Tuesday 18 August 2009

Gearing.


In its original state, my Husqvarna SMR ran a 48/17 sprocket combo. But that was with the dinky little 17 inchers. Now, with the huge 19-inchers, and the swollen Maxxis flat track tires, we need a different gearing altogether. There is no shortage of "gear calculators" to be found on the web and some are really OTT (here's one), but some simple maths is actually enough.

Here goes: The stock sprocket ratio is
48/17=2,83. The standard tire (150/60-17) had a circumference of about 1,92 meters.The new tire has a circumference of about 2,20 meters, i e about 15% bigger. That means we need to lower the sprocket ratio by the same amount (15%) to get the same "total gearing". 2,83/1,15 comes out to about: 2,46, which is our target ratio. I wanted to keep the front sprocket with 17 teeth and only change the rear sprocket. So I chose a 42 tooth rear sprocket, giving a sprocket ratio of 2,47 which is pretty close to what I need. I've always thought that the original gearing was a little long and I have a 16 tooth front sprocket in reserve should the gearing turn out to be on the long side.

BTW: does anyone know of a company that sells stainless Allen bolts with "countersunk" heads in 7/16''? The ugly hex bolts I'm using now have to go. I've been googling furiously, but to no avail.

10 comments:

  1. Hey, Checked with Siverts skruv in Stockholm? They have solved all my special bolt and screw requests so far... Any stuff to e-mail me soon? ;-) /Marcus (Marcus Moto Design)

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  2. I'll be sure to call them. Thanks. Imperial size bolts are a real pain in the ass.

    I've had a bit of a break over the holidays, but now it's full steam ahead again. Watch this space:-)

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  3. i've spied some hex socket cap and button screws but alas no counter sunk! keep hunting, persistence hasn't let me down to date. great blog BTW and glad to see that you're back in the swing of it. keep it coming!

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  4. I'd be surprised if I don't have some, I'll check on the weekend and get back to you.

    How long? and do you need nuts too?

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  5. I need 10 bolts. Around 30 mm long. No nuts. If you can find some it would be fantastic!

    /D

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  6. what length do you need? I get most of my imperial bolts through W&W cycles. We even have a flat track racer working there!

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  7. W&W cycles only carried chromed in 7/16''. And I don't think that fits on a tracker like mine.

    ACME Stainless in the UK has offered to make some for me. It costs a pretty penny, but there is no turning back now - every detail has to be spot on. Problem is, I will ride like a little old lady for fear of dropping it when it's finished:-/

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  9. I checked and to my surprise I have no 7/16" fasteners at all, it's quite an unusual size. None of my local suppliers list that size in CSK either. Sorry about that.

    McMaster-Carr (www.mcmaster.com) in the US stock them in stainless (1.25"lg) but they are not cheap at $4.20 each ($6.27 for a pack of 10 in black oxide is more reasonable!). Unbrako list them in their catalogue so that might be your best option.

    Remember to check the pitch before you order!

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  10. Thanks daws.

    I'll check with McMaster-Carr.

    /D

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