Author Topic: shimmies under acceleration (Read 3599 times)

salty · « **on:** September 15, 2007, 03:30:38 PM »

Hi people,1200 MK1 S Bandit has started to shimmie/weave under heavy throttle whilst cranked over? in a straight line its fine. i have checked the head bearings for play, they appear o.k. , as does the swing arm. Tyres are conti road attack rear half worn,front a bit more than half worn. i must confess that i have started to ride the bike harder now i'm used to it, but i'm sure it never did this before.the back end feels a bit hard,would softening the suspenstion on the back end improve this, or am i looking in the wrong place?
I'm open to any suggestions or ideas
thanks
Salty
BTW tire pressures 36psi front and rear cold

salty · « **Reply #1 on:** September 15, 2007, 04:03:43 PM »

Sorry forgot to mention the back end has been jacked up 25mm
Thanks
Salty

rkfire · « **Reply #2 on:** September 15, 2007, 06:45:51 PM »

Check chain too just for the hell of it. I still have the original macadams on mine, but from new my front tire doesn't give me a lot of confidence. I pumped it up to 38psi and it feels a lot better. It still isn't great tho.

Bikes can be sensitive to front tires, brand, type, psi, and wear (even if there's half the tread remaining). Could the tread be squaring off as the miles rack up? Cuppng? I think jacking up the rear will only make it more sensitive to front tire issues.

txbanditrydr · « **Reply #3 on:** September 15, 2007, 09:14:09 PM »

A few questions for ya....

What year bike (sorry for my ignorance)?
How many miles on it?
Stock suspension?
Present suspension settings? (preload & rebound/compression damping)
Have you ever set the "sag"? Read This...

Just trying to get some baseline information. TIA

salty · « **Reply #4 on:** September 16, 2007, 09:01:00 AM »

Thanks both, rkfire and txbanditrydr, firstly rkfire, checked the chain tension and alignment (with some wood for the later) both spot on, but after staring at the front tire, i think it may be cupped

However this sag thing has me intrigued and any help i can get to set this thing up better is very gratefully received, so to answer txbanditryder:
1. 2000 model Mark one old shape, actuly an N model but with the S Fairing on it.
2.Only 8000 miles
3.Stock suspension apart from the back end jacked up 25mm
4.preload and rebound were set to factory default by the dealer when i bought her a 9 months ago. i have left the rear as is,but set the front to two notches showing (was at four)when i fitted the fairing and lifted the rear.
BTW, i'm six foot one and about 245pound. tend to ride solo mainly.

Once again thanks
Salty

rkfire · « **Reply #5 on:** September 16, 2007, 11:14:31 AM »

I bet a new tire would make it feel like a sport bike, having said that, try a couple psi up front. If it does nothing, you waisted a few minutes time.

txbanditrydr · « **Reply #6 on:** September 16, 2007, 11:19:55 AM »

Salty... I don't profess to be an expert. I will offer some ideas based on information posted elsewhere on B/A and my own experience with setting up my new suspension earlier this year.

The Bandits have always suffered from less than stellar suspension components and were really tailored for a rider who weighed less than the majority of Bandit riders (myself included). Also, while 8,000 miles is not a lot of wear and tear, you should consider the "time" element of a nearly 8 year old bike.... things like fluids do degrade even when not being used. The rear shock is a sealed unit and cannot be re-built or repaired but the front fork oil can be replaced/changed and if it's never been done you might consider doing that.

Tire issues can play a significant role here and a new front tire will feel much better with all things staying the same. However, if you've never set the sag on your bike I'd encourage you to do so before making any real changes. The link I posted earlier will walk you through the process but you will need some help. Once this is set you'll begin to see the weaknesses of your suspension more clearly.

I'm going to predict that the stock rear shock factory-set preload setting will not provide the proper sag at your riding weight... you'll likely be cranking that up to position 6 or more from the preset at 3/4. The rebound damping adjustment will likely need to be raised too. The front forks will likely need to be cranked all the way in - there's not much you can do with the front since the only adjustably is pre-load.

Raising the rear end will create a quicker turning bike but at the risk of less stability... I don't really think you've raised it too much but you could go back to a stock height and try it again. That would be a cheap modification / verification process.

Riding harder will make the suspension weakness more pronounced, however a new suspension won't work any better if you're riding harder improperly. Check your body position, arm and hand tension, etc. as you corner - a lot of instability is caused by gripping the bars too tightly and inducing unwanted steering inputs. If you feel your riding habits are in good shape then you've earned yourself a suspension upgrade. You will not regret it either - I gained more corner speed through a better suspension than any jet kit / pipe mod.

So.... again, I recommend you start with setting the sag. Once you have those base numbers you'll know what the next step is. The whole suspension set-up process can be fun and awfully addictive - especially with good quality after-market parts. Try some of these ideas and report back. In the meantime - Ride Safe.

salty · « **Reply #7 on:** September 16, 2007, 04:57:19 PM »

Thanks for your prompt replys both, txbanditrydr, you've raised some very interesting points,I had forgot to add i had fitted 25mm bar risers (bad back and numb fingers) to slightly improve the riding position, and the fact its got renthals fitted.you may be right about the riding position, i need to pay more attention when i'm out next, as i have returned to biking after a seven year drought, you do loose the edge, this is also the largest, but not the fastest bike i have owned, but i'm not 18 any more(don't i know it!!). i'm resigned to a new front tire, and armed with the info you have provided it looks like i'm in for an interesting few weekends. i'll keep you informed on how i get on.
cheers
Salty
BTW, when i told the wife i needed to re-set my SAG, she looked at me knowingly and agreed," oh yes, you definatly do"...

rkfire · « **Reply #8 on:** September 16, 2007, 05:11:15 PM »

Quote from: "salty"

the back end feels a bit hard,would softening the suspenstion on the back end improve this, or am i looking in the wrong place?
I'm open to any suggestions or ideas

I don't think it has anything to do with the front end issues, but, thought I'd throw this out too. I thought the stock shock, stock setting felt pretty harsh even with a passenger especially on square edge bumps, or washboard roads. I lowered the compression damping setting one click, and raised the preload one click. It feels much better. The suspension is firm, but harshness is gone.

salty · « **Reply #9 on:** October 06, 2007, 12:25:01 PM »

Big thanks to all that replied,everyones input raised a valid issue that was playing an affect in one way or another.
txbanditrydr, your advice on setting the sag was spot on, now knowing the front tire was goosed, i did this anyway and was surprised at the positive difference in handling even with the front tire issue,so bitten by the bug,new rear shocker (see the BETOR Shock thread).amazed at the effect this and the sag thing has had on my bikes handling. the shocking thing (sorry i couldn't resist that one) was that after only 8000 miles the standard rear shock,with no leaks and looks mint,was the cause of my shimmies, all compounded by tire wear and incorrect initial set up of the suspension.
So big thanks to all and i cant recommend enough to go and set your sag, follow txbanditrydr link from this thread.Top Man
Cheers
Salty