It’s All About Length… (At 250MPH+)

追加: 2026-05-08

[00:00:00]I thought designing a body for 250 mph would be straightforward. Make it smaller, make it smoother, right? But what I actually found is that losing drag can make it less stable. And at 250 mph, unstable means one thing.

[00:00:15]So, in this episode, I've been using CFD to try and answer one question. What shape actually works? And then, turn that into a real body for my Project 250 machine.

[00:00:28]>> [music] >> I've been running aerodynamic CFD simulations, which are like using a virtual wind tunnel, not to chase perfect numbers, but to answer the important questions. Will it flip?

[00:00:40]Where is the downforce? And how much drag is there? Because at these speeds, you don't get a second chance.

[00:00:49]So, I started by analyzing the Beast, and this design was rushed.

[00:00:54]So, I thought, great, loads of easy wins.

[00:00:57]Yeah, that was optimistic. First step, baseline run. Then after that, start experimenting. Front downforce was already strong, so it was rear downforce and drag reduction that I aimed to target. I tried proper aerodynamic profiles. On the body, the fin, the canards. And the result? More drag and less downforce.

[00:01:22]Great. It turns out making things more aerodynamic can actually make them worse.

[00:01:29]I've been dumb. What change did what?

[00:01:32]The only way of really knowing was to only change one thing at a time. Which sounds simple, but if you're like me and constantly thinking of new ideas, it's painful. But it's the only way to actually learn what's going on.

[00:01:48]To get back on track and try and find some kind of improvement, I went back to the start and just added a supercar inspired diffuser on the rear end.

[00:01:58]I found good results with less drag and more downforce. A small win.

[00:02:04]Next, I removed one of the fins. Drag dropped as expected. But fins aren't there for drag, they're there for stability. Basically, keeping the car in a straight line.

[00:02:16]And at 250 mph, stability is everything.

[00:02:20]For the car to be stable, the aerodynamic forces need to act behind the center of mass.

[00:02:27]See how unstable it is? Now, if I shift the camera, center of pressure here, I'm blowing it.

[00:02:37]See, it's much more stable. Just the center of pressure is behind the center of gravity. Otherwise, the wind starts steering the car, which ends badly.

[00:02:49]So, I simulated a crosswind.

[00:02:52]Then, worked out the position of the center of pressure, just as I'd done for the downforce balance with the straight-line analysis.

[00:02:59]Twin fins, rock solid, center of pressure 100 mm behind the center of mass.

[00:03:05]Single fin, 50 mm in front of the center of mass.

[00:03:10]Yeah, that's not going to end well.

[00:03:13]So, it turns out that, even though it was rushed, I'd nailed the original Beast design. With the diffuser adding a bit more rear downforce, that's the best that I thought I could do.

[00:03:23]So, with those learnings, I moved back onto Project 250. I tested the new design.

[00:03:30]20% less drag. Great. But almost no downforce. And I genuinely thought I'd broken the entire project.

[00:03:40]After a lot of head-scratching, I realized I'd accidentally halved all my results.

[00:03:46]So, yeah, that was user error.

[00:03:48]But downforce was still down, not surprising given the narrower body.

[00:03:53]After fixing it, tweaking the canards, and sharpening up the rear end, the stability was good. But I'd only gained about 14% drag reduction.

[00:04:04]Maybe I could get some drag reduction by fitting smaller fins. But after designing an entirely new car, this was pretty disappointing.

[00:04:17]Then, I got a comment from Dave's Entropy. Make it more like a teardrop and extend the rear. And I thought, all right, let's try something a bit ridiculous.

[00:04:28]So, I added 100 mm to the rear and went for a single fin move right back and hit simulate. And when I saw the result, I genuinely didn't believe it.

[00:04:40]28% drag reduction. This was massive.

[00:04:44]Equivalent to driving the Beast with a 40 mph tailwind. But almost no rear downforce. So, not perfect. I tried extending the front by 50 mm just to understand if that helped, and I found a small gain. But it would probably make stability worse, so I decided to go back on this one.

[00:05:06]Tweaking the rear end once again, found a bit more rear downforce with minimal drag. And this thing was stable, even with the single fin, thanks to where the fin is, basically further backwards.

[00:05:19]Now the shape was done, I thought I'd try something.

[00:05:24]All right, with the center of gravity moved backwards, so this should be unstable.

[00:05:31]And it doesn't turn.

[00:05:34]There we go, instability.

[00:05:38]And then with the center of gravity where it's supposed to be, >> [sighs] >> stable. Yes.

[00:05:48]>> [snorts] >> A perfect demonstration, thanks to Burt Monroe.

[00:05:52]I feel a bit light-headed now.

[00:05:57]I got back into the CAD, making sure my chassis fits inside the body, splitting it up into five pieces for 3D printing, and adding lights and a service hatch.

[00:06:07]This was a lot of work. But what lights do I use? There's hundreds of options.

[00:06:14]So, I tested the four most promising.

[00:06:17]It's got quite a big spread of lights, so probably not ideal for speed running because you're a long way away. If it spreads the light, it's not going to you're not going to be able to see it from as far away. It's Oh my god.

[00:06:32]Well, these are surprisingly powerful compared to those motorbike lights which run on 12 volts.

[00:06:37]I mean, that's actually pretty good, isn't it?

[00:06:41]It gets very hot, though.

[00:06:43]That's a slight downside, I can Then this one is like a very focused spot.

[00:06:50]So, I think that one that one could be quite good. With their small size, good light output, and less heat, the mini 3-watt lights work best for me. And I'm adding side lights this time because last year I completely lost sight of the car mid-turn. I can't see it. Oh, no.

[00:07:08]Eventually, the design was complete.

[00:07:11]And it was time for loads of 3D printing.

[00:07:15]Because this body is a bit narrower than the Beast, it fits on my Bambu Lab A1, which gave fantastic results.

[00:07:52]Then, I could assemble it to the carbon chassis to check fit up before going any further.

[00:07:58]>> [music] [music] [music] >> And this is where I've got to now. It mostly fit as planned with one small area I'd got wrong needing a reprint.

[00:08:32]>> [music] >> Now, we're going to name this thing because Project 250 isn't going to cut it. Back in episode 1, you guys suggested some very good names.

[00:08:43]So, I've picked my favorites and I'm putting them in the comments. Add a like to the name you think I should go for, or add your own suggestion. Go and vote, and next episode, this thing gets its name and a new color scheme.

[00:08:57]I feel like we've learned a lot today.

[00:09:00]Length matters more than anything. A longer car equals lower drag. I shouldn't be surprised, but for some reason I am.

[00:09:08]Extending the rear helps more than the front and actually improves stability, so you need less fins, reducing drag even more.

[00:09:18]But with CFD, you must stay strong and change only one thing at a time.

[00:09:24]Otherwise, you've got no idea what worked.

[00:09:27]And honestly, I did this in the wrong order. I should have designed the aerodynamics first, and then built the car around it, not the other way around.

[00:09:36]But overall, I think we're in a good place with the design of this thing.

[00:09:41]Next up, we complete the build and test it on the runway at Race Around 1 because simulations are one thing, 250 mph is another.

[00:09:52]Subscribe so you don't miss it, and I'll see you in the next one.

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