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Offline markking

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Speedo signal
« on: February 03, 2020, 08:42:43 PM »
Where does the speedo signal come from on these bikes? - is it from the ABS module as on modern cars? I ask because when I hooked up my sat-nav for the first time the other day I noticed I've got a 10% speedo over-read. Sat-nav = 70mph, speedo = 77mph. It's a standard bike on standard gearing with the original tyres. If the signal does come from the ABS module I'm puzzled as to how it's affected by changing the gearing as I've read elsewhere on here - the wheels will still turn at the same speed whatever the gearing.

When I've geared down my R1 for track days by dropping a tooth on the drive sprocket I get a big speedo over-read (as expected) as the speedo takes its signal off the gearbox output shaft.

Looking forward to being enlightened - cheers chaps (and ladies?)  :112:

Offline Mister Paul

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Re: Speedo signal
« Reply #1 on: February 03, 2020, 09:26:05 PM »
It's illegal for speedometers fitted to cars to under-read. No speedo is 100% accurate so manufacturers have to design them with the margin of error above the real speed.

Offline markking

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Re: Speedo signal
« Reply #2 on: February 03, 2020, 10:16:24 PM »
Thanks for the reply Mister Paul, but I know that.

So back to my question, does anyone know where the speedo signal comes from?

Offline richardbd

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Re: Speedo signal
« Reply #3 on: February 03, 2020, 10:47:08 PM »
I suspect it's off the gearbox/driveshaft as I believe that changing the sprocket size buggers up the speedo reading.

Mind you, I'm no mechanic...

 :305:

Online Neox

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Re: Speedo signal
« Reply #4 on: February 03, 2020, 10:59:23 PM »
*Originally Posted by markking [+]

So back to my question, does anyone know where the speedo signal comes from?
It come from a sensor in the gearbox, just behind the cylinders on the upper crankcase.

Be careful, speed indication from GPS is not true as you expect...

Offline Roy McKenna

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Re: Speedo signal
« Reply #5 on: February 04, 2020, 03:48:04 AM »
From the VS (Vehicle Speed, most likely) sensor in the transmission. 

I have 2 Honda motorcycles, each with a GPS. The GPS and Speedo are within 1 mph at 60 mph...always.  That match at any speed.  When I have passed speed traps that post your speed on the side of the road... that matches also within 1 mph.  I'm not aware of any law that says a speedo can't be accurate in the US or my state. 
Roy McKenna

Online baldeagle

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Re: Speedo signal
« Reply #6 on: February 04, 2020, 06:20:59 AM »
I fitted a one tooth larger front sprocket on my 16.
Previously reading,on speedo was about 6 kph faster than accurate GPS.
After fitting the 16 tooth sprocket it was dead right.
Present bikes,MZes/250/2 Trophy ,CB500X and a Sym Cruisym 300 Maxi scooter plus a Suzuki Address 125.

Offline markking

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Re: Speedo signal
« Reply #7 on: February 04, 2020, 08:20:44 AM »
Thanks chaps. I had a thought driving to work this morning, there's a non-ABS variant right? which already made me think there must be a separate signal source for the speedo other than from the ABS, so the gearbox output source makes sense, and so it also makes sense that changing the gearing affects the speedo reading. Oddly, after discussing this with a mate (who's also just succumbed to 500X-dom), he strapped on his sat-nav but only got a 2% error at 70mph. His is an '18 model tho, mine's a '16.

I've got a 39T rear sprocket to fit shortly so roughly speaking that should get me around 5% back of my original 10% discrepancy, which is more reasonable.

 - I found the engine a bit 'busy' on the motorway, and as it's a fairly torquey little lump I'm sure it'll take a small rise in gearing without affecting the snappy way it seems to be able to pick up from almost any revs in any gear.



Look at that odo reading - more than3 yrs old and only 373 miles - don't blame me, I only just bought the bike.
« Last Edit: February 04, 2020, 08:35:38 AM by markking »

Offline richardbd

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Re: Speedo signal
« Reply #8 on: February 04, 2020, 08:41:18 AM »
Be interesting to see what you reckon once youve changed the sprocket size.  It seems quite a few people do it and then quite a few of them revert to the original, reporting various low-speed irritations.

I agree, its quite busy at motorway speeds and I was in the change camp.  However, having put 15,000 miles on it in all kinds of conditions - including like 3,000 miles of gravel and off-road, I got used to it and concluded that the Honda engineers know more about this stuff than me.

The issue for me was just that I was coming from a 1200 GS, so any small engine seems busy.  Theres actually nothing wrong with the CB, it was just my reference point.

In a similar vein, I also fitted a gear indicator as in the early days I was forever looking for 7th.  After a couple of weeks on the road I stopped ever looking at the gear indicator, just like I stopped wishing for a different sprocket...

Offline markking

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Re: Speedo signal
« Reply #9 on: February 04, 2020, 08:46:43 AM »
Thanks Richard, I'll keep an open mind re the sprocket then. 2-teeth down on the rear isn't as drastic as I've read some do on here which includes down 2 on the rear and up 1 on the front - blimey!  :033:

And as you found, the more I ride it the less I'm looking for 7th as well, and also I'm getting used to making sure I'm not in too-high a gear when exiting low speed corners - It don't drive smoothly from idle like a four-cylinder does - and the last twin I owned was an RD350LC in 1982.
« Last Edit: February 04, 2020, 08:54:23 AM by markking »

 


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