which prevented me from mantaining absolute simplicity. A must for me was 4-wheel independent suspension, so that kinda jacked up the price a bit.

This will be my first hobby grade R/C car, ever (the only other R/C cars I've had were years ago, and they were all toys) but I'm good with electronics and good with my hands. I know I could get a decent kit for the same price I will be dropping into this, but really the reason I want to build this is to build it. Very few peices will be pre-fabricated. The TX/RX of course, the throttle servo, the engine (will be getting one from a chainsaw) the wheels/tires, bearings, gears/rack gear will be the only pre-fabricated components. I will even be making a custom servo for steering from a controller board and a windsheild wiper motor. I haven't decided yet whether to buy actual U joints, use impact drill U joints, or make my own.

I've spent the last week or two drawing out the majority of it in Google Sketch up (nearly everything is in exact scale to use as a blue print, and if anyone happens to come across it and wants to use it as a base design, feel free) just so I could an idea of how it's all going to be put together.. Nearly everything you see is actually a modification of an earlier design.. The steering system alone went through 4 seperate revisions before I decided on one.

The actual sketchup file can be downloaded from here: http://sketchup.google.com/3dwarehou...3a90959f14b03f (The 3D view embedded in the site itself isn't showing up yet) and I've taken some screenshots and uploaded them to my photobucket for those who can't be bothered to mess with SU. Those can be found here: http://s670.photobucket.com/albums/v.../Project%20RC/

There are 4 sealed bearings in the front (2 for each wheel, placed back to back) and 4 in the back (one located just beside either side of the drive gear, and one located just below the lower suspension mount). There are also 4 U joints used in both the front and rear.

So, any thoughts?
Old 02-16-2011, 12:54 PM
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Default RE: Upcoming 1/4 scale trophy truck build

I think it looks good part from one thing...
What sort of gear ratio are you looking at there?

Judging by the drawings your ratio is way off.
[&:]


Depending on what sort of chain drive you're using you might have to consider using a second set of chain and sprockets to reduce the drive.





Pocketbike chain and sprockets are good as they're small and can be used straight off the bike, but mounting such a small sprocket on a chainsaw clutch can be difficult, if not impossible.



Depending on how the clutch is set up you might be able to do something like this:












That sprocket came from the cam in a dirt bike motor actually.




I'm looking forward to seeing more of this project though.


Old 02-16-2011, 01:29 PM
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IAmSupernova
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Default RE: Upcoming 1/4 scale trophy truck build

The chain sprockets/gears aren't to actual scale, I just used those to get a feeling for how to put it together.. I'm figuring on something like (but not exact) ratio of 3 to 1 (15 teeth on the axle, 44 on the clutch). I'm wanting to go for speed, I'm not too concerned with acceleration, or sheer power, this is just my personal project, it won't be competing in anything.

I made a Yahoo answers question about gear ratio trying to make sure I was gearing it correctly for the spectrum I want to hit, but it was brought up I may need to step it down. I was hoping to avoid that, I'm not expert, but as my mind conceives the design it would easily tack on an extra 100-200 bucks just for that. I have plenty of room to move the engine mount up, so I could add a bigger drive gear onto the clutch and increase the driven gear if that would help.

I have to admit, when it comes to gearing, I'm a bit inept, so I'm really welcoming any input on this. Let's just assume the chainsaw engine has a max rev of 8,000 rpms, and the tires are at their widest diameter is 8 inches, is there way to gear it using only 2 gears and a chain to reach speeds of around 30-50MPH (not scalee)? If not, how would I need to gear it to reach those speeds? It would be very easy for me add this in, but I'd rather not for cost reasons.
Old 02-16-2011, 01:48 PM
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Default RE: Upcoming 1/4 scale trophy truck build

Forget Yahoo answers....
If you want info about RC you should ask your questions on this forum.


I can tell you now that 3:1 won't work.
The car will stand still while you slowly burn your clutch to crumbs.

For a big car like that I'd go for at least 8:1.

Another thing to consider is at what RPM the clutch engages.
I've never had any experience with factory RC cars, but I know that a lot of them clutches engage at 6000 or 8000 RPM depending on what strength spring they use.

The problem with using chainsaw/weedeater motors is that the clutch engages at way lower RPM.
Depending on the clutch setup you may be able to

A) Get stiffer springs
B) Drill holes in the clutch shoes to make them lighter and in turn get the clutch to engage at higher RPM.


I say this to you now to avoid disappointment - don't worry about making calculations on paper about how fast the car will go.
Build it and be prepared to rebuild and redesign as you go.

Also, if you're worrying about the cost of adding an extra set of sprockets and chain in, you're on the wrong track.
Setting a budget before you've even started building is a bad idea.


Are you that same guy posting on RcGroups by any chance?





[EDIT]

Also, I just realised that you said 30-50MPH. [X(]
Unless you've got access to a private Nascar track or something, I don't think driving a large scale RC car at anywhere near those speeds is advisable.





Old 02-16-2011, 02:55 PM
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IAmSupernova
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Default RE: Upcoming 1/4 scale trophy truck build

Well, I don't nessecarily intend to run the car at max speed all the time, I want to be able to run the car at a nice idle speed and get an acceptable speed, rather than just run the engine at max output all the time, maybe I'm wrong, but I figure that'd be a lot better on the engine (I don't know anything about running an R/C car on a chainsaw engine, but I do know from plenty of other applications, cars, dirtbikes, go karts, that running the engine at redline all the time is really hard on the engine).

As for pricing, yeah I know that if I'm setting a price limit, I'm already doing it wrong, price is a relative thing when you get into something like this, but wherever I can cut costs, I will.. If I can acheieve the speeds I want, without redlining the engine without having to add an extre set of sprockets and chains, I'd rather do that.

As for gear ratio, I'm assuming (correct me if I'm wrong, again, gear ratios is where I fail miserably at) what you're telling me is I want a sprocket with 8 times the teeth on the clutch as the one that's on the axle, right?

As for the clutch, I couldn't tell you at what RPM it will engage atm, I have no solid ideas on what make/model of chainsaw I will be using yet. I chose chainsaw over weedeater because they already have the clutch (which means even if the clutch is wrong for my application, it's just a simple matter of fitting it with a new one) and they have plenty of torque to work with. I read up on a lot of using weedeater engines in cars, and everything keeps pointing to one conclusion.. Weedeaters are good for boats, without a decent amount of modifications they're no good for off road ground vehicles.

Just going off of rough guestimates based on average chainsaw engine RPMs, I'm going to guess they engage at 4k RPMs or less.. Based on what I've read that the average chainsaw engine has a peak RPMof about 8-10k. Again, that's a very loose estimate.

EDIT: Also, no I haven't posted on any other forum about this.. Well not an R/C one anyways.

Old 02-16-2011, 10:40 PM
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Default RE: Upcoming 1/4 scale trophy truck build

Lots of weedeaters/weedwackers have clutches.
With a bit of luck you'll find one that you can screw a pocketbike sprocket straight in to.


I have built a couple of cars using weedie motors.
(see attached videos)
I used angle grinder heads for primary gear reduction and sprockets and chain for secondary reduction.


I want to be able to run the car at a nice idle speed and get an acceptable speed, rather than just run the engine at max output all the time, maybe I'm wrong, but I figure that'd be a lot better on the engine
Yep, you're wrong....
Chainsaws and weedie motors work best at full revs, as they're aircooled.
The fins on the flywheel force the air across the cylinder head and keeps it cool.
This is why it's important to keep all the protective casing on the motor

Adding a second axle, or jackshaft is a good idea as you then have the option of playing around with ratios as you go.
It's all good calculating a top speed on paper based on the motor RPM and circumference of the wheels, but once it's on the ground it's a different story.
The weight of the car, the surface you're driving it on and wind resistance etc will slow it down.

I'm not trying to discourage you by any means, but these are all things that I've learned over the past couple of years of trying to build a decent homebrew car.

As I said, be prepared to make modifications and alter a few things.
I don't think anyone has successfully built a large RC car that worked straight away.
Check the sticky thread on top of this forum "List of home made projects"
Plenty of ideas in there.


Check out your local scrapyards and recycling places.
I usually get all my bearings from old angle grinders, fans, powerdrills etc.
Steel or aluminium for framing is usually available cheap if you know where to look.
The only things that really have to cost money with a project like this is the radio components and possibly the wheels.


Now get building!


[youtube]http://www.youtube.com/watch?v=ImT8UW9NZ0w[/youtube]





[youtube]http://www.youtube.com/watch?v=HdC4r5l_-j4[/youtube]





[youtube]http://www.youtube.com/watch?v=qwKJslE_ip4&feature=related[/youtube]








Old 02-17-2011, 10:48 AM
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IAmSupernova
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Default RE: Upcoming 1/4 scale trophy truck build

Thanks, I appreciate all the input. The problem for me with scavenging parts rather than buying them is that I live in a really small town, to stand any real chance at finding all the stuff I'd need I'd have to drive pretty far, and I don't have a license, so it's kinda hard for me to get out of town. But I did manage to find a place that sells bearings, even sealed ones, for pretty cheap (relatively speaking of course) so just buying bearings shouldn't be too much of an issue.

I also realize that research will only take me so far, and I am prepared to re-work things as needed.
Old 02-17-2011, 12:46 PM
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Default RE: Upcoming 1/4 scale trophy truck build

That's ok, just trying to help.










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