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More on Front shock

841 views 6 replies 5 participants last post by  Nick Goodwin 
#1 ·
In a recent post I indicated my Bonnie was "Walking" "wallowing" when
I hit small bumps while cornering fairly aggressively on highway turns.
My weight (250 in gear) seems to be the problem and several suggested heavier springs ( .85 instead of .75) and/or heavier shock
oil. Can someone explain what the effect of each of those changes would be ?
 
#2 · (Edited)
The instability you feel is the effect of the tire not having consistent contact with the road — and there are LOTS of things than can induce that.

The first may simply be that your shocks are bottomed out, so the tire is bouncing on the road surface, rather than the suspension being the thing bouncing. The stock .75 springs that came with your bike stock are nowhere near enough for your weight. Contact Race-Tech and get their expert advice on that.

Speaking of the spring, there is the issue of pre-load -- adjusting how much the spring is compressed just bearing the weight of you and the bike. That affects whether you are at the "top" of your spring with lots of room to move or at the "bottom" where the spring has little room to compress any more.

The other major factor is "damping" or "rebound" -- the ability of the suspension components to absorb energy and recover from being compressed or expanded. Think of this as the "bounce" part of your suspension dynamics. I'm not sure it is always correct to say use heavier oil. Oil that ties too light can have you bouncing, but heavier oil will "stiffen" the movement of the fork assembly, and could actually worsen your situation because the oil cannot move quickly enough through the orifices in the fork. Heavier oil is often used in racing, but you are looking at much superior road surfaces at a track thatn you find out on the road.

To complicate things, there is also the consideration of how much oil to have as a ratio of the total capacity of the fork (called "air gap.") Air squeezes more easily than oil, so more air gives you a more comfortable ride but not as quick a response as you might need in high-speed turns over a rough surface.

What all this means is that it might help to read some articles on the basics of motorcycle suspension. Each of these elements -- spring rate, pre-load, oil weight, air gap -- work together and have an effect on one another.

The outcome is pretty simple -- you want your tires on the road as firmly and as quickly as possible under all conditions, unless you're jumping the Snake River. Getting to that outcome is science and art. Your suspension is your lifeline to the road, so I have chosen to try to learn a lot about it rather than taking any one bit of advice in isolation. It's worth educating yourself on it to get to the set-up that works for you.
 
#4 ·
The first may simply be that your shocks are bottomed out, so the tire is bouncing on the road surface, rather than the suspension being the thing bouncing. The stock .75 springs that came with your bike stock are nowhere near enough for your weight. Contact Race-Tech and get their expert advice on that.
Race Tech will tell him .95 or 1.0..., which is MUCH too stiff, even for an aggressive old geezer. Maybe OK for the track and "juveniles". :D

IMO he should use the stock air gap and some Spectro 10W which will get rid of any stiction. I'm dubious about going straight to 15W due to the horrible hydraulic lock tendency of the stock fork valving! :eek:

Hit a big bump with that thing and it goes rigid. My 37-year-old kid called it a "torture device". (We've owned ~ 60 bikes between us, BTW. ;) )
 
#5 ·
Change the oil first. Don't do 2-3 things at once. Just do one thing at a time. If your forks only depress a portion of its travel it isn't bottomed out. And preloading it will just set the compressed hight higher. So the oil change is easier. I replaced the oil seals at 49,000 and used 15W oil. Yes it is a little stiff. I just got off of a rough road this morning. So maybe I'll try 10W oil. But for now the 15W fork oil is staying in there. But yes just the oil change made my bike more stable. I would describe it like the tire is staying the road more. Especially when cornering. No brake dive with the 15W oil either. The bike is more planted.


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#6 ·
His original post seemed to be looking for understanding, not recommendations. I still think there is a lot of value in understanding all the components and influences in the suspension before "trying out" any one recommendation. And, then, yes, start simple and do one thing at a time.
 
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