Hi fellas, is there a formula, so that i can calculate how many magnets needed to get my speedo working without a healer, i'm running 17" wheels and 40 tyres and a 3.62 diff.
Cheers Brad.

Unless you're very lucky I doubt you'll be able to correct it with more/less magnets.

If for example your speedo normally uses 4 magnets on the sprocket and the bike was originally geared to ~200mph at the limiter in 6th, putting only 3 magnets on will make it read ~150mph at full revs in top, and 2 magnets will make it read ~100mph, so you haven't got anything in between.

The only way you'd have a decent chance is if it used lots of magnets as standard, then removing one would mean a smaller incremental speed drop so more likely to land on the speed you need.

[Edited on 10/5/07 by ChrisGamlin]

I think someone on here used 9 magnets, but i think that was with a fireblade.
Did the magnets go on the bike sprocket.

rrv blade clocks, 9 magnets on prop with 3.62 diff & 185/60/13 wheels is perfect.

experiment and use a GPS (tom tom or the like) to compare or a £5 push bike clock and jack the wheels off the ground and spin them up.

You do not need to go at 100mph, just match the odo at low speed.

Regards Mark

dont mean to hijack the thread, but what magnets are people using on their prop shafts and where have they got them from? also how have you mounted them on the prop shaft? glue?

from what i've seen, araldite them to the prop adapter.

Make a bracket that will fit a hall sensor (old ABS pickup) and bolt to the holes left by the sprocket cover, araladite to the prop adaptor.

I used something like these Linky thing

As its a hall sensor I could have got away with bolt heads, maybe even lumps of weld. You do need to make sure the magnets all go the same way around.

Regards Mark

[Edited on 10/5/07 by mark chandler]

If i start with one magnet, running wheels from a pushbike speedo, knowing the diameter of tyers, i should be able to work out how many magnets are needed.

Do not be fooled into thinking its not working because the needle does not lift, wait for the odo to clock and use this as a gauge of accuracy.

Get them to both clock the same distance to set up, not speed.

I cheated and set up off the car using a lathe as a spinner. set the pushbike speedo wheel ratio to size, the added magnets, as you do not have a diff I clocked 3.62 mile on the pushbike clock to 1 mile on the bike clock. You could just as easily use a drill if you can support it solidly.






Regards Mark

[Edited on 11/5/07 by mark chandler]

quote:

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Originally posted by roadrunner
from what i've seen, araldite them to the prop adapter.

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RRT Blade with 3.62 Diff and 195.50.15's was within SVA tolerance (just) with 3 magnets on the prop adaptor...

hth
Steve