I was going HOW fast???!!

[NC Steve]

With my mostly useless speedometer, I need to figure out some way to calculate approximate speed using the tach.
Does anyone have a reference chart or formula to translate rpm to mph? I'm mostly interested in my speed in 5th gear, but it would be good to know what 3rd and 4th translate to also, for slower around town work.
Thanks much.

[msbuck]

Steve,

I'm afraid I can't help much because I have a jumpy speedo as well.   I vaguely remember that I used to run around 68mph at 4000rpm in 5th gear? Did you try to lubricate the little gear in the speedo?  It helped mine a little - now it only jumps about 5 mph at a time.  So I at least have a relative speed range.  I'm already on my 2nd speedo, so I'm not anxious to replace another one...

[Justin B.]

You really didn't state WHY it was worthless, but they are all inaccurate.  I did some timing on mine and found that it reads about 10% too fast.  I can do that math in my head as I ride...  But if your issue is other than this, well, I can't help ya!  

[Justin B.]

Aïda, I have usually found the speedo cable to be the fault of a "jumpy" speedo.  If the core starts to go bad it will get a strand or two (or more) sticking out that will snag on the cable sheath/housing.  This will cause the cable core rotation to not be smooth and constant and the speedo will jump around.  They will get really bad (jumping 15-20 mph) right before they seize up.

[Ed Miller]

I don't know if you guys have the speedometers that top out at 85, like mine (poor, slow bike!) or later ones.  On mine, when the speedometer worked, 4,000 rpm was about 63 mph.  You can do the simple algebra to figure the speed out for any other tach reading.

I've never ridden my Triumph with a working speedometer.  I'm not sure they exist.

[NC Steve]

Aida, no, I've yet to disassemble and lube the gears, although I saved your great details on how to do so. My schedule is such that I don't really have time to work on the bike during the week, and, unfortunately (   ), the weekends have been too nice to voluntarily choose wrenching over riding.
Justin, my speedo's doing exactly what you describe, a 15-20 mph "bounce", maybe more, plus my tripmeter's stuck, although the odometer is fine. I plan to eventually sent it off in a big box full of $$$, but can't quite bring myself to part with what Palo Alto and others want yet.
Ed, I think you're about right with your math too. I had 'er out this afternoon, and around 4000 rpm I felt like I was doing 60-ish, while 3000 seemed to be 45-50. When I'm in traffic I don't worry much about it, I just try to get out front and stay slightly ahead of the flow, but on some of the backroads with little other traffic, I'm just guessing.  

Thanks!

[Rob Valdez 79 R65]

Steve,

I will be happy to go out and take notes with my GPSr, maybe even tomorrow.  There is supposed to be a 4 hour window when it won't rain, and it is supposed to get up to 40F!  

If your speedo is bouncing that much, I would get a new cable (and boot!) for the bike.

[Justin B.]

Steve, undo the speedo cable at both ends and rotate it.  Any scratchiness or stickiness should be considered suspect.  Also, you can chuck a "strike anywhere" wooden match stick in an electric drill and use that to rotate the speedo, see if it reads smooth or still "jumps".  The '80 we have had sorta the same issue and it turned out to be the bushings that the drive gear in the speedo ride in were actually rusted to the gear's shaft.  No mean feat, considering the bushings are made out of nylon!  Being frozen to the shaft caused them to spin in the housing, hogging out where they rode, and made the gear mesh improperly.  Fixing this up smoothed it right out...

[msbuck]

Justin,  the first thing I tried was putting on a new cable.  We were out in Medford, Oregon when it started, so that was the easiest thing to do.  Didn't help a bit.  It was jumping ALL over the place!  Had to let my hubby take the lead for a change to keep the speed down.
[smiley=evil.gif]

Once we got back home we did some lubrication and did the check with the drill.  It's much better now, but not quite there yet.  I still have my original speedo (this one is my first replacement)...maybe I should send that one off to be rebuilt.  When my original went bad, it was 'slightly' more for a brand new one verses sending it off.  So I opted to replace it.  Not sure that would be the case anymore, though.   [smiley=shocked2.gif]

[Justin B.]

Hard telling, prices have went crazy on most hings lately...

[NC Steve]

I think I'll try replacing the cable and lubing the innards 1st, as an inexpensive possible solution to my speedo's twitchiness, and use Rob's GPS rpm to mph calculations (thanks, Rob!) as a guideline in the meantime.
Barring that cure, though, guess I'll eventually have to take out a 2nd mortgage on my house so's I can ship the thing off to Palo Alto...

[Rob Valdez 79 R65]

While a bicycle computer can be a PIA to set up correctly, I think it is a very viable alternative to the stock unit.  Not as much fun as a GPSr (IMHO), but a lot cheaper ($20), and you don't have to wait to acquire a signal before it works.  And it is less tempting to thieves.
And it gives you a clock, and tripmeter and odometer and kilometers/miles and and and...

[NC Steve]

A bike computer does all that, and for 20 bucks??!!
I guess these things are available in any good bicycle shop?
Why are they such a pain to initially set up?

And, how is the visibility factor, both in placement and while riding in bright sunshine?
Thanks for any more info, Rob.

[Rob Valdez 79 R65]

I don't think visibility in the daylight is as much of a problem as night time.  AFAIK, none are backlit.
You do need to make sure it will function beyond 55 or 60 mph.  Some do not.  A bicycle dealer can answer those questions.

I do not have one.  Since I already had a GPSr, and wanted to mount it anyway, that was my solution.

Here is a link to an installation http://www.geocities.com/toms_toys/bc80.html

I believe trolle has one mounted on his, but I just looked at his website and didn't find the info.
Several folks over at Boxerworks forum have them installed.  I suggest a search there for "bicycle computer" or "bicycle speedometer".  LOTS of info there.

[NC Steve]

Rob, thanks again for the great info.
I see what you mean about the PITA part, but still, all that info, and accuracy to boot, for 20-some bucks is hard to argue with!
There's a serious bicycle shop not far from me, and I think I'll definitely swing by there next weekend on the bike, weather permitting. I know a couple of these folks thru my son and my personal trainer/bi-athlete neighbor, and I think they might get a kick out of this!
Hmmmm, maybe I can even pull a Tom Sawyer and "let" them help me set the thing up...