Speedometer question

[dragonrider]

First things first..... just want to say a big thank you to all who have helped me since I joined. My husband has a bike that is not a gs and he is actually envious that there is a forum like this where people actually seem concerned and helpful.  My question has to do with the speedometer on my bike. I have just started riding and my husband goes with me when I ride. Before the problems I started having, we were riding along a very busy highway and I thought I was doing the speed limit (55) When we got home my husband told me I needed to pick up my speed and he said I was only doing 45. The head mechanic at the local cycle shop said that some bikes could be off the correct speed by 10 to 15 mph and it wasn't worth replacing the speedometer, just be aware that it was off. This seems a little dumb to my way of thinking and I was just wondering if anyone else had this problem. Thanks

[John Bates]

I get the feeling from reading other peoples posts on speed that the GS spedo is not to be relied upon.  Most refer to their speed as "indicated speed"  not "true speed" in recognition of the fact that speedometers are in general  not very accurate.  True on some bikes as well as some cars.

Since the speedo reading is derived from front wheel rotations, the size of the front wheel has a major effect on accuracy.  If your front tire has been changed from the original (in size) that could explain some error.

[cummuterguy]

also, the lower the tread depth of the front tire, the less accurate it gets... not by much, but it does

I find my speedo to be about 10% off (showing faster than I am going) on a 50% worn tire (never checked it when the tire was new)

[mastrind]

I think of as a nice addition....if I think I'm going faster then I am, then I am less likely to get a speeding ticket....I've been told the rule of thumb is this:

actual speed is 10% to 15% less then indicated speed (depends on bike, year, tire tread and make, etc,).....avg is about 10% less.....so if indicated 55 mph.....more likely to be 47 to 50 mph....

....that's just what I've heard.....

[coll0412]

I think the tach is pretty accurate, maybe we can figure out the speed basd on the tach and gear ratio for the bike,

I guess everyone has different size tires, but maybe someone can give some specs on the gear ratio, and make an equation to find the actual size with the two variables being RPM and rear tire size

[weaselnoze]

my dad has a gps that has perfect accuracy for speed in mph.  i should try it sometime

[cummuterguy]

or, just get a gps unit and mount it on the bike somewhere..  I wonder how accurate those are?
I just borrowed one from a buddy to take with me on my trip to s. dakota next month. maybe i'll test it on the bike this week...

editage....   weaselnose types faster than i do...lol

[coll0412]

So instead of doing homework I quick derived the equation, with the help of gsJacks complation of stats on the GS

here it is


(rpm*.261825*diameter of tire(in))
-----------------------------------------      =mph
(gear*primary*secondary*88 )

Gear
1st=2.461
2nd=1.777
3rd=1.380
4th=1.125
5th=.961
6th=.851

Primary(stock)
2.714

Secondary
2.437



This should be right, anyone with corrections please post

[coll0412]

Also if you dont want to measure your tire diameter then

((width of tire(mm))*(aspect ratio) *.078740)+ 17=diameter(in)
       

i.e stock 130/70 17

so 130mm width
aspect ratio is .70

which gives us a diameter of 24.16(in)


again if this is incorrect please post corrections

[vfrocket]

or, just get a gps unit and mount it on the bike somewhere.. I wonder how accurate those are?
I just borrowed one from a buddy to take with me on my trip to s. dakota next month. maybe i'll test it on the bike this week...

I have a Garmin E-trex, and I have found it to be pretty accurated on speed. Maybe 1-3 mph off of the indicated speed on my rangers speedometer.

[starwalt]

I just love this group and am happy to see another nutcase, like me, who would think about equations to determine things like mph based on rpm! That's what college courses do to a mind. Neat equation coll, I will add that to my collection.

Regarding external checks for speed with a GPS:
any late model GPS is very accurate when it comes to speed. I've taken my Magellan Platinum all over the east coast and even Haiti. Plus or minus 1 mph every time, every where.

Regarding the speedometer accuracy:
This mechanism has been discussed in other threads. CLICK HERE for a representative drawing of how the 2002 and older tach and speedometer works. There will be an inherent slipping of the input with the output with time, temperature, and phase of the moon (just kidding   ).

[gsJack]

Every time I pass one of those advisory type police radar carts along the road, I hold it on 40 mph.  They always read 36 mph on the 97 GS.  Very consistant 10% off. They are always in the crowded urban areas around here so I haven't tried them at much higher speeds.

When I got the 02 GS, it was a little less off than the 97.  My first tire change was to the 110/80 and 140/80 AV45/46 tires and now the speedo is only maybe 1-2 mph off at 40 mph with the larger front tire that drives the speedo.   Not bad at all.

[addison]

Well, this explains alot. I live in at the beach in Florida where people generally drive slow - retirees and old people....But it seems when I am on my bike people are driving ALOT faster according to my speedo. I think the speedo on my '04 is way way off. Damn, I thought I was actually going 120 the other day...lol.

[starwalt]

Well, this explains alot...the speedo on my '04 is way way off. Damn, I thought I was actually going 120 the other day...lol.

The 2003 and subsequent models introduced a major change in the ignitor circuit. The tachometer is now an electric meter rather than the eddy current type mentioned above.

Alas, the speedometer is the same old technology. Perhaps you have some slippage or linkage issues with the front wheel and drive cable? I know of no test setup to check it. Find/make a police friend and get checked like gsJack.  My money is on it not being linear either.

[Spitfire]

Folks, I too have heard that most speedos are 10 per cent out, but is it always down rather than up? My missus was following me in the car once, and she said I was going over 100 mph when the bike speedo never crossed 95? Also, the book top speed of the gs is around 110/115 but I can't get mine over 105, even downhill with a following wind. It makes me think mine is wrong the other way? Is this poss?

[fettcols]

I know mine has to be really slow... I was on I71 going just over 80mph and got passed by a ratty old (overloaded) jeep wrangler towing a camper!

I would love to get a nice GPS but don't really want to worry about if it's still on my bike when it's parked out in public...

[starwalt]

... but is it always down rather than up?... Is this poss?

Either one is possible given the nature of the device. The output needle of the speedo has two sources of resistance to the spinning magnet turned by the front wheel. The primary source is the spring. Too tight/loose = slow/fast error.

The secondary source that I have noticed after taking them apart is, for the lack of a better term, clear viscous goop. The goop is between the top plate in the following pic and the larger diameter shaft insert between the spring and the top plate. The goop is glommed all over the shaft. Here are pics of the tachometer. The speedometer has the same mechanism in addition to the odometer. I am showing you this because it is easier to see the part we are talking about.



Here's the same tach as viewed from the other side. You can see the space between the larger diameter shaft insert and the dial face mounting plate better. I think the goop dampens the movement somewhat. It sure is sticky.


Being a mechanical gadget by design and subject to the rigors of heat, cold, vibration, abuse, etc. I am amazed they are as accurate as they are. About the only dead on item when it comes to the "clocks" is the odometer. Even that is not immune from Srinath's tinkering!  

[FFDougK]

I picked up a eTrex Legend for hiking and whatnot.  When I tried it on the GS I got the same 10% or so difference between actual and indicated.  What bothered me was when I put the GPS in my Honda Civic...that thing is also off indicating 80 when I'm only going 72!  So much for my favorite automaker, I guess I'll let it slide though.

[dragonrider]

Ask and ye shall receive. You guys are awesome. My husband is sitting in a corner crying because his bike doesn't have a support group like ya'll. We know a couple of cops here where we live, I might ask one to use the "gun" on me when I get her back up and running, otherwise I'll use the ignorance is bliss excuse and just think I am speeding when I am actually not (although getting passed by a horn honking granny is sooo humiliating)    ps... my husband is thinking about using the gps on his bike. Thanks again guys, you are the best!!!

[cernunos]

What bike does hubby ride? Love the Baby G and the forum.

C.......