was wondering what sprocket combination to put on my 280 pro..at present its got a 41 rear and a 9 front..this is what it came with ..but i have a new 42 rear in the shed and was wondering what sprocket to get for front and how many links in chain..maybe a silly question but never bought one before..

A one tooth increase at the back wouldn't drive any change at the front. It will make the bike marginally slower but on very noticable. you currently have a ratio of 1:4.55 changing to a 42 rear makes this 1:4.66 but then putting a 10 tooth on the front take you back to 1:4.2 which is probably too quick for normal use if your used to 1:4.55. Basically you can see that every tooth increase at the back decreases the ratio by 0.11 but tooth increases at the front are increase it by 2.66. Obviously this depends of the sizes of both front and back and the figures I've given are with what you've got.

Just divide the rear teeth by the front to work out the ratio and remember that what you started with was 1:4.55 i.e for every full rotation of the rear wheel the front spocket has to rotate 4.55 full turns.

Bottom line is try the 42 and see what you think. It will be fine in sections but if you use the bike on the road it will slow your top speed down. For trials like the SSDT I like to go down a tooth at the rear but some guys go up one at the front. I tried this once and couldn't ride it in tight sections as it was too fast.

Your chain adjuster should cope with a one tooth drop at the rear and still leave adjustment unless the chain is well stretched.

thanks for that i will order up same size front as i have then and a chain...