The race that stopped a nation
Well, I went to Maryborough. We worked hard. We lost. We learnt. We had fun.
We worked frantically on our four-wheeled carbon-fibre beast - arriving at Maryborough on Thursday morning, working all Thursday on it, all Friday on it, got it past the scrutineers on the Saturday morning, worked till 1pm on the Saturday. The raced started at 1:30. We pulled it off the track at about a quarter to two and let it back on the track at quarter past four. And then there were numerous hindrances that slowed us down.
Essentially, the drive-train was crap. To give you an idea, I was doing 3min30 laps(1.3km track) during the afternoon. We gradually improved things and I got it down to a 3min02 lap. The other rider managed a best time of 3min20 on that vehicle. He then completed a few laps with another vehicle from Tassie and completed a 2min17 lap - knocking 1min03 off his previous best and set a Rosny College record for fastest lap time. The drive-train of the quicker vehicle was by no means good.
Anyway, we raced til about 3am or so before hitting the hay. We got up at 7 to give it another go. Unfortunately, the drive-train had suddenly worsened. I managed two laps and Reuben (the other rider)after much coaxing for the sake of photos, managed three. The rear sprocket was wobbling really badly, suggesting a bent rear axle. Nobody knows what incident could have caused it.
On the plus side, the four wheels meant that rather than taking the racing line, you could accelerate into the corners, throw yourself around and keep accelerating, without getting off that inside line. We never looked like rolling.
So for my 3min02 lap, the average speed corresponded to 26km/h. For Reuben's 2min17 lap, it corresponds to about 34km/h. The lap record was 1min45. That corresponds to 44.5km/h. The track speed limit is, in theory, 60km/h.
So, looking at the efficiencies, based solely on times, the drive-train efficiency (including extra wheel - all the other vehicles were 3-wheelers) was ~25% worse with our machine. However, the fact that our machine could take tighter lines, it had reduced frontal area, reducing aerodynamic drag and was about 5kg lighter, meant that the drive train would be about 35% worse. Assuming that the drive-train of the better vehicles was about 80% (finely tuned drive-trains have efficiencies upwards of 98%, but they cost big bucks, and experts are tuning them, furthermore, in the race there would have been significant deterioration, so I'd say 80% efficient is a conservative estimate), then the efficiency of the drive train of our 4-wheeler would then be around 45%. We'll say 49% to be generous. Now, assume power input is proportional to velocity (which is not quite right, I know). So if we doubled the drive-train efficiency, we would say that we could double the average speed to 52km/h, which corresponds to me setting a lap record.
But, unfortunately, aerodynamic drag is proportional to velocity cubed... so on a bicycle, this means that, while pedalling at 30km/h, 80% of your input energy is going into overcoming aerodynamic drag. So, 52km/h average speed is no longer a realistic target, even with the reduced frontal area and drag coefficient of the HPV (They are both linearly proportional and aren't reduced quite as much as is required to make our machine record-breaking.)
So, room for improvement all round.
We worked frantically on our four-wheeled carbon-fibre beast - arriving at Maryborough on Thursday morning, working all Thursday on it, all Friday on it, got it past the scrutineers on the Saturday morning, worked till 1pm on the Saturday. The raced started at 1:30. We pulled it off the track at about a quarter to two and let it back on the track at quarter past four. And then there were numerous hindrances that slowed us down.
Essentially, the drive-train was crap. To give you an idea, I was doing 3min30 laps(1.3km track) during the afternoon. We gradually improved things and I got it down to a 3min02 lap. The other rider managed a best time of 3min20 on that vehicle. He then completed a few laps with another vehicle from Tassie and completed a 2min17 lap - knocking 1min03 off his previous best and set a Rosny College record for fastest lap time. The drive-train of the quicker vehicle was by no means good.
Anyway, we raced til about 3am or so before hitting the hay. We got up at 7 to give it another go. Unfortunately, the drive-train had suddenly worsened. I managed two laps and Reuben (the other rider)after much coaxing for the sake of photos, managed three. The rear sprocket was wobbling really badly, suggesting a bent rear axle. Nobody knows what incident could have caused it.
On the plus side, the four wheels meant that rather than taking the racing line, you could accelerate into the corners, throw yourself around and keep accelerating, without getting off that inside line. We never looked like rolling.
So for my 3min02 lap, the average speed corresponded to 26km/h. For Reuben's 2min17 lap, it corresponds to about 34km/h. The lap record was 1min45. That corresponds to 44.5km/h. The track speed limit is, in theory, 60km/h.
So, looking at the efficiencies, based solely on times, the drive-train efficiency (including extra wheel - all the other vehicles were 3-wheelers) was ~25% worse with our machine. However, the fact that our machine could take tighter lines, it had reduced frontal area, reducing aerodynamic drag and was about 5kg lighter, meant that the drive train would be about 35% worse. Assuming that the drive-train of the better vehicles was about 80% (finely tuned drive-trains have efficiencies upwards of 98%, but they cost big bucks, and experts are tuning them, furthermore, in the race there would have been significant deterioration, so I'd say 80% efficient is a conservative estimate), then the efficiency of the drive train of our 4-wheeler would then be around 45%. We'll say 49% to be generous. Now, assume power input is proportional to velocity (which is not quite right, I know). So if we doubled the drive-train efficiency, we would say that we could double the average speed to 52km/h, which corresponds to me setting a lap record.
But, unfortunately, aerodynamic drag is proportional to velocity cubed... so on a bicycle, this means that, while pedalling at 30km/h, 80% of your input energy is going into overcoming aerodynamic drag. So, 52km/h average speed is no longer a realistic target, even with the reduced frontal area and drag coefficient of the HPV (They are both linearly proportional and aren't reduced quite as much as is required to make our machine record-breaking.)
So, room for improvement all round.
posted by BSJ-rom at 7:36 pm
5 Comments:
Sounds like you had fun. Pity about the drive train. It's annoying when it's just one thing holding you back like that. Good effort though. I am wanting a few pics of the four-wheeled carbon-fibre beast.
Hi Jerome. I admit, I'm slacker than you. I'll try and work on it. :)
wow i'm lost. you'll have to get in quicker next time to have a brownie :P
No photos, who could be bothered reading all that?
You would be better off with a bicycle, but it's allready been developed so no fun for "engineers".
Whats all the talk of drive train efficience? I thought you were just using normal bicycle parts? A regular bike mechanic could probably fix your problems.
Ever tried riding a bicycle at 52km/h on the flat? Put the calculator down for a bit and get a "feel" for power needed and wind effects. You can't calculate everything. You first need practical experience, and then use the calculator to approximate the main factors.
I've ridden a bicycle at 52km/h down hill, and it was a bit of work... thing was, riding down hill the top gear was incredibly stiff, and the bike was geared too low, which meant that we couldn't get up to a decent top speed (30-odd km/h is a little slow for a top speed) so we couldn't take advantage of the aerodynamic advantage that we would have over a bicycle.
There is no doubt that a regular bicycle would have performed better than the vehicles we raced in, but, I don't know that they would do much better than the more efficient vehicles. I mean, 44.5km/h average speed on a 1.3km track by decent amateur cyclists who are maybe 18 years old is none too shabby.
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