Bike Rumor: just wow.

Questions about bike hire abroad and everything light bike related. No off-topic chat please

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youngs_modulus
Posts: 586
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

TLN wrote:
Wed Nov 27, 2019 7:34 am
youngs_modulus wrote:
Wed Nov 27, 2019 4:03 am
Interesting! Got a source for that?
I'm skeptical, but let's see what you've got.
Oh, no source.
I know. ;)
TLN wrote:
Wed Nov 27, 2019 7:34 am
But they're hitting doot panel and there're no damage. I assume same applies to hood and other panels, that won't deform on impact.
All the modern cars, unibody or body on frame covered with frames (bumper, hood, side panels) that will absorb some impact.
I follow why someone might think that, but it turns out that's not how this works. First of all, everything deforms under every load; the only question is how much it deforms. To paraphrase Robert Hooke, all the world's a spring.

Even if you designed a car not to crumple in an impact, you wouldn't be able to sell it for street use--you'd never pass any crash tests. An infinitely stiff car would stop instantly in a collision, producing enormous G forces and killing the passengers.

As you say, the bodywork on all cars--including Tesla's future truck--absorbs impact energy by permanent crumpling or bending, AKA "plastic deformation." When an impact doesn't bend metal permanently, it still deforms, springing back into its original shape. Even this elastic deformation helps, spreading the impact energy over a longer period of time. This reduces peak Gs even if the bending isn't permanent.

But a dent-resistant body panel can actually absorb more impact energy than a standard panel. As I said before, hardness can be a tricky concept--it's probably easier to think of hardened stainless (especially the 300-series alloy in question) as strengthened stainless. Hardened/strengthened stainless takes more force to bend permanently, so it absorbs more energy than non-hardened stainless in the process of deforming.

In fact, car manufacturers use especially high-strength steels in crumple zones for exactly this reason. That's one reason the BR article ticked me off--the author casts a gimlet eye at "hardened body panels" without realizing that unibodies have been using increasingly hard/strong steel for the last 40 years.

So a thick panel made from strong steel resists dents from sledgehammers (~10 kilos hitting at ~10 m/s, or 1000 Joules) but still deforms plastically under bigger impacts (a rider/bike at 200 kilos and 60 KPH, or ~28,000 Joules). I'm glossing over some of the details here, but the Tesla truck is not going to be objectively more dangerous to cyclists than any other big truck.

As others noted, any cyclist hit by a truck is having an extraordinarily bad day, no matter who built the truck or how.

I agree with TheRich that the BR author seems to have some axe to grind with self-driving cars, Tesla in particular, Musk or all of the above. And I share TheRich's bafflement—there's plenty to criticize about all of those things without imagining nonexistent threats. If you hate Musk or fear self-driving cars, just say so.

joejack951
Posts: 745
Joined: Fri Feb 18, 2005 6:50 pm
Location: Wilmington, DE

by joejack951

youngs_modulus wrote:
Wed Nov 27, 2019 7:47 pm
But a dent-resistant body panel can actually absorb more impact energy than a standard panel. As I said before, hardness can be a tricky concept--it's probably easier to think of hardened stainless (especially the 300-series alloy in question) as strengthened stainless. Hardened/strengthened stainless takes more force to bend permanently, so it absorbs more energy than non-hardened stainless in the process of deforming.
Rather than 'strengthened' which to the masses likely isn't much different than 'hardened', how about simply 'more elastic'? I know it doesn't as cool or techy but that's what it is. For a given panel thickness, it can deform further while remaining elastic (not deforming permanently) but due to the fact that virtually all steel has the same stiffness, it doesn't take any more force to create this deformity than it would for non-hardened steel.

Someone should make a Youtube video with a side by side comparison of full soft 300 series stainless wire next to cold drawn spring-tempered wire. Use the same diameter and length of wire and show how a deflection of some distance permanently deforms the former while the latter springs back. Use a force gauge to show the relatively equal forces to create the deflection.

I only have full soft wire on hand otherwise I'd do it right now.

by Weenie


youngs_modulus
Posts: 586
Joined: Wed Sep 20, 2006 1:03 am
Location: Portland, OR USA

by youngs_modulus

My only issue with that is that "more elastic" sounds a lot like "more flexible" in much the same way that "harder" sounds like "stiffer." Hardened steel is just as stiff as annealed (full-soft) steel--it's just stronger.

But I don't mind if anyone prefers to think of hardened steel as "more elastic" than the annealed stuff. "More elastic" is certainly accurate in a technical sense.

That's certainly more than I can say the Bike Rumor article. ;)

froze
Posts: 166
Joined: Thu Aug 05, 2010 3:47 am

by froze

I don't care how they make a car or truck to be safe for cyclist or pedestrian impact, if you get hit by one it's going to be a painful day, or worse.

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