Ironman sprockets for WR250R

Stock gearing on the the WR250R is a 13T counter sprocket with 43T rear sprocket. This gives a final ratio of 1:3.31. While this is fine for cruising on pavement, it's a bit tall for trail riding on a bike that's already anemic for torque. Consequently most dualsport riders gear down their WR and wring out the engine's high-rev capability to make highway speeds. The question is what ratio to choose?

Many riders online recommend a 13/47 combo for a ratio of 1:3.62. Consensus is this gives a good compromise between highway and trail gearing on the WR. Those favouring more of a trail-oriented ride opt for a 48 in the rear. The important thing seems to be sticking with a 13T up front rather than using a 12T to get lower gearing. Otherwise there is a risk of the chain wearing through the rubber guide and into the swingarm metal itself. Despite having just one less tooth, the 12T is a smaller diameter gear than the 13T and this significantly narrows the chain path in a critical area.

My bike was used when I bought it and it came with a 12T up front and the stock 43 in the rear. This gives a ratio of 1:3.58--almost the same as for 13/47 (1:3.62). Although I haven't had the wear problem that many other riders report when running a 12T, my rubber swingarm protector was well worn and I would probably have gone right through it and into the swingarm in another week of trail riding.

Time to switch to a 13/47. It'll put less stress on the chain and sprockets and probably run quieter too. I can also vouch it's a decent compromise when there are long highway sections of 100km/hr before getting to the fun stuff.

Ironman sprockets from Dirt Tricks are highly rated for their durability. After shopping around no one seemed to have stock or good prices, so I ended up ordering them direct from the company along with some new bolts. Apparently the MSR sprockets which look the same are made by Dirt Tricks.

These are some nicely made parts using hardened tool steel. The rear cut-out design results in the 47T weighing only 570g compared to the OE 43T which was almost a kilo! Not like I'm a weight-weenie for motorbikes or anything...

I hadn't really thought about clearances when I ordered these parts because no one else mentioned them being an issue. As you can see, the larger diameter of the rear sprocket puts the chain much closer to the lower chain guide and upper chain guard (not mounted in this pic). I'm a little concerned about wear on the lower guide. Chain line around the counter sprocket seems good though. 

As you can see I've also mounted a new Pirelli MT43 on the rear. This is a taller tire than stock and it has a fairly square profile. It's actually almost exactly the same rolling length as the Heidenau K60 (207cm vs. 209cm) which is also known to be taller than stock. Nevertheless, the MT43 has a squarer profile which rubbed on the inner fender where the K60 didn't. I had to move the axle back about another centimeter compared to the K60. Now it's a drag bike. 

To tie it all together I cut the chain to 112 links and used the new factory recommended method to set chain tension (8-10mm of clearance under the midpoint of the swingarm when chain is pushed up using 36lbs of force).

Even though there still a foot of snow on the ground I couldn't resist going for a rip around the block last night. Everything worked perfectly.

Next up is a Scorpion Rally for the front. Once I have some miles on the tires and sprockets I'll update my review. Meanwhile...

Quick update: Took another, longer rip on the bike and have to say the 13/47 gearing is a lot smoother than the 12/43, especially at low speeds and when slowing down at low rpms. Better control, less jerkiness. Much easier to control the bike while standing.