35cc Honda 4-Stroke= 30mph!

[Bear45-70]

Wheels out of alignment = lower MPG

[Petrock's 1984 Spree]

Yeah were trying to get them as close as we can.

In other news, the other side of the car has gotten the majority of my attention
From BobCad
0401111657 by jpetrak, on Flickr
To the mill
0401110724a by jpetrak, on Flickr
0401111529a by jpetrak, on Flickr
To being mounted on the rear tire's disk brake mount
0401111656a by jpetrak, on Flickr
0401111655 by jpetrak, on Flickr

Now all that is left to CNC is the giant 138 tooth sprocket
I think that might be a 10 hr. job.
It'll be amazing once its all done though!

[noiseguy]

Ah, this is a tricycle then?

Definately would like to have the front suspension sturdy; even if its aligned statically if the front hangers are wobbly the wheels will be going in/out of alignment and scrubbing off power. A top bar would really help this and wouldn't add that much weight.

You should be able to get adequate alignment of chain by setting a couple of string lines. Nothing complicated, and using a chain allows for higher tolerance of mis-alignment than, say, gears. Also just about the most efficient power transfer. Wonder, though, if you could have used a gearbox on the back (Sturmy-Archer 7 speed or like) to allow you some more ratios. Is this a single speed event (in other words, start up and get going and whoever runs the farthest wins?)

[Petrock's 1984 Spree]

Yep, tricycle

Okay, i'll lookj into that top bar. I dunno how much material we have left though. That aluminum is pricey stuff when your running on a low budget. Yeah the alignment technique were using is called the acrimen (sp?) steering angle.
Last year using that techinque got us close enough, although it didn't matter much because our vehicle last year was a whole lot rougher than this one.

You should be able to get adequate alignment of chain by setting a couple of string lines. Nothing complicated, and using a chain allows for higher tolerance of mis-alignment than, say, gears. Also just about the most efficient power transfer. Wonder, though, if you could have used a gearbox on the back (Sturmy-Archer 7 speed or like) to allow you some more ratios. Is this a single speed event (in other words, start up and get going and whoever runs the farthest wins?)

Yep, chain it is. I dont think we could get the engine close enough to the rear tire to make the drive train out of gears unless the engine rode on top of the tire. You know we looked into some in-hub 3 speeds and the gear ratios just weren't tall enough. To get the tiny engine to move the car you need a VERY tall gear ratio.

On the Spree i believe its around 12, on our vehicle its 13.8
Yeah, the rules are you have to stay above 10mph the entire time, and your not supposed to kill it and coast. But other teams, with ceramic ball bearings, can coast for over half the track! So, if ya can't beat em, join em!

We dont have the cermaic bearings, which help out tremendously, but we did modify the engine to take electric start.
So the 650 mpg high is probably only 325 from the efficiency of the engine and the car. The other half of that would probably come from killing the engine and coasting around the track

[Trafficjamz]

so how is mpg calculated in this race?

do you only get 1/2 cup of gas or something?

[noiseguy]

Ackerman. That's for setting your trailing arms:

http://en.wikipedia.org/wiki/Ackermann_ ... g_geometry

With the 3-speed hub, the goal would be to keep the engine at the optimum fuel-efficiency RPM. Ratios on a 3 speed are typically an underdrive, a direct drive, and an overdrive. Ratios something like 1.33, 1.0, and .75. You would run the hub as part of the drivetrain, and shift as needed. What you guys did is probably fine for the contest at hand.

[burnt_toast]

so is this for high school or college? just curiosity, maybe I missed that somewhere

looks like a nice design so far

[spree-rider]

HS

[Petrock's 1984 Spree]

You get 8 ounces of gas to go as far as you can. Once you stop, they tally up how many laps you. Then they do some math and figure out how many mpg you got.

Ackerman. That's for setting your trailing arms:

http://en.wikipedia.org/wiki/Ackermann_ ... g_geometry

With the 3-speed hub, the goal would be to keep the engine at the optimum fuel-efficiency RPM. Ratios on a 3 speed are typically an underdrive, a direct drive, and an overdrive. Ratios something like 1.33, 1.0, and .75. You would run the hub as part of the drivetrain, and shift as needed. What you guys did is probably fine for the contest at hand.

Yeah thats it! Last year we just pulled strings back from the steering arms and made sure the intersected at the rear tire. Huh, that might be something to think about then. Man a three speed hub that also has a disk brake mount, that might be a little difficult to find. Its definately worth it if there is something like that out there.

Well, the sprocket didn't go quite as i had hoped
Our mill says it is rated for steel, but only being able to take .0025 per pass is a load of BS in my book. Its really wierd too, most mills seldom have the option to reach 10k rpm, and the max ours has is 21k. Its a glorified router. Great for wood, not so great for chewing steel.
After 5 hours of figuring out the speed and feed rate, and 2 cutters later this is all the mill was able to do in 40 minutes of cutting the steel. And it roasted the cutter just doing that.
0402111251 by jpetrak, on Flickr
0402111545 by jpetrak, on Flickr

So were going to have to send the steel out to some place that can cut it for us

[noiseguy]

Odd, looking at your tracing on the steel, appears that the sprocket teeth face inward?

Appears you're trying to mill it all at once from a plate. Would be tempted in your case to cut the body of the sprocket out with a bandsaw (or torch and grind depending on thickness) and use the mill only to cut the individual teeth.

[Petrock's 1984 Spree]

Odd, looking at your tracing on the steel, appears that the sprocket teeth face inward?

Appears you're trying to mill it all at once from a plate. Would be tempted in your case to cut the body of the sprocket out with a bandsaw (or torch and grind depending on thickness) and use the mill only to cut the individual teeth.

Oh, nah sorry the teeth face outward, its just a really bad trace. We had cut out a masonite prototype that i used to do the tracing with just for a crude idea where i should put the screws so the mill wouldn't try to cut through one of them.

Right, thats always an option. But doing the teeth is the longest program. Its somewhere around 3-4 hours and if we blew through 1 cutter in 40 minutes then we would have to burn up 6 cutters just to get the teeth done. And thats only half of the sprocket.

I was doing a lot of research on 7075-T651 aluminum to see if that would be a little easier to do. Its got a BHN (Brinell Hardness Number) of 150, compared to the steel's 95. But its machinability rating is 70%, where as the steel's is 55%.
The higher the percent, the easier it is to machine. so i think it might be worth a shot.

Does anyone here have an aluminum sprocket on one of their motorcycles? How do they hold up?

[noiseguy]

OK, getting outside what motorcycles use, aluminum would be the obvious choice for your application... not sure why I didn't think of that. Steel is wild overkill for your application. Especially for the low torque that you're transmitting with it. Your sprocket at the motor should be steel though. Think of the way a quality bicycle is built... aluminum large front chainring with steel rear sprockets. Al is easier to machine and will weigh a *lot* less.

Since it only needs to last 1 race, long-term durability isn't a concern. Do some research on what alloys are used for bicycle front chain rings.

The biggest potential issue would be the sprocket folding... but your hub is * huge. If the sprocket is too floppy, you could reinforce it with a spider that grabs it out farther from the hub and closer to the chain. I honestly don't think it'll be a problem.

[Petrock's 1984 Spree]

Well this is really stupid. It turns out the mill will cut the steel!!!
The mill has this stupid little setting buried in its long list that controls the mill's speed and feed rate. No matter what you enter into your program, the mill will over-ride it with its pre-set rate. Once we figured out how to change that preset rate everything is humming along the way its supposed to!!

Only a tiny problem. There is a little allan key that locks the little cutter in place, and it started to let go half way through. So it didn't cut it all the way through. So some blue loctite and we should be in business!

0401111657a by jpetrak, on Flickr

0404111627 by jpetrak, on Flickr

0404111741 by jpetrak, on Flickr

[noiseguy]

That big ole sprocket's going to be like a flywheel...

[Petrock's 1984 Spree]

That big ole sprocket's going to be like a flywheel...

Its actually worrying me a little bit. I dont know if the added weight will be good or bad.

Good= The little engine will have enough "umph" to lug it from a dead stop and getting going up to speed so the rotational mass will let the vehicle coast for a while without the engine

Bad= It weighs too much and we just can't get it to move.

At this point im not putting my money on either because i've had my fair share of surprises with this thing