Speeds on Strava

[Drago]

My Missus did some walk for life thing. She tracked the journey in Emdomondo and apparently at one point was walking at 44mph.

 

[young Ed]

Stop in 2 cm? Is that really what you meant to say?

no, i meant to say i can stop in 20mm! i genuinely reckon at anything upto 20mph i could stop on a 2 pence although it wouldn't be normal braking and would involve the rear wheel jumping up in the air!

I don't think it is a brilliant idea to support downhill records on open roads!

i don't ever encourage other riders to go for high speed, downhill records on a public road.... it makes it easier for me to get the KOM that way!
Cheers Ed

 

[JMAG]

i genuinely reckon at anything upto 20mph i could stop on a 2 pence although it wouldn't be normal braking and would involve the rear wheel jumping up in the air!

I'd love to see that. Make a video and if you can stop your bike from 20mph in 20mm, I'll buy you a lollipop.

If you're still at school, find a physics teacher and I'm sure he'll make a bet with you too

 

[young Ed]

I'd love to see that. Make a video and if you can stop your bike from 20mph in 20mm, I'll buy you a lollipop.

If you're still at school, find a physics teacher and I'm sure he'll make a bet with you too

can i exchange the lollipop for a lemon sherbet?
just passed my physics so now i've no got a physics teacher to also make a bet with
the video may have to wait though as the roads are wet (flooded by a few inches in places) and until i've worked out how to test my stopping distance accurately enough...
Cheers Ed

 

[JMAG]

You passed physics? You should be able to follow this then.

20mph = 32.186kmph
32.186kmph = 8.9405 m/s or 8,940.5 cm/s
To stop in 2cm, you would need to stop in 0.002237 seconds. This deceleration would be approximately 6666.7 mph/s or 303.03 g's.

No go and look what astronauts and fighter pilots can withstand

 

[Spinney]

I used to be a physics teacher* - I'll make that bet with you @young Ed

Also, assuming young Ed and his bike have a total mass of 70 kg, at 9 m/s his momentum is 630 kg m/s

I made the stopping time 0.004 s, so the braking force would be change in momentum/time, so 630/0.004 = 157 500 N (or the same as the weight of a 15 tonne mass, very roughly).

This is the kind of thing often known as a back-of-the-envelope calculation. But even if the figures are out by quite a long way, I think you may have rather over-estimated your stopping power.


*and therefore hoping I've got this right so I don't make a total arse of myself!

 

[Spinney]

I'd love to see that. Make a video and if you can stop your bike from 20mph in 20mm, I'll buy you a lollipop.

If you're still at school, find a physics teacher and I'm sure he'll make a bet with you too

or she!!

 

[ianrauk]

Does Ed means he can stop from 20mph in the width of a 2p piece, or does he mean he can stop ON a 2p piece?

 

[Spinney]

The former. His first claim was ambiguous, I can see, but see this one...

no, i meant to say i can stop in 20mm! i genuinely reckon at anything upto 20mph i could stop on a 2 pence although it wouldn't be normal braking and would involve the rear wheel jumping up in the air!

i don't ever encourage other riders to go for high speed, downhill records on a public road.... it makes it easier for me to get the KOM that way!
Cheers Ed

Even I could stop ON a 2p, as long as it was sufficiently far away to allow for normal braking!!

 

[ianrauk]

The former. His first claim was ambiguous, I can see, but see this one...



Even I could stop ON a 2p, as long as it was sufficiently far away to allow for normal braking!!

OK.
Yup if it is the former that he means, then no way in a million years could he stop in that distance.
Ed. You are so wrong young man.

 

[Sea of vapours]

...
Also, assuming young Ed and his bike have a total mass of 70 kg, at 9 m/s his momentum is 630 kg m/s

I made the stopping time 0.004 s, so the braking force would be change in momentum/time, so 630/0.004 = 157 500 N (or the same as the weight of a 15 tonne mass, very roughly).

And for extra marks: assuming Young Ed has decelerated that rapidly, approximately what area. perpendicular to the original direction of motion, will what-used-to-be-Young-Ed now cover?

 

[Andrew_P]

You see I always knew there would be a use for my Physics classes, pity I never listened...

 

[JMAG]

I used to be a physics teacher

Oooh, can you calculate how fast Young Ed's buttocks will be travelling when they hit the back of his head?

 

[Dan B]

You passed physics? You should be able to follow this then.

20mph = 32.186kmph
32.186kmph = 8.9405 m/s or 8,940.5 cm/s
To stop in 2cm, you would need to stop in 0.002237 seconds. This deceleration would be approximately 6666.7 mph/s or 303.03 g's.

No go and look what astronauts and fighter pilots can withstand

I don't think it'll end up making that much difference, but this calculation assumes that the bike stays on the ground. If you allow the rear wheel to rise, the CoG of the bike will be able to decelerate over a substantially longer distance - until the weight is over the front axle, at which point you can't control which way it comes down. So, at a rough guess, 20mm plus (length-of-bike/3)

I'd still very much like to see it though;-)

 

[Spinney]

Oooh, can you calculate how fast Young Ed's buttocks will be travelling when they hit the back of his head?

Mind bleach time!!!