Speedo ratio

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Mjolinor
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Speedo ratio

Postby Mjolinor » Fri Aug 24, 2018 2:13 pm

According to:
http://bmwmotorcycletech.info/ringgears.htm

I have the correct speedo to go with my back axle, 1.224 and 37/11 but I have a 1970 engine and a five speed gearbox. Did the output shaft / speedo drive remain unchaged throughout?

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Re: Speedo ratio

Postby barryh » Fri Aug 24, 2018 2:31 pm

The number of gears won't make any difference as the speedo drive is taken from the output shaft.

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Re: Speedo ratio

Postby Mjolinor » Fri Aug 24, 2018 4:04 pm

It can if the five speed box has a different speedo drive ratio to the four speed box.

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Re: Speedo ratio

Postby barryh » Fri Aug 24, 2018 6:04 pm

It can if the five speed box has a different speedo drive ratio to the four speed box.
You mean the ratio between the speedo drive gear and the worm drive on the output flange ? I suppose that's possible if the gearbox is from a /5 as the speedo drive gear is a different part number as is the output flange. Odd that BMW would change the ratio though.

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Re: Speedo ratio

Postby Mjolinor » Fri Aug 24, 2018 7:02 pm

It can if the five speed box has a different speedo drive ratio to the four speed box.
You mean the ratio between the speedo drive gear and the worm drive on the output flange ? I suppose that's possible if the gearbox is from a /5 as the speedo drive gear is a different part number as is the output flange. Odd that BMW would change the ratio though.
That is what I am asking. To me the speedo seems low and before investigating a fault in the speedo head I would like to know if a ratio change is possible.
I guess after 50 views it is pretty safe to assume no change or someone would have known.

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Rob Frankhamr
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Re: Speedo ratio

Postby Rob Frankhamr » Fri Aug 24, 2018 10:42 pm

The engine has no part in this equation... Assuming that the speedo is the correct one for your /6 rear drive and that (as you say) you are using a five speed box (any 5 speed box) the correlation between the speedo ratio and the rear drive will be correct.

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ts
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Re: Speedo ratio

Postby ts » Fri Aug 24, 2018 10:43 pm

For a while, when I had the 4-speed gearbox out of my /5 for an overhaul, I had a 5-speed box from a /7 installed. I cannot say I noticed any significant difference in what the speedometer showed (didn't get any speed tickets either). So my guess would be that the worm drive ratio for the speedo cable is the same.

Functionally, it felt like it was possible to accelerate quicker, which could be explained that I was going up though lower ratio gears when accelerating.
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CharlieVictor
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Re: Speedo ratio

Postby CharlieVictor » Sun Aug 26, 2018 7:10 pm

Dunno if it helps, but my R100S speedo was always optimistic by 15 to 20 km/h depending on the speed. We found out the driveshaft came from an R60 with a gear ratio of 37/11 instead of the original R100 of 32/11.
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Harleyman
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Re: Speedo ratio

Postby Harleyman » Sat Apr 25, 2020 5:41 pm

Apologies for resurrecting this old thread, but I'm hoping that someone can enlighten me.

I have recently (at vast personal expense!) acquired a MPH speedometer for my Danish import R100RS. The W number on the MPH unit is 1224 which indicates a 37/11 ring gear ratio; the original KPH speedo says W 691 which equates to 33/11 ratio.

I'm aware that the gearing on the bike is higher ( or is it lower?)than it should be for that particular speedometer, but can someone with more mathematical nous than I have, please tell me whether it will make the speedometer read fast or slow, and by approximately how much in percentage terms?

I'm not overly bothered about OEM perfection; as most of you will know 1977 RS speedometers are one year only and already rarer than hens' teeth. The one I have acquired is in excellent condition and the odometer works; oddly enough I sourced it from Norway! However if I can get some idea of how far out the speedo is it will help me to avoid an expensive mistake should a van be lurking somewhere.

Thanks in advance.
1977 R100RS
1988 R80RT
Several old Harleys and a Honda CB350

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Re: Speedo ratio

Postby barryh » Sat Apr 25, 2020 7:45 pm

The key to your question is to understand that the W number is the number of speedo cable revolution per mile or Km traveled. Converting Km to miles will give you a comparison:

W1224 speedo needs 1224 cable revs per mile

W691 speedo needs 691 revs per Km or 1112 cable revs per mile

That tells you that your new W1224 speedo requires more cable revs to read the same as the W691 so the odometer will read low and the speedo will read low, both by approx. 10 % In practice that could mean your speedo is quite accurate as they usually read up to 10% fast. The odo meter will record 10% less miles though.

Harleyman
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Re: Speedo ratio

Postby Harleyman » Sat Apr 25, 2020 8:59 pm

Thanks Barry, that'll do for me; it concurs with what I'd read, just needed it explaining in layman's language.

I'm not overly worried about the recorded miles; the original was suffering from the usual slip so I've absolutely no idea of what the actual mileage is. The new clock reads just short of 60K. Judging by the footpegs and the fact that it has been rebuilt at some time, I suspect it's done a bit more than that; but it goes well!
1977 R100RS
1988 R80RT
Several old Harleys and a Honda CB350

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7omly
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Re: Speedo ratio

Postby 7omly » Mon Jun 15, 2020 9:08 pm

I'm glad I found this thread. The speedo on my R80RT is marked 1.244 and I know it is correct for the bevel box which is 37/11. The speedo is so inaccurate that at 80 mph I'm actually only doing around 56 mph, I know this because I use a GPS app on my phone to check it. It's so bad I purchased a GPS speedo from eBay and I rely on that now.
I was wondering if I got a different airhead speedo would the fact it's fitted to the wrong bevel box ratio bring it nearer to the the correct speed.
I've even considered purchasing a police spec speedo from ebay.
I've seen a pattern part speedo from India and it has the 1.244 marking on it but if these are direct copies they'll probably be just as inaccurate.
Any advice from the experts would be appreciated.
Stay safe.

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george baker
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Re: Speedo ratio

Postby george baker » Mon Jun 15, 2020 9:45 pm

Hi
I bought a gps speedo from eBay and when it got here I found it doesn't have an odometer even though the advert said it had that feature
:-(
G
Member 21, R100R, and an under used K75

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7omly
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Re: Speedo ratio

Postby 7omly » Tue Jun 16, 2020 12:00 am

Mine doesn't have an odometer either but it's nice to know how fast you're actually going. Someone joked to me the other day that's why BMW riders never get caught speeding.


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