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Thread: Aftermarket Speedometer Conversion (1978 R100RS) - hints or advice for success?

  1. #1

    Aftermarket Speedometer Conversion (1978 R100RS) - hints or advice for success?

    Hi, longtime BMW owner (R1200C) but new to BMW MOA and the forums. Also a new owner of a '78 R100RS (though the fairings have been replaced to look like an 'S'). The speedo on it is busted. The previous owner said he'd actually fixed it once but it broke again soon thereafter and he gave up. He mounted a GPS unit, which conveyed with the bike. I'd rather get a working speedo put in.

    OEM/OE speedos go for $250 on e-bay, and I'm not willing to spend that on old parts when I can likely get newer [aftermarket] parts for much less. I also would prefer to update the instrument panel to a more modern and slimmed down look anyways.

    After doing a bit of research, I've become familiar with the caveats with putting non-original speedos in. Unfortunately, most manufacturers don't seem to give much guidance on working around those caveats, mostly seems to be set ratios. I think I am settling out with the Acewell digital multifunction speedo and tach from Dime City Cycles (DCC): https://www.dimecitycycles.com/black...eedometer.html, especially since it provides both speedo and tach and most of the indicator lights (although turn signals work on the bike, the instrument panel indicators don't, so that's another thing I'd like to fix). So I think I can replace the entire dash with that one meter.

    Their speedometer-only analog meters are set for 60mph @ 2240 rpm, which doesn't seem to work for this bike (I read elsewhere on the forum anecdotally that the R100 is about 70mph @ 4000 rpm. even allowing for some uncertainty, those ratios are just too far apart. It's also shown with a sticker on the back "M=2.1" which I assume is the ratio, whereas the R100s have either 3.0 or 2.91)

    So, questions:

    - I spoke to someone at DCC and they seemed pretty confident that I would be able to get the digital meter (but not the analog) working with the bike, via the programming functions. But I thought I'd ask around before dropping the $150 (or more). Does anyone have experience with converting over to this instrument? Have you had success?

    - The digital multifunction meter comes with parts to install a magnetic sensor on the front wheel. Alternatively I can get the Acewell Digital Speedometer Conversion cable ($19) and hook it up between my existing mechanical speedo cable and the meter, and use the programming functions for calibration. For $19, that seems preferable to installing another sensor and having to get that calibrated correctly, though that calibration seems to be just a matter of programming the front wheel circumference into the meter. Suggestions or recommendations?

    - I have *not* done research on tachometer replacement, so I don't know what the potential caveats are there. (Honestly, I'm not too concerned with having a working tach, I'm not going to race it; but if I can get one working "easily", I might as well.) Is that a straightforward conversion/hookup? Can I just rewire the input to the existing tach into the digital multifunction meter?

    - Are you aware of other aftermarket solutions that work out-of-the-box (and that cost less than $150)?

    On a sidenote, it seems like speedo conversion should not be so difficult! Maybe it's just the engineer in me, but even for analog meters, I'd think you could just reprint the gauge face with a different range to have it reflect speeds for different ratios. In other words, if speed is linear with (proportional to) some frequency input, why couldn't you just change the spacing between the dial marks to "calibrate" the dial for the right ratio? You might peg high, or the reproportioned gauge may reflect speeds you can never reach, but that's something I'd live with to get a working speedo that is somewhat accurate. It'd be an ugly hack-job if I did it myself, but seems like something the manufacturers could pick up on....

    anyways, thanks in advance for any input!

  2. #2
    Liaison 20774's Avatar
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    Welcome to the forum! Lots of research there...I can't help much with some of those details. In your research, did you run across the function of the charge light in the instrument housing? It's critical to the charging system. There are ways to deal with it. There are speedo rebuilders. One of the better small shops is Terry Vrla. His contact info along with other speedometer resources is in the link in my signature line.
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
    Quote Originally Posted by 20774 View Post
    Welcome to the forum! Lots of research there...I can't help much with some of those details. In your research, did you run across the function of the charge light in the instrument housing? It's critical to the charging system. There are ways to deal with it. There are speedo rebuilders. One of the better small shops is Terry Vrla. His contact info along with other speedometer resources is in the link in my signature line.
    krikeys! Thanks for the head's up. no, I have not done broader research on whole instrument panel replacement, so I guess I will have to.

  4. #4
    Liaison 20774's Avatar
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    If the PO has something non standard in place, maybe he took care of the issue. A resistor placed in the right spot can correct for the absence of the charge light. Snowbum discusses this on his website.

    Http://bmwmotorcycletech.com
    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  5. #5
    Quote Originally Posted by 20774 View Post
    If the PO has something non standard in place, maybe he took care of the issue. A resistor placed in the right spot can correct for the absence of the charge light. Snowbum discusses this on his website.

    Http://bmwmotorcycletech.com
    the instrument *is* all original, the previous owner tried a speedo rebuild. I'll look into the charging light issue, I may even be able to use the meter depending on the resistance requirement.

  6. #6
    Liaison 20774's Avatar
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    Kurt -- Forum Liaison ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  7. #7
    Rally Rat 1074's Avatar
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    Speedo Rebuild

    If the PO tried to rebuild the speedo himself then the problem may be in the rebuilder himself! I am NOT a speedometer rebuilder and although it's been said that I'll try anything once, I'd just as soon give my instruments to someone who knows what he is doing!
    With that said, there are three professionals who come to mind for this job. Palo Alto Speedometer, North Hollywood, and Overseas in Austin, TX. When it comes back it'll be like new so I wouldn't worry about the age of your unit.
    I have used all three of these companies in the past and can recommend all of them. Do the research in that direction and you won't have to worry about adapting and/or jury rigging something to make it work almost like the original unit. Plus, most likely it will cost less for you to get more.
    Boxerbruce

  8. #8
    I had good luck with Overseas. I'd bet they'd fix it for under $200. Call and see what they charge for an estimate. Alternatively, there is a good article on how to fix the odometer somewhere (and someone will chime in with the url). I'll bet that's what the PO was doing when it broke. You could go in and see if you can see the problem (and perhaps still pay someone to fix it correctly). BTW, hooking a drill to the drive is a good way to check operation.
    '61 Clubman's Gold Star, '13 690 Duke, '13 Daytona 675R, '14 Street Triple R, '17 1290 GT (gone but not forgotten: '76 R75/6, '84 R100, '76 R90S)

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