this post was submitted on 03 Oct 2024
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Aotearoa Weekly Kōrero 4/10/2024 (self.newzealand)
submitted 2 days ago by Dave to c/newzealand
8 comments fedilink hide all child comments
 

Last weeks thread here

Welcome to this week’s casual kōrero thread!

This post will be pinned in this community so you can always find it, and will stay for about a week until replaced by the next one.

It’s for talking about anything that might not justify a full post. For example:

  • Something interesting that happened to you
  • Something humourous that happened to you
  • Something frustrating that happened to you
  • A quick question
  • A request for recommendations
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  • A picture of a cloud that kind of looks like an elephant
  • Anything else, there are no rules (except the rule)

So how’s it going?

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[–] flambonkscious@sh.itjust.works 2 points 1 day ago (6 children)

So stupid question - why does my speedo read ~3km faster than a radar thingy on the road?

Is that a safety margin thing or have i got the wrong tires installed (or similar)?

[–] BikerChiwi@mastodon.nz 4 points 1 day ago (5 children)

@flambonkscious @Dave The guy who services my car told me many moons ago that some vehicles are set to read slightly higher than they really are - as a safety thing.
¯\_(ツ)_/¯

[–] Dave 2 points 1 day ago (1 children)

Ooh that's interesting. I have seen this in almost every car I've ever driven past one of those things. There's actually one car that I remember borrowing one time that stands out in my mind as it matched the speed sign thing and that was unusual enough to still remember it years later.

I just assumed that since the speed is read internally (gear box or something?), as the mm if rubber come off the tire the speedo would over read more and more.

[–] flambonkscious@sh.itjust.works 2 points 1 day ago* (last edited 1 day ago) (1 children)

Maybe? Surely it's quite a few mm to get 3km/h over though?

Sounds probably like I need to learn math again... Eww!

Actually, surely tires aren't all the same size? Ours are recently installed and the measured difference hasn't changed.

[–] Dave 2 points 1 day ago (1 children)

Just found this reddit post. A comment says:

F1TV did a tech talk recently where they measured some tire wear. I don't know how many laps it ran, but the measurement was 0.6mm difference. If the pit limiter was calibrated to exactly 80kph for a new tire, that would equate to 79.87 kph on that worn one. I don't think that's enough for them to account for

I think 0.6mm isn't enough, but we can expand on that. A search shows a new tyre might have 8-9mm of tread. And legal minimum is 1.5mm, so that's 6.5mm or so difference, or about 10x the test above. Still, that would only make a difference of about 1km/h in the reading over the life of the tyre.

So seems 3km/h over is not explained by tyres. I guess F1 car tyres are smaller, but I'd think the impact would be smaller on larger tyres.

Actually, surely tires aren't all the same size? Ours are recently installed and the measured difference hasn't changed.

I thought you had to get the exact same sized tyres, because they had to fit the wheel.

Reddit seems to think it's calibrated to the factory installed wheels.

[–] flambonkscious@sh.itjust.works 1 points 3 hours ago

Thanks Google/Dave! - 0.6mm is a wild amount of wear but they are racing the shit of them...

Are you endlessly curious or something??

I did find this pdf from an nzta page that suggests there's a range of tire sizes available that a given wheel can accommodate (within their range of tolerance as well): https://www.lvvta.org.nz/documents/infosheets/LVVTA_Info_01-2009_V3_Tyre_Size_to_Wheel_Size_Compatibility_Guide.pdf

The 2nd page is a big honking table of different sizes, profiles and rim widths, so I gather there's a degree of anything goes and maybe next time we wear ours out, I'll try to be a little more attentive.. Maybe I can get some accurately sized ones (probably not worth it - getting pinged for going a few kmh over the speed limit would really suck!)

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