Kent K,, 400 Sprocket ratios

[tlanders]

Thank you for the GREAT articles in Still Keeping Track. Question: You are using a 14/57 sprocket ratio for the 250 per the last article. Are you using the same ratio for the 400? I am also using 14/57 on my 250 and I think it's perfect for that bike, but I'm using 14/53 on the 400. My thinking was it might help keep the front wheel on the ground. I started with a 14/51 because Mike Lenz said he likes a higher ratio for the 400 and I had a 51 rear sprocket, but it didn't suit me and so I moved to the 14/53. Mike or anyone else, please chime in here. Thanks for the input, Teddy

 

[Knute]

Hi Teddy,  

Yes, I also use the 14/57 on the 400.  The 400 engine we're using is a very early 1974 version, which has no bottom end power, so this gearing helps it out a little.  It's also nice because we don't have to carry a bunch of different size spares to the races.  In fact, we just take an old rear wheel assembly (with a used tire and sprocket) that can be used as a spare for both the 250 and the 400.  

If you want to play with gear ratios and don't have a 57 laying around, a 13 countershaft with your 53 (13/53) will yield the same ratio as the 14/57.  I think the 14/57 is slightly better though, because it offers more clearance as the chain passes over the swingarm.  The larger countershaft sprocket is also easier on chains.  Let us know what you end up with!

Kent

 

[tlanders]

Kent,

Thanks for the response. I have a 55 tooth, I would try it first with the 14 even though right now I love the way it runs. Using the 14/53 ratio, the only time I didn't get a hole shot with it last year is when I put it in neutral at the last Casey Regional-what a surprise!!!

I re-read all three of your articles and again want to thank you for providing us all with the fruits your hard earned experience. I have made copies of all three of them and put them in three ring folders and am giving them to friends as Christmas presents. I hope Al B. doesn't get me for copyright violations!!!!

I forgot that you are using reed valves. Is this on both bikes or just the 250? Were you able to do a comparison before and after putting on the reed valves? If so, how did they help? My understanding of them is that they help the bottom end but get in the way at the top end. Is this true?

Last question (sure Teddy!!), did your engine rebuilder dynamically balance your engines? I have heard that it is a good idea to dynamically balance the 400 since they have so much rotating mass and tend to vibrate so much.

Merry Christmas!!!!

Teddy

 

[Mike Lenz]

Ditto to Kent. Even after all these years of racing these bikes I learned a new trick or two! For gearing I use a 15/48 combo. This puts FIRST gear up where most MX bikes gearing is. With the other gearing I tend to get into first once a while, which is totally useless on a mx track with that gearing and totally frustrating when I do it. However I feel the hot set up would be to convert it to a 5 speed, which I intend to do with the next rebuild [if it ever wears out before IM dead!]. Another plus with this gearing is the thing will do about 100!

 

[Mike Lenz]

BY the way in my inventory I have a 16 tooth counter sprocket! I remember getting it back in the 70's but it doesnt fit. I always wondered why ktm made it?