RussB wrote:
Moving the rear wheel forward/backwards in the swingarm alters the front/rear weight bias, as well as the wheel base, both of which effects stability, cornering, front/rear grip.
It also changes the amount of leverage on the rear shock, having the rear wheel all the way back will make the shock softer, further forward makes it stiffer.
When you put on a new chain next, add an extra link and see if you can tell the difference with the rear wheel far back in the swingarm. If you don't like it, simply remove the extra link and return to a more centered position.
You can also adjust it by changing your sprocket sizes, but this is more costly and effects your final gear ratio.
^ ^ ^ ^ ^ THIS... ! !
It's amazing what small changes in wheelbase can do to the personality of a bike even while adjusting other things so that the wheel base is changed without affecting other things like effective rake, sag and shaft speed in the shock.
Most people only focus on what changing the rear wheel position will do to the head angle / rake of the machine. And yes, If you move the rear wheel without making a change to shock spring preload it WILL change the effective angle that the forks attack the ground (not the actual head angle/rake of the frame). I often see people trying to improve the behavior of the front end by moving the rear wheel forward in the chassis, this does sometimes work for what you're looking for...... BUT...
....Here's a different approach to the situation from my personal experience and results that is counter intuitive to most people The front wheel on my YZ125 has always been a mythical creature. I've always heard of legends and lore that there is a front wheel on my bike. I've even caught a glimpse of it here and there if I look at it from the right angel while it's on the stand. But I'll damned if I could tell it was there when riding the machine. No feel of the front end what so ever, no traction. If I wasn't wagging the rear tire around like a speedway racer the bike would not turn. Forget about the front tire holding a deep rut without sitting on the fuel cap. I tried all manners of things aimed at changing the effective head angle through sag, spring rates, fork oil levels and shortening the rear wheel base. Never quite got there.... But, I added links to the chain and move the wheel as far back as I could (maybe 3/8in difference) and increased the preload to maintain sage level... Whole new world. My front tire came out of the darkness. I could feel the front tire, turning improved, confidence in the front end improved, lap times dropped. I experimented with this approach on 2 other YZ riders that were struggling with similar feel/traction issues, their lap times decreased as well. I get that body weight and riding style / skill has a huge impact on what approach may work best.
Ultimately for me, with the longer wheel base, it meant a tiny jump up in rear spring rate to maintain free sag with a high race sage and a few clicker changes. Amazing what a small change in only the wheel base will do to the personality of a bike.