Sideways Pitch Of Rear Wheel.... Sudden Tightening Of Line

[blarson]

The longer I've been riding the more I've begun to take notice that the more rapidly I change direction the faster the rear end of my bike pitches to the side. This accompanied by a sudden tightening of the line I'm taking through the corner without any additional steering input. This characteristic displays itself only when I change direction quickly and with a fair amount of lean angle and sometimes manifests with the additional feeling of the rear end tucking under (twisting feeling). None of these odd characteristics apply at modest (street legal) cornering angles. Obviously, the amount of variables that might be contributing to this steering characteristic are indescribable within the confines of this forum. However, I'd greatly appreciate if some of the more knowledgeable individuals would take a crack at some of the possible explanations. Unfortunately, the advice I receive will be the best bet I have for figuring this out as I cannot afford any riding school until I finish my bachelors degree and make some money.

 

That being said, here are some details(and my life story) that may help with analysis. This bike in question is a 01' ex500 that is mostly stock in hardware except for the forks being filled within the last 2000mi with 20wt oil. The tires are have only about 2000 miles on them and less than a month old they do not display any cupping or deviation from the original profile. The front is a dunlop qualifier and never feels like it's pushing unlike the pirelli sport-demon tire I had on just before. The rear is a dunlop D208SM sized one up (130x70 to 140x70) from stock. I would have never up sized the rear tire except that doing so was the only way I could put a radial tire on the bike. With these tires the bike will hold at least 10 degree more lean angle without feeling upset compared to the bias ply tires on there before. I run high 20's-low 30's for pressure in the rear and changing tire pressure does not seem to change the aforementioned handling issue. Sag is set correctly. The rear shock is past it's prime leading me to believe that it is at least in some way responsible for the tucking feeling but does not explain the sideway pitching. This bike has been my daily driver for over a year but I won't rule out rider error. That being said, I have made every effort to isolate any action on the bars by holding the bike with my lower torso to the best of my ability. My body weight is around 200 pounds wearing full gear. I am also generally riding straight up and without hanging off although the problem is still there when I hang off as well. Off throttle application is as smooth as I can make it from near the apex of the turn. The bike generally reacts smoothly when I put on the gas mid corner. I don't feel the rear tire sliding nor am I laying down any rubber.

 

My relatively (un)educated guess is that I am experiencing a high degree of chassis flex due to increased cornering loads from the modern tires. This explains to me why the line would tighten as well as why it would pitch to the side. I think the twisting or washing out feeling might be from a combination of mis sized tire and crappy shock and unrelated to the pitching. Ohh wise ones, what do you think? Sorry for the novel. Thanks for the advice.

[bellevuetlr]

do you have any pictures of yourself on a corner? too much lean maybe and not enough weight on the inside? are you shifting really late and not feathering the clutch ? hows your body position? since you said you sit straight up on the bike, do you lean your upper body to the inside or do you push the bike down away from you?

[blarson]

do you have any pictures of yourself on a corner? too much lean maybe and not enough weight on the inside? are you shifting really late and not feathering the clutch ? hows your body position? since you said you sit straight up on the bike, do you lean your upper body to the inside or do you push the bike down away from you?

 

Taking a few pictures may be worth a thousand words. I'll have to see if I can get my girlfriend to sit at a good corner and do that. In the meantime.....

 

When this pitch is happening my upper body is leaning to the inside of the corner but I'm keeping my rear in the seat. I'm generally holding onto the tank with my knees to take pressure off the bars. I still get the same feeling when I'm hanging all the way off as well. I set my body position before corner entry. Clutch is fully engaged as well. In fact, I'm using just a slight (maybe) 10% throttle opening all the way to around the apex. I do that because the throttle response on these bikes is horrid so I found that holding a steady 10% throttle reduced drive lash when I get on the gas around the apex. I usually apply slight pressure on the outside peg ala' the pivot steering introduced in twist of the wrist 2. In short, it doesn't feel like I'm pushing the bike underneath me.

[racer]

The rear shock is past it's prime leading me to believe that it is at least in some way responsible for the tucking feeling but does not explain the sideway pitching.

 

My body weight is around 200 pounds wearing full gear.

 

My relatively (un)educated guess is that I am experiencing a high degree of chassis flex due to increased cornering loads from the modern tires. This explains to me why the line would tighten as well as why it would pitch to the side.

 

I think the twisting or washing out feeling might be from a combination of mis sized tire and crappy shock and unrelated to the pitching. Ohh wise ones, what do you think? Sorry for the novel. Thanks for the advice.

 

 

 

My best guess:

 

 

Although they do offer more grip and are better at absorbing bumps, I find radials give much less feedback and a leave me with a sense of being "out of contact" with what the tire is doing while the radial carcass *deforms and flexes* during steering changes before the tires "settle in" to a corner. In fact, I found the switch from bias ply to radials a bit unnerving until I learned to "trust" what the tires were going to do as I couldn't really "feel" them like I could a bias ply anymore. For a while, I didn't know if I was falling or not until I had made the corner...lol.

 

You said that changing tire pressures didn't alter the feeling ... this also seems to fit the general profile of radial carcass flex. However, the carcass flex of a tire like the D208 definitely increases at lower pressures. I'm not familiar with the new Qualifier. The last one I had on a street bike was bias ply...lol.

 

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I seriously doubt you are experiencing much frame flex. That phenomenon was generally associated with very powerful engines in weak frames. And neither of those conditions are associated with the EX500. So, unless there is a broken weld joint somewhere, I wouldn't worry about that. That said, have you inspected all of the frame's weld joints?

 

 

 

One potential trouble spot could be the rear arm and link bearings. The bike is six plus years old, have you ever checked for play in the rear arm bearings or shock link bearings? These need to be serviced with grease to keep them tight. Steering head bearings, too.

 

 

 

Why are you running 20 wt fork oil?

 

Using heavy fork oil is not a substitute for proper spring rates. I would consider a new, perhaps heavier (aftermarket), fork spring rather than running heavy fork oil if you are having front end issues. I don't know what the recommended fork oil is for the EX500 (probably 10wt?), but, maybe try going with a 15wt oil for more damping rather than the 20wt which is *pretty* heavy fork oil, IMO, and could degrade the proper functioning of the fork altogether, as well as the damping system. You might also consider running a bit more oil outright to reduce the compressible air space in the fork in lieu of a new spring. I would certainly go that route before running 20wt oil, but, that's me.

 

 

 

What makes you say the rear shock is past its prime? Simple age or mileage? Or something more significant?

 

Anyway, if I had to offer another guess, if it isn't simply a matter of radial carcass flex or a busted frame or loose arm/link bearings or some combination of all of the above ... it might simply be an unbalanced suspension response due to a worn out rear shock and heavy fork oil causing a "sinking" feeling in your rear end. Erm...in the bike's rear end that is.

 

Or perhaps a bad shock combined with the mis-matched rear tire (size AND profile!) causing a sudden change in the suspension and/or geometry during steering/throttle inputs that is altering where the contact patch is located on the rear tire (like at a steeper point of the profile) due to the shock compressing more than the fork during whatever inputs you are making?

 

(That one feels like a stretch.)

 

However, sometimes a different size tire is enough all by itself to create weird handling or traction, add a different profile and you double your odds of having weird stuff go on. When you inspect the tires, do you notice a difference between how much of each tire is being used? Is one "chicken stripe" wider than the other? Is one carcass maybe stiffer than another?

 

 

At the end of the day, at 200 lbs, you are near the upper limit of the standard suspension rates for an EX500 anyway, and that is if the components are fresh and in good condition. Depending on how much mileage is on the bike, you might want to replace the fork springs and/or rear shock anyway. I know that probably sounds about as likely as buying a new bike to you right now, but; fork springs should be fairly cheap. Progressive's probably run about $50 new for an EX500. Make sure you specify your weight and what sort of riding you do, ie. racing or street. (Fast street is still street.) Sometimes a good place to scrounge a "new" stock shock and fork springs is a low miles 'wrecker' machine at the local boneyard. That shouldn't run more than $200 for the shock with spring. But make sure the machine is low miles and there is no damage to the shock.

 

In the meantime, you might try cranking up the rear compression damping (if you can) to try to compensate for or "match" the heavy fork oil up front to see if that helps with any "rear end sinkage", but; that would not be a real long term solution (especially if the shock is bad or has no compression damping adjuster, in which case it won't do anything). You might also try cranking the spring a tad to run a bit less sag in the rear, but....again, this is taking you further and further from the real solution and will, at the end of the day, be trading off essential suspension characteristics to work around something that *may or may not be a real problem*.

 

 

Cheers,

 

r

[racer]

I did a major rewrite and edit on the long post above. I also read back through your original post, blarson, to refresh my memory of what all you said in your long post...lol.

 

Something that didn't occur to me because it seems obvious (to me) but might not be so for everyone would be to inspect the rear axle and nut to make absolutely certain they are really tight and that the axle is not sliding around under cornering forces.

 

The rear axle nut tends to be the highest torqued on any motorcycle. And even though it may seem snug with the old crescent wrench, it generally takes about 50-70 ft lbs of torque to secure properly. That is like 70 lbs of force applied to the end of a foot long wrench. A "close enough" measure might be to carefully use your foot or lean on a closed end wrench (like the closed end axle wrench with handle extension that is usually supplied in the under seat emergency tool kit) with your hand until you grunt if you don't have a proper torque wrench and socket kit. (I cannot overstate the value of that kit. If yours is missing, you'll want to order a new (or used) one and/or even create your own emergency tool kit.)

 

You could use a socket on a medium/large ratchet handle in a pinch if you have nothing else, but; that kind of torque can damage the ratchet so it is best to use a fixed wrench or "breaker bar" if you don't have a proper torque wrench.

 

Also make sure the rear wheel is as close to perfectly in line with the front wheel as possible. The chain adjuster hash marks aren't usually very accurate.

 

Ok, sorry if I am insulting anyone's intelligence, but, when I bought my first street bike at 19 years old, I certainly did not know this stuff and learned much of it the hard way.