Just wondering how fast my wee bike was going?

back-on-two-wheels

Quite some time ago, I had a lovely sprint down a long, fairly steep hill on my trek 1.5 however I reached a point where I couldn't pedal any faster. Even though my legs were hammering round more quickly than comfortable, the chain was 'slack' so I'm guessing I ran out of gears :shock:
I'm sure there's probably a mathematical method of working out my approx speed - but as I don't really know how fast my legs had to pedal to 'run out of gears', I reckon there's a component missing from the equation.
So, has anyone conveniently had a computer fitted in similar circumstances... and what did the digits read?

Giant Phil

There is a hill near me commonly know as the 2 mile bends, where I spin out on my roadie, and continue to do around 50 mph.

If you are DIEING to know exactly how fast you were going, you could take the easy method: Do the hill again with a bike computer, or the hard method: using the ratio of the gears, and your approx rpm (if you span out, I would say around 180rpm) to work it out.

When I am in top gear, my crank does 1rev for every 4revs that the back wheel does.

The diameter of a wheel is 70cm, so 70 x Pi gives you the circumference of the wheel - 2.19m.

So, at 180rpm you would complete 10800 revolutions in one hour. If you times that by 4, 43200 you get the number of revolutions your back wheel is doing in one hour. Times that by 2.19m = 94608 meters in one hour.

Convert that into miles an hour, you have 58.7mph.

(Slightly unorthadox way of doing it I'm afraid) But that isn't far off, you can play around with the approx rpm you think you were doing, to maybe get a better Idea.

Phil

dennisn

You need to know 3 things. Circumference of you rear tire, gears you were in(i.e. 52-12),
RPM's of the crankset.

Dennis Noward

itboffin

back-on-two-wheels wrote:

Quite some time ago, I had a lovely sprint down a long, fairly steep hill on my trek 1.5 however I reached a point where I couldn't pedal any faster. Even though my legs were hammering round more quickly than comfortable, the chain was 'slack' so I'm guessing I ran out of gears :shock:
I'm sure there's probably a mathematical method of working out my approx speed - but as I don't really know how fast my legs had to pedal to 'run out of gears', I reckon there's a component missing from the equation.
So, has anyone conveniently had a computer fitted in similar circumstances... and what did the digits read?

Here's a link (one of many) to an online gear inch calc http://www.velobase.com/Resource_Tools/GearCalc.aspx

I had the same problem spinning out on the down hills with my 50T big ring, now upgraded to 53T which helps but in the long run improving my spinning technique had a bigger effect, even so there are only so many RPMs I can manage before feeling sick and if you're going that fast it's unlikely that continuing to pedal is going to add any extra speed, just tuck get low and enjoy the speed.

Living in Wiltshire myself I know there's some real monsters out there, great fun, well the down hills anyway and really helped me with the HOTA sportive last Sunday even Cudham 'Test Hill' at 25% wasn't that bad.

In case you're interested a bunch of us from the commuting thread will be doing a leisurely fun ride on the 8th March in and around Marlborough & Pewsey

http://www.bikeradar.com/forum/viewtopic.php?t=12596650

John C.

IF you are running 52-12 gearing
a candice ( crank revolutions per minute) of 60 relates to about 20mph

I think I can spin fairly quickly and I can spin at just over 120 rpm.so I can peddle down hill at just over 40mph. I do feel at this speed as if my legs are about to be ripped off.

CiB

53-11 gives me 30mph at about 82rpm, but I used to run out of legs at not much more than 42mph on the 52-12 that was on the bike, and I wouldn't be surprised if that's what you (OP) were hitting - around the low to mid 40s.

cougie

Impossible to say really - we dont know how fast you can spin, or the gears you were using.

John.T

I have to hit the brake at just over 30 mph on my 65" fixed. 150+ rpm but I could not hold that on gears. I have spun out on 53/13 at around 53 mph trying to stay with someone on 53/11 in a hilly TT afew years back. Down hill with a gale behind us.