Monthly Archives: March 2007

last builders meeting

was yesterday…so the next time we see each other will be on practice day. I wonder how many have started construction? I really found it useful to make a full sized sketch of the car (plan, elevation, and end views). For me it is clearer than computer sketches. I used heavy cardboard that are used to separate feed bags on pallets. Sometimes you can find those sheets made out of plastic, even better for template material.

Here is a good site: Analytic Cycling “provides technical methods for evaluating and estimating cycling performance and parameters. It makes extensive use of differential equations and advanced numerical methods”.

quick and easy ways to faster wheels

Lets assume you have made the wise choice of spoked bicycle wheels for the car, here are some things to make them faster.

1. True them, or get a bike shop to do it. Remove hop and wobble.

2. If hub uses the old cone type bearing races, wash out grease and use oil for lubrication, then adjust them.

3. Research tire choice. hey, I’m just sayin’….

4. Psst… some say a little talc on inner tube makes the tire run faster.

5. Consider wheel covers. I think we were the only team to cover our spokes last year, (wrongo, Ed.). I bet more will do it this year. Its a no-brainer.

6. Wheel alignment – they all have to be pointed in the same direction. Setting aside steering complications, all four wheel’s axles should be perpendicular to the car’s centerline. Here is a simple set-up to check alignment (and also allows toe-in measurement, if you want).

Find a nice flat working area, concrete floor garage is good. Strike a line on the floor, this will be an important reference line. Rest the car on some supports so that the wheels are off the ground (no need, Ed.). Align the center point marks on the car (didn’t I mention you should transfer that construction centerline to the outside of the car at some point?) directly over the reference line on the floor. You can use a plumb bob for that. Now string some fishing line between between jack stands or similar heavy objects, see the diagram. You want the fishing lines to be equal distance from the centerline, and parallel. Also the line should be axle height.

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Now using a machinist’s ruler, you can measure from the wheel rim to the fishing line. Measure from two points on each rim, 180 degrees apart, ie 9 o’clock and 3 o’clock. The two measurements should be the same if the plane of the rim is parallel to the centerline of the car (assuming rim is not bent).

Well that pretty well covers the obvious, I’m not going to say anymore about wheels. Esoteric discussions will have to wait until after the race.

still on construction

a few random thoughts…

Using, as we did last year, a chassis base of 3/4″ baltic birch plywood, gives you a good foundation onto which you can attach the mechanicals. For axle to base attachment, we used one of the methods shown in soapbox derby construction manual. The plywood base itself is not a stiff enough structure for a good racer, but with the sides attached the structure is very stiff (in essence a monocoque).

Remember to draw an accurate centre-line on the base, very important for accurate alignment of axles.

The picture below is of the car mostly assembled but not painted. You can see the foam nose (glassed over) and the extra bit of side material scabbed in. The plywood bottom was 96″ long, because the sides curve, an extra bit of 1/8″ ply is needed. The foam nose brings the overall length to 100″ (which was the max last year).

The blue top is simply 2″ foam, carved to shape and then glassed over. Obvious to point out here that polyester resin would melt the polystyrene foam, you have to use epoxy.

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back to construction tips

Topic jumping again. Here are a couple of pics of our car in the initial construction stages. Like most teams last year, we did not have a heck of a lot of time to make our car, so we chose a very quick (and inexpensive) method. The template was made and cut out, and the shape transferred onto 2 pieces of 3/4″ “baltic birch type” plywood (2′ X 8′). The shapes were cut out producing 2 identical parts. One would be the base, and one would be the substructure of the top. Sides were cut from 1/8″ luan plywood, often sold as “door skins”. The base plywood was laid on a flat floor and the sides were epoxied and stapled to the plywood edge. The top had access areas cut out and then similarly glued and stapled to the top edge of the sides. A strip of fiberglass tape was epoxied to where the sides joined the top and base. You can see me doing that in the pictures. When the epoxy cured the resulting structure was quite stiff.

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overview of wheels

All other things being equal (hub bearings, tires and tire pressure), a larger wheel “rolls better” than a smaller wheel. The maximum common wheel size is 27″ or 700c. One could make the case that a 20″ wheel has a smaller frontal area than a 27″ wheel and would be just as fast. I don’t know.

Bearing friction on bike wheels is pretty darned low. To get to that better happy place, try replacing the grease with oil, use high quality ball bearings (do caged bearings really have lower friction than loose ones?), adjust the cones, and if using cartridge bearings, cut away the rubber water seal.

The rolling resistance (this discussion limited to rubber tires on asphalt) is also affected by tire pressure and tire tread pattern. Higher pressures = lower rolling resistance. Another caveat, maybe “too” high a pressure will result in a rougher ride and a loss of horizontal velocity. Perhaps there is a sweet spot in tire pressure, high enough to reduce rolling resistance, but not as high as to eliminate shock absorbing. No tread (slicks) are faster than treaded. Narrow tires faster than wide.

The cross-sectional shape of the wheel rim in a spoked wheel has a large effect on the air resistance of the wheel. A box section rim will have a higher resistance than a deeper and tear dropped shape rim. Fewer spokes will also mean less air resistance. Disc wheels, especially lenticular cross section ones, have the lowest air resistance (see links under “blogroll”) Look at time trial and triathalon bikes to see what the current thinking is. Do we all get the hints here?

Top of the line disc wheels or those fancy reduced spoke wheels are pretty expensive, and if bought new would blow the Canary budget limit. In my next post I will talk about inexpensive ways to make your wheels as fast as they can be.

more on the purpose of this blog

Walt the Race Director mentioned in a couple of the team meetings that some companies have expressed interest in entering the 2007 race but don’t know where to start in car building. I volunteered to contribute some of what I know, and so this blog was started. Its certainly not meant to be an authoritative nor exhaustive study of Canary Derby Race Car construction. I hope it gives new teams a bit of an assist in getting started.

I really would like other teams to comment and contribute. Yes its a race, and so we won’t share all our ideas, but in keeping with the purpose of the Canary Derby I think a bit of collaboration will help all of us, especially new teams.

Sometimes I imagine that this year all of the cars will look similar, a natural selection process (after all, the cars will be subjected to the same conditions). But then I remind myself that the people in the companies entering all have strong ideas and opinions as to what makes the best car.

I hope there will be a diversity of approaches, its much more interesting and fun that way.

So please, if you have any tips or resources you would like to share, let me know and I will publish them here.

digression into aerodynamics

From the very start you have to have an aerodynamically clean design. Its a challenge to do that with the dimension rules in the Canary Derby, and also with the budget limit. At the speeds at which the competitive cars were running last year (34-37 kph at the corner, probably faster at the finish line), aerodynamic drag is the major force on the car. I’ll get into a wheel discussion later, but rolling resistance is a minor component of overall drag (except for the cars last year using soapbox and skateboard(!) wheels). The Carmanah Technologies car, had in my opinion, the best running gear in the field. They used racing wheel chair wheels and tires (700c?) and the test runs the week before the race were done with no body and were quite fast. But the body shape they chose was not very good, and I think that cost them the race. The Aspreva car had slower wheels (20″) but a faster shape (albeit not the best shape possible), and won.

design and construction tips 2.

The previous entry about laying out a plan view of the car is directly applicable when you are making a car with a flat base to the chassis, ie. cut from a sheet of plywood. If you are going all out and will be making a “curved in all 3 dimensions” body, its still good to have that plan view layout. It may not be the base of the car but will probably be a plane on which the axles will be mounted. Its important to have a reference plane if you want front and rear axles to be aligned. The soapbox derby construction manual on classic soapbox racer construction says more about this. Its a very good document to read.

design and construction tips 1.

some approaches and techniques I have found useful in building a Canary Derby car…

Determine what position the driver is going assume in the car (head first not allowed). Have the driver assume that position and measure total height, width and length. Having the driver do that on a long bit of cardboard or paper allows you to scribble down the stats right on what you will be using as a template.

Now decide on shape, starting with plan view, that will enclose the measurements you just made. There are a couple of things to remember in this layout process:

1. reducing frontal area reduces drag
2. a fair curve is a fast curve (fair means smooth curve transitions)

Strike a centreline down the length of the cardboard/paper template. Lay out the measurements from that centreline, but only do one half, as if car was sliced longways down middle. Then draw the plan shape on one half. You can use a long bit of 1 x 2 wood as a batten to spring a nice curve.

You then can fold along the centreline and cut out the template to make a symmetrical pattern. A plan view outline of the car.

An elevation view follows more or less the same procedure. But here you have to decide how much of the driver will be sticking out of the car, will there be a canopy etc? Just remember that reducing frontal area will reduce aerodynamic drag (its a mantra I will repeat often). Also consider hand placement for steering, knee and foot room, and ingress/egress.

The plan view template is useful even if you are making up a welded tube type car. It allows you to experiment with the placement of axles, steering gear, back rests, ballast points etc. For me its more “real” than using Autocad or Solidworks or even hand draughting. You can make cut outs of component profiles and place them on the template, have the driver on it, move positions, see what works and what doesn’t (interference issues do arise) before cutting building materials.

satisfies the rules?

Volvo’s 2004 extreme gravity racer does have 4 wheels, but as the race director notes in the comments, the headfirst driving position is illegal in the Canary Derby. Too bad.

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Red Bull Ljubljana

Even more fun (lots more Red Bull race videos on YouTube)

some British cars

A couple of pics from cars entered in a race held at the Goodwood Festival of Speed a few years ago. Note that the cars in those races had to negotiate a slalom portion of the course, as a pack, at speed. That explains the side rails (to prevent wheels interlocking, the wheel spats might be a rule requirement too, to reduced wheel to wheel unpleasantness) and the sophisticated suspension and steering. Also their rules dictated a minimum car height. Click on image for full size.

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aerodynamics and steering

on the side are links to some articles about bi/tricycle aerodynamics and steering. They are very much applicable to CDSBRBs.

construction manual

soapbox derby construction manual a very good manual (pdf) on how to design and construct the more “traditional” soap box racer. Much of it is very useful for Canary Derby builders