Triumphrat

djkennedy67

I have just changed the rear sprocket to a 46T from 43T due availability of NOS parts on my 1962 Bonneville.

The gearing I believe will be slightly lower, but will I have to have the speedo adjusted as it runs off the gearbox and presumably is now overeading.

Any advice welcome.

Regards

David Kennedy
Surrey UK

Plewsy

Hi djkennedy67 welcome to TR
technically yes, but you can getaway with this,
get someone to clock you and see what speedo reads
then dot the speedo 10mph increments.
I think you will find the dots fairly close to original numbers.

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djkennedy67

I thought as much.

I believe the speedo is probably inaccurate any way, so all within 5 mph probably.

Thanks

David

Plewsy

I am certain you are allowed a 10% error factor.

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pookybear

Plewsy,

I am missing something here, I think it is because it is a 62 model.
Is the speedometer drive off of the sprocket? Because if it is off
of the axle through a tang drive, the change in sprocket will not
change the speedometer. Only a change in tire size would do that.


And of course a change from a 46 tooth to a 43 tooth sprocket on
the rear is an overdrive effect.

Pookybear

Plewsy

Hi Pookybear djkennedy67 mentioned the speedo running of the gearbox.
so I just took it for granted it does
You are spot on! if it runs of a wheel, a sprocket change will make no difference to speedo reading.

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djkennedy67

Pre unit Bonneville's speedo drive comes off the offside timing gear drive which has a constant speed in relation to the gearbox.

Hence final drive is lower with increase in rear wheel sprocket size.

Regards

David

jimmy bush

Just FYI: I looked in my "Kim the CD Man" Triumph edition and found the speedo drive in the inner gear box cover of the unit 650's until 1965. 1966 had it in the rear wheel.

dogtired

I have found my GPS will suction cup to my tank and sit there quite well, I used it to check my speedo. (And found the speedo very accurate.)

Just find a reasonably flat spot and stick it there, like in this pic.


It travelled quite well, and did not show the least inclination to fall off. might be worth a try if you have a GPS or have someone who will lend you one that shows actual road speed.



p.s. check out my brand new clip on mirrors, they are very sexy and provide excellent vision.

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pookybear

Thanks David,

That is what I was wondering, I was missing something.

Pookybear