Steering/Fork Caster on motorcycles

[dna9656]

The handle bars start to shake violently when taking both (to get them out of the cold or to rest my arms) hands off. What I have read says that motorcycles use positive caster (or rake and trail) to make steering easier. Would a negative caster be caused by worn/out of adjustment triple tree bearings?

Edited March 8, 2015 by dna9656

[Peder_y2k]

No.

 

What are you doing taking your hands off the bars? Firing a machine gun over the w/s? I should have known.

 

BTW, you can achieve negative caster by slamming your bike into a wall doing 100 mph. Otherwise, negative caster ain't gonna happen.

 

http://yospeed.com/wp-content/uploads/2013/02/100304-wheel-alignment-caster-positive.jpg

 

-Pete, in Tacoma WA USA

[cowpuc]

Hi Doug,

Having played around building Choppers for a number of years (nasty habit I started back in the early 60's with bicycles and was never able to get rid of) gave me the opportunity to learn some things about motorcycles and their steering geometry called "trail". Basically, if you draw a line thru the center line of a motorcycles goose neck and follow that line till it contacts the floor surface and make a mark - anything rearward of this line is considered "Positive trail" - forward ( away from the bikes frame) is considered "negative trail". Now draw another line thru the front axle of the bike that is perpendicular with the floor and make another mark where that line meets the floor and you will notice that it will land behind the mark you made from the goose neck - all rideable bikes must have positive trail. An interesting thing that I discovered many years ago with Chops, the longer the frontend, the more "rake" (angle in degrees reguired to maintain proper amount of trail) you must have and the more rake that you add the more crucial hugging minimal trail becomes - 18 inch over front ends require as close to zero trail as possible without going into negative territory.

Having modded a number of stock frames thru the years, I discovered that 3 to 6 inches seems to be a standard amongst all manufactures of stock bikes. Less trail being common on quick turning canyon carvers and more trail on bikes that have the feel like they could run the highway with no one onboard. There is simply not enough area of movement within a worn bearing to effect trail enough to cause an issue.

This does not mean though that your steer bearings arent out of adjustment/worn out and causing (or adding to) the violent shake you are talking about!! You didnt mention what model/year bike we are talking about here. I have some experience with the 83/84 Ventures and can tell you that nasty headshakes at 42ish mph have been a problem with all of mine, especially after the front end bushings get tired (or swingarm bushings get tired, or fork springs get sacked, or brakes start getting sticky, or tires get a lot of miles on em, or steer head bearings get out of adjustment, or wheel bearings get sloppy).

Personally, if it were mine (because I am a little crazy and like to go no handed sometimes) I would start the diagnoses of your bikes shake by looking real closely at how much travel you have in the forks WITH NO AIR IN THEM. Then, raise it off the ground, spin the front wheel - now take a rubber mallet and smack calipers and spin it again - see if you have a brake drag going on. Spin it again and check front tire carefully for out of round or cupping. Check the fork brace for looseness. Check side to side wheel bearings. Check deflection on the steer bearings. Grab the down tubes and see if you get any lateral movement inside the fork legs (bushings).. Now do the same process

on the back end! Look it over, you will find it!!

Edited March 8, 2015 by cowpuc

[dna9656]

Pete;

SO what your saying is a 35-40 to MPH impact with a small Nissan truck is NOT sufficient to adjust the caster?

 

I offer the photo below in dispute of that allegation.

 

[ATTACH=CONFIG]97987[/ATTACH]

 

 

No.

 

What are you doing taking your hands off the bars? Firing a machine gun over the w/s? I should have known.

 

BTW, you can achieve negative caster by slamming your bike into a wall doing 100 mph. Otherwise, negative caster ain't gonna happen.

 

http://yospeed.com/wp-content/uploads/2013/02/100304-wheel-alignment-caster-positive.jpg

 

-Pete, in Tacoma WA USA

Edited March 8, 2015 by dna9656