rpm at speed

[Guest captain nemo]

I went out today to a bicycle shop and picked up a Sigma BC 800 speedometer that I will mount on the bars. You take a measurement of your front tire by rolling it out on the pavement and then entering the distance. You attach a magnet to the front rim and the thing gives you an EXACT speed in mph or kph. If you keep the same tire pressure, this is a highly accurate figure. I will soon see when I have time how much error is in my own speedo. The thing has many functions. One of them I like is that it will record your highest speed and freeze it so that if you really feel like a salt flats experience, you don't have to look down - and the evidence is there FOR ALL to see when you pull into the cafe. It also serves as a trip meter and will figure your ave. speed and body fat also I think.

[dlaing]

Thanks for the post Captain! I have been wondering how these speedos work.

Obviously we don't have 27 inch wheels with 1 inch tires...

[Guest J.R.]

I have without a doubt,hit the limit of the speedo and then someon the Lemans.

One of the longest stretches of straight road we have in these mountains.

Believe me, it didn't take that long to bury the speedo and keep on chucking for more.No real numbers,but as fast as I ever want to go.When I had the balls to look down,it was reading 140.Optomistic? Who cares, at that speed the world goes by VERY fast.

 

J.R.

[al_roethlisberger]

Yeah, at those speeds, who really cares... plus, especially on our bikes, there's always someone "faster"

 

I did my one time "high speed" runs some years back on Hwy50 in Nevada, the "Lonliest Highway in the World" :

 

Nevada's Hwy 50

 

According to my FJ1200's also I'm sure very "optimistic" speedo, my friend and I hit 155mph. We even did a 160 mile run from one moutain pass, across the bottom of the valley to the horizon up the next moutain, in 80 minutes. You do the average speed math

 

In all these cases, the bike was rock solid, and I think it had a bit more oomph to go. But especially in retrospect I realized how close I came to dying. If *anything* had happened, and we saw a lot of *anything* on that trip, deer, vultures, tumbleweed, you name it...

 

One of those *anythings* was when we were cruising along at about 80mph outside of Area51, when a dust devil came along and literally blew my friend off the highway into the desert. Although he pulled it off(but almost dumped it), the VFR obviously wasn't designed for "off road" use

 

 

Anyway, not being judgemental, and to each their own.... but I personally don't think I'll be doing any top-speed runs again any time soon. Been there, done that, and although nothing happened, hindsight tells me that I was very fortunate. So I'll count myself lucky, and not push that luck again.

 

I've just seen too many motorcyclists in my area die over the last year or two, and it's sobering....

 

 

al

[Guest captain nemo]

Yes, I hear you Al. What I really want is accuracy which is why I bought this Sigma. I haven't put it on yet - it is designed for spokes - but I think with epoxy I can get the little magnet onto the front rim. We'll see.

 

I think about 100 mph is tops for me - and in short bursts. That still feels 'reasonable' to me under good road and seeing conditions. But, again, not for long. This is really at the limit of my 'seeing.' I simply enjoy how fast the Guzz will get there. When you've been cruising at 70-75 mph, a short burst to 100 when passing is fun and not too insane.

[callison]

I wouldn't glue the magnet to the rim for a number of reasons. Mine is mounted to a short piece of aluminum that sits under one of the rotor bolts for the front disk brake. It's 1 A.M., so I'm not going to run out there to take a picture, but tomorrow I'll give it a shot. There was a web site for sports bikes a few years ago with some very good explicit instructions on mounting these gadgets. It may still be out there.

[Guest Jeff Kelland]

I have seen 140mph indicated on my 02 lemans. I was suitably impressed with it's stability. I am not an advocate of indiscriminate speed runs, but every once in awhile when the conditions warrant.....well I just can't seem to help it! I'm guessing maybe low 130's.

[Guest captain nemo]

Carl, I would very much appreciate any pics you can take of the Sigma set up. I don't see how the magnet can be attached to the wheel without 'glue' of some sort. I think I can attach the pick-up around one of the Brimbo lines. I'll wait to hear from you....

[dlaing]

Keep in mind that the tire circumference grows with the increase in velocity.

So, the faster you go, the slower than actual it will read.

What the difference is? I have NO idea.

Perhaps someone with a GPS and a Sigma could do a comparison.

[callison]

I don't even have to take photos or explain it. Just look here...

http://more.sportbikes.com/reading/cycle_computer.html

or here...

http://www.geocities.com/MotorCity/Downs/3.../3772/Bc700.htm

and here too...

http://www.fmack.org/mc/cbr_mybike/index.html

and a not so good set of pictures...

http://www.geocities.com/toms_toys/bc80.html

[docc]

Keep in mind that the tire circumference grows with the increase in velocity.

So, the faster you go, the slower than actual it will read.

Seems like if the circumferance grows, the tire covers more ground per revolution and the speedo discrepency would diminish with increased velocity.

 

Of course, some calculus genius will tell us that the speedo is dead on at 476 mph.

[callison]

I think you're going to have a hard time attaining a speed that will dramatically change the circumference of the tire and maintaining that speed long enough to create a substantial error. Five years ago, I got to witness an LSR bike tire rotated to 275 mph. It grew 4" in diameter overall!!! Scary to be near. Our bikes aren't likely to attain half that speed for any appreciable length of time and I kind of doubt that the tire growth is very much anyway. I have these Sigmas on 3 of my bikes and they're orders of magnitude more accurate than the one on the bike. If they don't drop under 1% error in the final measure, they're still an improvement and I couldn't care less about the difference at that level as the Sigmas are so flexible and useful in other ways. I use them to track total tire miles and a bunch of other things.

[Guest captain nemo]

Thanks Carl, I'll look at those links tonight. I'm out the door early A.M. again to go to my Motorcycles Safety Class. Three days - two of them LONG days of riding and testing and it is FRICKING COLD. Good thing my riding gear is here.

 

Talk about the obsession with speed - one of these guys is new to bikes and he was talking to me about his squid friend and how he got a ticket in Nevada for doing 160 mph. He said his friend was only in 4th gear on his Suzuki 1200. (I thought 'sure, ok'). He then went on to say that the bike would do 220 mph. (I thought, 'sure, ok'). I didn't dispute it or say anything. You see how nice I am in person? But, anyway, this obsession with speed is everywhere and everyone has some ridiculous notion about it.

[callison]

One of the forums I read had a posting a few years ago about European specs for speedometes. +7% fast, -0% slow. Making a speedometer that reads slow allowing you speed is just plain verboten. But you can make a speedometer that reads 7% fast meaning that you're careening down the road thinking you're going 70 and you're really doing a tad over 65 mph. An it's a legal speedometer! But not a technically accurate one. So, given that the speedometer can be anywhere within a band of 7%, tire circumference changes as the tire wears (and the rear tire changes most, and the most rapidly [typcially], and that's the one driving the speedometer) and rear wheel slippage that translates into a higher speedometer readout, it should be a relative no-brainer that a bicycle speedometer driven by the front wheel will be much more accurate than the one that comes on the bike. Especially since you have control over the calibration.

 

So, if a guy says his bike goes 220 mph and the allowable error is 7% (and I'll almost guarantee the bike manucturer pushes it this way because it allows for market specmanship for top speed), then his top speed is more likely to be somewhere just over the 200 mph mark, which is dead on for a stock Suzuki Hayabusa. Too damn fast for me. I'll stick with the Guzzi's!!!

[dlaing]

I think you're going to have a hard time attaining a speed that will dramatically change the circumference of the tire and maintaining that speed long enough to create a substantial error. Five years ago, I got to witness an LSR bike tire rotated to 275 mph. It grew 4" in diameter overall!!! Scary to be near. Our bikes aren't likely to attain half that speed for any appreciable length of time and I kind of doubt that the tire growth is very much anyway.

At half that speed we'e be doing nearly 140MPH. If the diameter expanded 2 inches, then the circumference would increase over 6 inches (2*pi) which is about a 7 or 8% increase.

So if the Sigma is reading 140MPH, you may in fact be going about 150MPH, so the Vague-lia may be more accurate for high speed phreaks.

OK, so maybe the front tire is not going to expand an inch in radius(otherwise it would hit the fender).

In any case the Sigma is not the final verdict for measuring top speed, although it is a great way to monitor trip mileage, fuel consumption, velocity through speed traps, etc.

Even Land Speed records do not determine true peak top speed, they measure the average speed through the traps.

Even GPSs have limitations, but are generally accurate to 0.5MPH up to 999MPH.

On a related note, here is a "this should be illegal" product for speeders in the UK:

http://www.geodesygps.co.uk/links.htm