Steering Stem Offset

[wobblywalrus]

What happens when you have too much offset?  Some background information.

My bike is a 2003 Triumph Bonneville with 59 inch wheelbase, 29 degree rake, and 4.61 inches trail.  I run as a modified partial streamliner and it handles great up to and through the mile.  During one one out of every four shutdowns I get a bad tank slapper speed wobble.

This winter I lengthened the swingarm 3 inches and installed a set of A & A racing triple clamps for a Harley flattracker.  This was not a bolt-on fix.  It was a big project.  Now I can adjust the fork offset.  The offset is the distance in millimeters between the triple clamp stem axis and a line through the fork tube centers.  Here are my impressions of various offsets.

67 mm  Almost impossible to ride.  Unstable.  I stained my shorts after a blast around the neighborhood.  This offset uses the A & A 7 mm offset spacers set forward.

60 mm  Twitchy.  Sort of like a Vespa scooter.  Fine around town but real scary on the highway at speed.  This happens with the 0 mm offset A & A spacers.

56 mm  OK geometry for the street.  Standard Triumph triple clamps.  These clamps are flexy and not as strong as the A & A clamps.

53 mm  More stable on the road with heavier steering than other setups.  Works good with the radial tires I use.  The rear end tends to push out more when blasting through 90 degree street corner turns.  This would be my favorite setting if I did not race at Bonneville.  Real stable on the freeway and in side winds.  This offset happens when the A & A 7 mm spacers are installed backwards.  See photo.

A successful tuner that I trust, and who is familiar with new Triumphs, recommends a 34 to 40 degree offset.  I  see how small offset changes make a big difference and I am nervous about this big change.  I will be making these clamps by hand and I can only make one set before the speed trials, so I do not want to get too radical.  Also, this bike without the sheet metal is my basic transport during the off season.  I will need to turn occasionally.

 

[Freud]

If you aren't too far from Aurora, OR, go talk to Treit. He will know.

FREUD

[willieworld]

maybe this will help  http://www.rbracing-rsr.com/advchoppercalc.html     the offset isnt measured in degrees  --the  less the offset (steering stem with fork tubes ) the more the trail if nothing else changes---another thing is the swingarm it has to be very ridged --no flex--or the tail will wag the dog   willie buchta       

[John Burk]

By the photo on the wall and bike in the foreground this guy in the 1860s was making progress in understand trail and rake . 

http://www.crazyguyonabike.com/doc/page/pic/?o=3Tzut&pic_id=120227&v=Gr&size=large

[Blue]

Changing "offset" is changing trail.  Try 5 to 10% of your wheelbase.  10% should be so stiff that you can't steer and <4% is where head shake starts.  Remember to measure the trail difference due to weight (and pitch) shift power-on vs. power-off.  Note that the trail decreases power-off and that's when we get the tank slapper.

[wobblywalrus]

Thanks to everyone for advice.  Memories of Timbo Horton's mishap, which appeared to be a broken swingarm from the photos, influenced the modification.  Lots of metal was used in the extension and it is real strong.  I do not need to add any ballast.  The swingarm weighs enough.

After work this afternoon I put a wooden crate on the luggage rack, filled it with junk, put on my padded jacket, and took off down the Interstate.  The freeway section was between Woodburn and Salem.  Lots of trucks, truck ruts, fast traffic and a side wind.  The bike has always been a handful on this road.  With standard 117 mm trail (55 mm offset) it wiggled and hunted a bit due to wind turbulence and the ruts.  The box on the back always made it worse.  I could not ride the bike on this section with the flattrack triple clamps set at 113 mm trail (60 mm offset).

This afternoon the bike was stable.  The triple clamps are set at 53 mm offset (119 mm trail).  I rode along in the ruts, around all the trucks, through the sidewinds, no problem.   One handed while picking my nose.  The bike has the standard swingarm and the trail to wheelbase ratio is 8.0%.  A happy day.  Wobblywalrus lost his wobble.

Some late night calculations show that a set of clamps with a 48 mm offset will give me an 8% trail to wheelbase ratio with the extended swingarm.  I will build my clamps using this offset.

There is nothing wrong with the 60 mm offset on the Harley flattrack triple clamps.  The oval racers have much less rake than my Triumph and this large offset is OK with their fork geometry.  If I knew then what I know now, I would not have used these clamps.

This is a humbling experience.  Before this, in my dreams, I could see myself building a frame.  If such small geometry changes give such large differences in handling, I will never get everything figured out when I build a frame.  That job is best done by experts and the money they charge is well earned.     

     

[panic]

The stem offset is only directly proportionate to trail if the steering axis is vertical. As you tilt the triangle backward (more rake) the effect on trail is obvious.

[willieworld]

there are lots of things that can cause the wobbles besides trail  --loose spokes in front or rear--low tire front or rear--fork flex--steering head flex--frame flex--swing arm flex--bad wheel or swingarm bearing--out of balance wheels--loose axle or steering head bearings--broken triple tree or frame--or a combination of the above---and a little here and a little there will add up fast--  just some thoughts willie buchta

[bak189]

"That job is best done by experts and the money they charge is well earned"

Thank you.....we appreciate that statement.....................sound like you got it sorted out!!!!!

[bak189]

But HEY, what is with all this metric stuff.....this is America, Boy.......we are not yet on the Metric system
( but they certainly are trying to get us there).............when I am in Holland I use the Metric system.....
USA I use inches.......I consider myself a multi measurement master..................................................

[panic]

Tony Foale did some interesting tests, which showed how little effect rake has if everything else works.
http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm

Thanks, Willie!

[willieworld]

panic   it doesnt sound like we read the same article---my take on the article was that rake didnt matter much as long as you have enough trail--if you look at the pics in the article you will notice that all of the bikes have good trail no matter what the rake --in a couple pics you will notice that the front end is on the bike backwards to gain positive trail---harley davidson did that years ago on all of their touring bikes--and still do ---my conclusion from the article (and was before i ever read it )that although rake and trail are related you can change one without the other---dont worry so much about the rake  worry about the trail---i would like to see tony ride a bike 100 mph with 0 trail  he would only do it 1 time   just some thoughts   willie buchta

[wobblywalrus]

Thanks for the comments about other problems that cause wobble.  Last year when we got back from Bonneville I took the bike apart to convert it back to a steet bike and I found a notched steering head bearing. This winter I fixed that.

The advice about trail is good, too.  I had the suspension set wrong at last year's speed trials.  Lots of preload in the rear and I had one inch of fork tube sticking up through the tops of the triple clamps.  The trail was about zip when the bike nosed down after the end of the mile when I shut off the throttle.  That is when I had the speed wobble.  This year I will back off on rear preload and move the triple clamps up near the tops of the tubes.  That should give me more trail during the shutdown.  Also, I will add a half inch the the fork spring spacer to make it ride higher in the front.  The fairing weight compresses the forks too much.

Triumph had a bias ply front tire and a radial rear as standard equipment and that was what I was using.  The bike had a light steering feel and worked great at the speeds it was designed for, but it felt strange when pushed hard.  I could not pinpoint the exact problem.  Now I run radials on both ends and the tires grip much better and they are more stable, but this introduces another problem.  Rigidity.

The triumph frame and swingarm are nice and strong as they come from the factory.  The triple clamps, both standard Triumph and flattracker, are flexy and the radial tires accentuate this.  In comparison to bias-ply tires, the radials tend to track in the direction they are pointed and it takes a more rigid fork to crisply shift the front tire off-line for countersteering, in-corner corrections etc.  Right now I can feel the forks flex when I work the bike hard.  Some nice strong clamps will give me more steering precision and confidence. 

[panic]

!!!

You're right, I inverted it - duh, senior moment, thanks.

[bak189]

All this talk about speed wobbles and trail enc. brings up a interesting point ...........when Tech. is done on your bike it is mainly to check for safety, with this in mind, would it not be a good idea to have at least one of the inspectors present understand what M/C frames and set up are all about?
The example being the Tri. "set up" of Wobblywalrus that he rode last year at Bonneville ...........now he understands what he did wrong.........but as he noted, he came close dropping the bike at speed.
Over the past many years a lot of mishaps could have been prevented in LSR if at least one of the Tech Inspectors understood the "rideabillity" of M/C's

PS. This also goes for sidecar Tech.