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Spartacus
8/3/2015 2:34pm
8/3/2015 2:34pm
Edited Date/Time
8/4/2015 2:11pm
I figured I better post this here rather than the main forum so I don't have several 13 year old telling me I don't know what I'm talking about......
Back in 89 I had a cr250 that was SUPER sensitive to the rear axle position. Forward, the bike turned great. If I had to pick up some slack in the chain and moved the axle back 3/8" that damm bike tried to climb up every berm/turning rut I encountered. Move the axle forward just that small amount and it was back to cornering just fine.
I've been struggling with the same thing with my 83 r480. No matter where I sit or how adjust my body position, it wants to climb up out of the last 3/4 of any turn. I'd raised the fork tubes up in the clamps and it's been much better on my home hard-packed track.
Last weekend though I fought the living hell out of that bike in the sand at Southwick. The only way it wouldn't climb out of the last 3/4 of a turn was to not rail and to drive in, manhandle it around and square out. Not the hot set up for keeping momentum in the sand. Honestly, some slow rutted turns had me sitting and paddling around like it was first time I'd ridden.
I tried sitting on the gas cap, I tried sitting in the middle and I tried sitting on the rear fender. Lots of power, coast through and praying to the Lobster God, nothing seemed to work.
I'm taking a link out and seeing what happens with the axle forward but was interested in the input here too.
I'd figure I was just an old wanker and blame myself but I know how touchy my 89 was and figure the answer is somewhere to be found.
Yeah, I fooled with tire pressure too.
Comments.
Suggestions.
Mocking.
I blame Newman.
Back in 89 I had a cr250 that was SUPER sensitive to the rear axle position. Forward, the bike turned great. If I had to pick up some slack in the chain and moved the axle back 3/8" that damm bike tried to climb up every berm/turning rut I encountered. Move the axle forward just that small amount and it was back to cornering just fine.
I've been struggling with the same thing with my 83 r480. No matter where I sit or how adjust my body position, it wants to climb up out of the last 3/4 of any turn. I'd raised the fork tubes up in the clamps and it's been much better on my home hard-packed track.
Last weekend though I fought the living hell out of that bike in the sand at Southwick. The only way it wouldn't climb out of the last 3/4 of a turn was to not rail and to drive in, manhandle it around and square out. Not the hot set up for keeping momentum in the sand. Honestly, some slow rutted turns had me sitting and paddling around like it was first time I'd ridden.
I tried sitting on the gas cap, I tried sitting in the middle and I tried sitting on the rear fender. Lots of power, coast through and praying to the Lobster God, nothing seemed to work.
I'm taking a link out and seeing what happens with the axle forward but was interested in the input here too.
I'd figure I was just an old wanker and blame myself but I know how touchy my 89 was and figure the answer is somewhere to be found.
Yeah, I fooled with tire pressure too.
Comments.
Suggestions.
Mocking.
I blame Newman.
Maybe Trey Jorski will chime in. He goes "kinda" fast on a 83 CR480.
First try adding preload to the rear shock. Then I would actually move the rear axle back in the swingarm to give the bike more trail. I'm having a similar problem with my 79 yz400 but I know it's my dual rate spring setup so I've gone back to a factory Yamaha progressive spring and will test it out this weekend.
If you can't dial it in with fork height, tune the shock spring.
The Shop
Wheel base is different for different types of competition. Trials and Short track prefer some thing shorter to allow them to turn. They tend to be shorter wheel based bikes. Desert racing is looking for high stability with minimal turning. They prefer the longer wheel base. Each time you adjust the chain you are either making the wheel base longer (tightening) or shorter (loosening). I am not aware if your 480 is that sensitive to the change, butit sound like it.
As far as Jorski goes, I think he typically shortens his throttle cable about 3" to insure the slide is safely out of the way in the carb.
I'm only a clubman rider, but through experience do know this.
You should be setting your sag every time you adjust your chain, or have a big breakfast / take a King Kong sized crap.
Having the correct spring rates and setting the race sag are always your first port of call as all your front end geometry starts from the angle of the bike as dictated by the rear end.
If he really is super tall, he's going to need much stiffer springs and more rebound damping if he hasn't already done so. Note that the shocks from this era have very little rebound damping so stiffer rebound is a must. I usually use 7wt shock oil in lieu of shuffling shims. Pro suspension guys will scoff at this approach, but it works for great for vintage racing, where the motos are short.
Taller riders put more load on the suspension, so correct springs & damping rates are critical, especially the shock.
The suspension/spring rates are good and the sag is well within the ball park but I'll fool with it a bit and see how it effects the handling.
So many variables, so little knowledge.........
Since I'm taller than the person the bike was designed for I've found that I need less sag. I suspect you are in the same boat. On my modern bikes I'd swapped the springs out for my weight and then would go test to determine baseline sag settings. I wpuld start with what the book says to run and then ride it. Then i would add two turns of preload and go test again. I would repeat this till the bike was unstable or the front end knifed, then back it off to the previous setting.
At our size no bike is setup for you and I.
Sag was 110 mm and I've screwed it down to 100mm ( one full turn) and will report back because I know you're all breathless to know the results.
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