F1 in Schools - Wheel System

[aaronTAR]

Hi guys,

My name is Aaron and I'm new to this forum so if this is posted in the wrong place I'm sorry! I'm taking part in F1 in Schools for the first time this year, and my team has qualified to the National Final and we have 7 weeks until race day. We won the fastest car award at our regional final, and I've been reading through the old threads on the issue.

One thing I'm still confused about and not sure how to improve is our wheel system. At the regionals we used a solid 3D printed wheel (had to be sanded thoroughly as the printer has a resolution of 0.2mm) made of PLA, with a carbon fiber axle. We used fully ceramic bearings. To set them up, we placed the bearing into the slot in the car where most teams put the axle. We then put the axle inside the bearing and into the wheel. The axle was 2mm thick. We then had washers to keep the spacing and alignment all ok. The system was designed to be more stable and better aligned than other potential systems, but it did feel dodgy and was very fragile.

I'm pretty much a total beginner in this area, so any suggestions or tutoring would be of great help.

Can you guys give me some pointers on where to go with the wheel, like what areas I could develop? Also if anyone has any articles where I can read up on wheel design in relation to this competition or in general I'd appreciate it.

Thanks,

Aaron.

[Lightspeedrt]

Hi Aarron,

Id start by not 3d Printing your wheels. 3D Printing cannot print circles very well and you would get a hell of a lot better result if you had them lathed. Id suggest Acetal as a good wheel material, it is light, strong enough and has a low cof. Id also suggest redesigning your wheel so the bearing is inside the wheel and not spinning the axle, thatll allow for less rotating inertia so your wheels will spin up faster. I could post a cross section of one of my designs although itd be more beneficial if you posted yours.

Dylan

[tok-tokkie]

I question your statement that placing the bearing inside the wheel reduces the angular momentum. With the bearing in the wheel it is the outer race that is rotating. With the bearing in the body it is the inner race that is rotating. Density of ceramic & acetal is not much different? But an acetal wheel can have a really thin flange between the hub & tread so there is very little material so low angular inertia compared to a ceramic bearing.

2mm bearing gives a spindly carbon axle. How about 3mm & use a hypodermic needle for the axle to save some weight while having something less fragile? 3mm ceramic bearings are cheaper than 2mm I think - but still expensive.

[Lightspeedrt]

With the bearing inside the wheel only the wheel is turning not the axle as well, although depending on your axle it might be pretty similar results, however with the bearing in the wheel it will be much more stable because your reducing the moment arm is shorter. I never commented about the ceramic bearings.

[aaronTAR]

Hi guys,

Could you suggest an article or piece where I could do some background reading to get up to speed on the terminology etc.?

What would be involved in machining acetal and how can it be sourced? Is there any other components you feel should be added to the system to improve efficiency? The Irish winners from last year mentioned something about a suspension system in their wheels - you can read up about it at http://www.f1inschools.ie/resources.php (It's AutoLaunch Racing, under the exemplar portfolios heading).

As regards the axle, the 2mm carbon fiber is surprisingly sturdy and lightweight. We had another set of axles machined from a more heavyweight metal, but decided to go with the carbon fiber on the basis that it was lighter. This was as we were trying to get our car to the bare 52 grams required in the competition. However I'll take the suggestion on board and look into it. Do you think it will increase weight that much?

For the bearings, I have read that they spin faster on the outside rather than the inside. However, when you take into account the stability that having them in the body brings I wonder does it even out over the course of the race? Any suggestions on how to get the best of both worlds are welcome.

Would you guys go with a solid wheel or a hub cap that doesn't rotate with a thin wheel around it? Or just a a hollow wheel? And why?

Furthermore, if you gives could give some pointers on what areas to research so I can improve the entire wheel system I'd be really grateful!

Thanks,

Aaron.

[aaronTAR]

Hi guys,

I've just had the option of having wheels laser cut from acrylic made available. Would you recommend this above a wheel which has been cut from ercelin? And why would you choose one over the other? Thanks!

[tok-tokkie]

I am surprised that the cf axles are sturdy. Excellent choice in that case.
Acetal is readily available from plastic suppliers to the engineering trade. Common trade name is Delrin (duPont). it is the nicest material to machine that I have worked with.

[aaronTAR]

Hi there,

I'll try and source that then, thanks for the info! Would you recommend laser cutting the material or using a milling machine?

[tok-tokkie]

Laser will not give as smooth a surface finish as cutting with a tool. Circular things are usually turned on a lathe but they can also be cnc milled easily.

[aaronTAR]

Thanks for the help! Did you have any thoughts on the suspension system I mentioned? Do you think it would be feasible? Considering the car is so light etc.

[tok-tokkie]

I downloaded the Autolaunch pdf. I had no idea that this competition results in such a high standard of presentation and project management. My eyes have been opened.

They talk about the moment of inertia of their wheels yet they chose to have a stationary axle so the outer race of the bearing has to rotate and thus is included in the moment of inertia. The moment of inertia of a wheel = mass x radius ^2 (radius squared). If the outer race of the bearing is rotating it is the piece with the greater mass and the greater radius. Conversely if the inner race is rotating it is the lower mass and much smaller radius. Since the inertia is proportional to the radius squared it is much better to have the inner race as the rotating element.

Here we are simply considering the energy required to spin the wheel up to speed. Besides that there is the linear acceleration of the whole car. You have chosen small 2mm shaft size bearings which are lighter than 3mm bearings. This lets you keep your car down to the minimum mass. That is the greatest imperative for meeting the challenge that has been imposed = a simple F=ma challenge. But detailed consideration of this challenge is interesting as is discussed by Autolaunch - even though their conclusions are suspect.

They chose to use PTFE wheels. That allows the wheels to slide on the track. That probably is an advantage. But PTFE has a much higher sg (specific gravity) than most plastics so their wheels were a bit heavier. But, in their discussion of the wheels they say that they chose acrylic.

Suspension. Are the joints in the track a problem? If not then no suspension is required. If they do disturb the car then suspension would be an advantage. You are using thin (2mm) cf axles. Are they not flexible enough to give you inherent suspension? The mass of the car is very small so it would require pretty soft suspension for the wheels to move while the car body is undisturbed.

EDIT: I wrote that before going past the CFD section of the report. I see their axles were acrylic rods inside grooved holders so they could only flex vertically. Nice detail. They discuss their selection of PTFE for the wheels. Consider constraining your cf axle to move upwards & backwards on a 45° angle. It will ride over bumps more easily I believe. If it is not constrained I think you may have dynamic problems.

[aaronTAR]

Hi there,

I too couldn't believe what I had signed up for!

Thanks for the equation and insight there. So for the direction we're trying to go with the car, the inner race is the way to go? We also considered this in our proposal which is similar. However we concluded that a lightweight car was probably the way to go, considering the cars are still going at a high speed towards the end of the track. We feel that a focus on acceleration rather than the inertia of the car is more beneficial.

Would you still recommend the Acetal as you have mentioned above? Against a PFTE wheel?

It really depends from car to car. It seems to have an affect on some more than others, however I think they can be a problem. I didn't notice our own car jumping hugely but I would imagine it was a small bit.

The axle at the moment is pushed through inside the bearings, so it's a pretty snug fit. At the moment we're thinking of 3D printing a holder for the bearing (which holds the axle) which would be more accurate. When machined out of the balsa, there was problems with alignment and some of the wood chipping. Would you recommend making an area for the entire bearing to slide up and down to act as the suspension? (The grooves you have mentioned) Furthermore, if that kind of system was used, wouldn't a soft material have to placed at either side of the gap to make sure the bearing doesn't come down hard?

Thanks again for the help. Would you be able to PM me your name so I could put you down as an adviser? The competition asks for students to learn from people in the field. However if you can't that's fine!

Regards,
Aaron

[Blanchimont]

Some general remarks.

Are you at minimum weight limit minus the tolerance allowed?
Did you exploit the tolerances on the different dimensions/shapes of the car and the wheels?
Saving weight at the rotating wheel parts should be more useful than saving weight on non-rotating parts.
Did you consider using axles without bearings as the Pinewood Derby cars do( http://en.wikibooks.org/wiki/How_To_Bui ... _Car/Axles )?

Do you have any information on how much energy the CO2 cylinder contains, how big the initial acceleration, the maximum speed and speed at the finish line(if lower than the max speed) is?

[aaronTAR]

Hi there,

We have exploited the minimum weight. In fact, we just about made it up to the tolerance. Why does saving weight on rotating parts become more useful? I had thought that saving weight on the body to allow more inertia in the wheels would work better (whilst still keeping the car very light of course) but I'm open to suggestions on that!

Some cars do that. However, the bearings really do seem to improve performance and any team with serious ambitions tends to use them.

The canister contains 8 grams of compressed CO2. It shoots air for about 0.3 seconds from what I've found throughout my research. I don't have exact figures on the speeds, as I don't have access to a testing track!

On the point about the canister, I've seen some suggestions regarding a de laval nozzle. Would you recommend trying to pursue that line of thought? I've also seen a different type of nozzle at the back of the car. The Irish Champions from two years ago used it - available to see here http://www.f1inschools.ie/resources/f1_ ... racing.pdf on page 12. The idea is based around that of the thrusters on a rocket. Can you please advise me on the reasoning and advantages/disadvantages of these systems?

[Blanchimont]

The rotating parts should be as light as possible and as close to the turning axle as possible, as it requires additional energy to rotate them. tokkie mentioned the formula above.

A car braking with locked wheels contains the following kinetic energy at the speed v: 1/2*m*v^2
If the car is driving with rotating wheels at the same speed: 1/2*m*v^2 + 4*1/2*I*omega^2 (where I is moment of inertia of one wheel and omega=v/r)

You see that the car with rotating wheels contains more energy at the same speed because of the additional second term in the equation. If the maximum energy would be limited for the car, as it is in your competition where all the energy comes from the CO2, the speed of the car with rotating wheels will be lower. To increase the speed of your F1 in schools car, you need to minimise I*omega^2. I depends on the mass of all the rotating parts(wheels and bearings).

A wheel is basically a combination of a hollow cylinder and a disc inside to connect the cylinder to the axle/bearing, you can read about the equations here( http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html ). My idea to go for Pinewood Derby axles was to get rid of the rotating parts of bearing and therefore eliminate the energy to spin those parts. It could be worth considering, but you have to some research and calculations to be able to compare the gains in reducing the rotational parts with possible losses in the friction.


What about exploiting the rules on the wheel surface that is in contact with the track. Instead of having a constant diameter over the whole width, you could try to use a smaller diameter in the middle of the wheel. Of course only within the tolerance allowed. Maybe this trick does reduce the rolling resistance a bit?

How do you current wheels look like? Are they as narrow as possible?