yz handling at speed


37 replies to this topic
  • Jim813

Posted September 11, 2009 - 10:37 AM

#21

Found an article about having too much sag and how it effects the front end of the bike.
http://motocross.tra...per-sag-effect/
I was wondering if this could be a potential solution to my problem? But, I start to wonder why bikes set-up for light riders and similar speed that I have ridden don't have this effect.

  • grayracer513

Posted September 11, 2009 - 12:14 PM

#22

Yes, that could be part of the problem.

There are a couple of interesting things at work here. I'm going to make the assumption that the springs are still stock. At your weight, that would make them slightly too light, which in turn requires more compression damping to compensate. If you're 16 and weigh 205, you're probably fairly tall, too. Tall people tend to place weight further aft than shorter guys.

So, you are running the forks up in the clamps 8mm, and then running the sag at 110. Comparing this to a bike with the forks up 6 and sagged at 97, it's as if you were running 108 with the forks flush, which is guaranteed to aggravate a push/washy feel at the front.

But there's more at work here than that. With the springs too soft, and you (assumably) taller than normal, the rear will crush even further in proportion to the front in a dynamic sense, aggravating the matter more.

Then there is the stock valving. Late YZF's tend too have rear rebound that is way to loose initially, and fork rebound that is just the opposite; it ramps up too rapidly. In the rear, this often leads to the suspension being adjusted so tightly in trying to control the kick on jump faces and in whoops that it skips around in the rocks, or if it's loose enough to control the small stuff, it's a rodeo ride in the larger bumps.

At the front, we already talked about the way the weak springs play off against the compression damping. If the springs are light, the fork rebounds less than it should already. Couple that with the aggressive base stack in the rebound valve, and you have a fork that feels harsh and bounces the front end around. It seems odd to think of rebound causing harsh compression "feel" in very small bump situations, but it does. What happens is that the fork absorbs the first rock, but doesn't recover fast enough to keep the front of the frame from dropping onto the next one, which leads to the bars hopping around, and way too much mass in motion to feel stable. Neither issue is really something that can be effectively dealt with by adjusting the clickers or front or rear ride heights.

So, it seems to me that it's a suspension problem, and the only real way around it is to address it from that standpoint. Springs, however, are by far the most important issue with suspension. If they aren't right, you can't fix it any other way. So get that corrected first, dial in what you have, and go from there.

  • Jim813

Posted September 11, 2009 - 12:16 PM

#23

Thanks for all the input. The fork spring are stock, but the rear is a stiffer 57N (5.8kg/mm). I am about 6' even, which may be a bit taller than the average. I may be lacking to give out all my info , so here is everything I can think of to post.

Fork/shock rebuilt and re-valved at 67.4 hours on bike( currently 89.1 hours on bike)
Dunlop MX31 front tire
RTT triple clamp/stabilizer (with more forward and taller mounts)
Steering bearings checked and tightened a bit tighter that OEM specs.
Triple clamp bolts tightened to OEM specs.
Stock front fork springs (.470kg/mm)
KYB stiffer shock spring (5.8kg/mm)
All bearings working properly


From the recommendations on this site,
I checked:
Fork compression setting: both at 15 out
Fork rebound: settings both at 10 out
Fork height in clamps: both at 8mm up, set back to stock setting at 5mm above clamps. One thing I have to do is compare the stock top triple clamp to the RTT top triple clamp and see if there is and height difference that could be giving me a different reading that with the stock top triple
clamp.
Sag: set at 101.5mm, pre-load at 12mm, static sag at 25mm

Checked for fork binding: Didn't notice any fork binding, but I did take the front wheel off, checked steering bearings, checked wheel bearings, make sure there were no burrs on the axle, and put everything back together. I made sure that the fork didn't bind when putting back together by following the steps in the manual. I don't know if this matters or not, but on the right fork lug has some scratching inside where the axle slides through.

I will try the bike this weekend and see if anything is different than before.

  • harrymank

Posted September 11, 2009 - 04:20 PM

#24

I have an 08 YZ450F and it also wanders around a bit but only on the dirt road. I only way maybe 170 and the bike is in stock form. traveling down the road it gives you an uneasy feeling having the bike wander underneath you.

  • Jim813

Posted September 11, 2009 - 06:56 PM

#25

The RTT top triple clamp is about 3mm taller than the stock top triple clamp, which should mean the forks 5mm up in the RTT clamp is 8mm up in the stock clamp...

  • Sider_steve

Posted September 11, 2009 - 07:31 PM

#26

That sounds right,so when you had them up 8 mm it was really like being up 11 mm in the stock clamp

  • Jim813

Posted September 11, 2009 - 08:30 PM

#27

Well more like 11mm, but yes.

I am starting to wonder if this could be caused from previous damage to the left inner fork tube. Last June I wrecked when I first got the bike and put a small divot ( most likely from rock) in the inner fork tube about 2.5 inches from the top of the inner fork tube. I had the divot smoothed and re-polished, but I wonder if that crash could have done more damage than easily visible. When I had my suspension rebuilt and re-valved, I was informed that my left free piston was cracked. I don't know when the damage happened, but it makes me wonder if something is not mechanically sound with the left fork. Any thoughts?

  • Sider_steve

Posted September 11, 2009 - 08:59 PM

#28

Well more like 11mm, but yes.

I am starting to wonder if this could be caused from previous damage to the left inner fork tube. Last June I wrecked when I first got the bike and put a small divot ( most likely from rock) in the inner fork tube about 2.5 inches from the top of the inner fork tube. I had the divot smoothed and re-polished, but I wonder if that crash could have done more damage than easily visible. When I had my suspension rebuilt and re-valved, I was informed that my left free piston was cracked. I don't know when the damage happened, but it makes me wonder if something is not mechanically sound with the left fork. Any thoughts?


It is possible that the ding could be part of the problem.
How much did you ride the bike after the crash and before the suspension work?
And was the same type of problem the reason you sent the forks off to get redone?
And was the cracked part replaced?
If I understand correctly you just had the forks reworked.
Then while they were getting work done you added new top clamps.
Then when you got the forks back on the bike you set front and rear sag.

I find when it comes to suspension, try not change too many things at once.
I probably would have put the new top clamp on then installed the forks.
Then went for a ride, that way if things were not working right I wouldn't have so many things to look at.
Even a new front tire could change the way the bike works,it may have stiffer sidewalls and a few less PSI of air might fix it but it is a change in the way the front end works.
when trying to improve it just work your way backwards from the changes you made to get it where it is now

  • Jim813

Posted September 11, 2009 - 10:06 PM

#29

It is possible that the ding could be part of the problem.
How much did you ride the bike after the crash and before the suspension work?
And was the same type of problem the reason you sent the forks off to get redone?
And was the cracked part replaced?
If I understand correctly you just had the forks reworked.
Then while they were getting work done you added new top clamps.
Then when you got the forks back on the bike you set front and rear sag.

I find when it comes to suspension, try not change too many things at once.
I probably would have put the new top clamp on then installed the forks.
Then went for a ride, that way if things were not working right I wouldn't have so many things to look at.
Even a new front tire could change the way the bike works,it may have stiffer sidewalls and a few less PSI of air might fix it but it is a change in the way the front end works.
when trying to improve it just work your way backwards from the changes you made to get it where it is now


I damaged and repaired the inner fork tube in June 2008 with 2-4 hours on the bike total and rode the bike for another 65 hours before sending the suspension off to be set-up.

I sent the forks and shock off because I was completely amazed how well my friends yz250 worked after he had his suspension re-worked from the same tuner I used. I never really realized how bad my suspension was( for the record, I am still experiencing most of the problems I had before my suspension was re-valved and rebuilt) until I had ridden his. I had no idea anything was damaged or cracked, aside from the divot in the left inner fork tube I had already repaired. The cracked free piston was brought to my attention from my tuner, and he replaced it with a new 2009 free piston modified to his specs. I had the forks back from his shop in June of 2009, and since have put 22 hours of testing on the suspension.

I had previously installed the RTT stabilizer/ top triple clamp ( it comes as one unit) in December 2008.

Yes, after I got the suspension back I re-set sag and realized the stock rear spring was too soft, so I started to test stiffer rear springs, which brings me to now.

Thanks for the advice.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Sider_steve

Posted September 11, 2009 - 10:28 PM

#30

I damaged and repaired the inner fork tube in June 2008 with 2-4 hours on the bike total and rode the bike for another 65 hours before sending the suspension off to be set-up.

I sent the forks and shock off because I was completely amazed how well my friends yz250 worked after he had his suspension re-worked from the same tuner I used. I never really realized how bad my suspension was( for the record, I am still experiencing most of the problems I had before my suspension was re-valved and rebuilt) until I had ridden his. I had no idea anything was damaged or cracked, aside from the divot in the left inner fork tube I had already repaired. The cracked free piston was brought to my attention from my tuner, and he replaced it with a new 2009 free piston modified to his specs. I had the forks back from his shop in June of 2009, and since have put 22 hours of testing on the suspension.

I had previously installed the RTT stabilizer/ top triple clamp ( it comes as one unit) in December 2008.

Yes, after I got the suspension back I re-set sag and realized the stock rear spring was too soft, so I started to test stiffer rear springs, which brings me to now.

Thanks for the advice.


Ok that info really helps.
It tells us the problem was there before the forks were changed.
Did you buy this bike new?
And did the problem first show up after you damaged the forks?
Or has it been there since you have owned the bike?

And this question is way out there.
Would your friend let you put his forks and front wheel on your bike to see if the problem went away?
And if he will leave his wheel and tire on his forks slide them in your clamps.
If it works great try it with your front wheel installed.
If everything works good then you have narrowed it down to the forks.

  • grayracer513

Posted September 12, 2009 - 06:56 AM

#31

Free piston failures are not uncommon:

http://www.thumperta...ad.php?t=650136
http://www.smartperf.../YZMODASSEM.htm

A dinged or dented inner tube will either cause the fork to bind as the dent passes the bushings or it won't, and this can be easily checked by removing the spring and working the fork through the stroke by hand. Otherwise, the only thing it will do is cause a leak.

  • Jim813

Posted September 12, 2009 - 09:34 AM

#32

Ok that info really helps.
It tells us the problem was there before the forks were changed.
Did you buy this bike new?
And did the problem first show up after you damaged the forks?
Or has it been there since you have owned the bike?

And this question is way out there.
Would your friend let you put his forks and front wheel on your bike to see if the problem went away?
And if he will leave his wheel and tire on his forks slide them in your clamps.
If it works great try it with your front wheel installed.
If everything works good then you have narrowed it down to the forks.


The bike was bought new. To be perfectly honest I don't know when the problem first occurred, and I never seriously noticed the problem until I had ridden my friends bike. The problem is I wrecked on my first real ride on the bike, and never got a good feel for the stock suspension before I wrecked.

  • LVThumper

Posted September 12, 2009 - 02:21 PM

#33

Someting to consider is your choice of rear tire. I, as well as many other desert riders I know, have ditched the Maxxis IT (both Desert and regular IT) as they have a tendancy to "wiggle" at high speeds. It almost feels like a flat! I run a Dunlop 745 up front and have switched to the Michelin MH3 out back. This has been the best combo I have found so far! :busted: The front sticks and turns, while the grip / traction and long wear of the MH3 has been great.

Oh yeah, I have an '01 426 set up for my size (6'5" 250lb) with proper suspension, high bars with risers set in the stock location, Rekluse ZStart, and a 3.1 gallon desert tank. It is WELL planted at high speed (I gear it up for high speed races to close to 100 mph) and I DO NOT run a stabilizer. I know, everyone says I'm crazy, until they ride my bike and realize how stable it truely is. :thumbsup:

In my experience with several Maxxis IT rears, they all last a long time, but they are unstable at high speeds.

Good luck and try a different rear tire. That may be the hot ticket!! :banana:

  • Jim813

Posted September 13, 2009 - 09:11 PM

#34

Thanks for all the input. The fork spring are stock, but the rear is a stiffer 57N (5.8kg/mm). I am about 6' even, which may be a bit taller than the average. I may be lacking to give out all my info , so here is everything I can think of to post.

Fork/shock rebuilt and re-valved at 67.4 hours on bike( currently 89.1 hours on bike)
Dunlop MX31 front tire
RTT triple clamp/stabilizer (with more forward and taller mounts)
Steering bearings checked and tightened a bit tighter that OEM specs.
Triple clamp bolts tightened to OEM specs.
Stock front fork springs (.470kg/mm)
KYB stiffer shock spring (5.8kg/mm)
All bearings working properly


From the recommendations on this site,
I checked:
Fork compression setting: both at 15 out
Fork rebound: settings both at 10 out
Fork height in clamps: both at 8mm up, set back to stock setting at 5mm above clamps. One thing I have to do is compare the stock top triple clamp to the RTT top triple clamp and see if there is and height difference that could be giving me a different reading that with the stock top triple
clamp.
Sag: set at 101.5mm, pre-load at 12mm, static sag at 25mm

Checked for fork binding: Didn't notice any fork binding, but I did take the front wheel off, checked steering bearings, checked wheel bearings, make sure there were no burrs on the axle, and put everything back together. I made sure that the fork didn't bind when putting back together by following the steps in the manual. I don't know if this matters or not, but on the right fork lug has some scratching inside where the axle slides through.

I will try the bike this weekend and see if anything is different than before.


I got a brief chance to test the new settings, although I didn't get a chance to ride at a race pace. The bike stayed a bit straighter, and I didn't notice much difference cornering the bike. The farther I leaned over the handlebars the front seemed to stay a little straighter and react better to bumps, but it got to the point where I could look down over the front of the fender, and I don't feel very comfortable riding that far forward over the bike. The problem still remains.

  • Sider_steve

Posted September 13, 2009 - 09:38 PM

#35

I got a brief chance to test the new settings, although I didn't get a chance to ride at a race pace. The bike stayed a bit straighter, and I didn't notice much difference cornering the bike. The farther I leaned over the handlebars the front seemed to stay a little straighter and react better to bumps, but it got to the point where I could look down over the front of the fender, and I don't feel very comfortable riding that far forward over the bike. The problem still remains.


Sounds like having your weight over the front end helped,but your not happy riding there.
So you need to change the center of gravity on the bike, making it sit so more weight is in the front end.
There are 2 easy ways to do this,raise the back end of the bike,or lower the front end.
Since raising the front forks in the triple clamps will cause the bike to corner better,yet high speed stability will suffer. I would try less sag in the rear,your set a 101.5 mm now. Stock settings call for 90-100 mm, you can split the difference or go for 90 mm.
Then take it for another ride,sounds like it has improved since last time you rode it.

  • grayracer513

Posted September 14, 2009 - 06:49 AM

#36

Raising or lowering either end of the bike has no effect on how much either end actually weighs. Having the rear relatively higher than the front compared with a given baseline increases the amount of weight transferred to the front during braking, but the effect is dynamic and transient. The biggest real effect that pulling the forks up in the clamps or decreasing the race sag has is in changing the head angle of the bike, and either of these has the same effect on stability at speed because they both do exactly the same thing to the overall steering geometry.

The '06+ YZ450 is exceptionally sensitive to small changes in setup, too, and front tires selection is very important.

Also, the farther you lean the bike, the more willingly it will turn, so lean the bike farther than you do. For instance, the bike at 50 degrees to horizontal, and your upper body at 65 degrees, closer to upright than the bike is. Your tires have to be up to this.

  • Jim813

Posted September 15, 2009 - 10:26 AM

#37

After talking ot my suspension guy, he has a feeling the fork may be too stiff initially. I am sending the forks out this week and will see how it performs when they get back.

  • harrymank

Posted September 24, 2009 - 10:52 AM

#38

This last weekend I was riding next to my buddy's corvette on the pavement and the front wheel wouldnt stay steady, the bars were shaking pretty good, but of course we were doing 80 mph





Related Content

 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.