Motorcycle Rake and Trail

[ThumbBiker]

I have a question on the rake and trail of a motorcycle. Working on a 1972 Benelli/Motobi 175cc for M-PG. Took the following “production” measurements – wheelbase = 47.3”, rake neck 26 deg, fork length = 24.6”, rear tire dia = 24.0”, front tire dia = 23.6”, triple clamp offset = 3.25” and plugged these dimensions into the “RB Racing Professional Rake and Trail Calculator”. This gives a trail of 2.14” ( ~ 4.5% of wheel base ). This also correlates with ‘actual’ trail measurement.  Class record is ~72 mph and I remember reading an old road test that sounded like the bike was a little ‘wobbly’ at top speed – not sure what that means but doesn’t sound encouraging.  I haven’t ridden this bike so have no actual experience and am working on the engine now. So, I’m looking for some input from the racers with experience as to whether this is enough trail or do I need to increase it. I have heard from another racer that he has 42 deg rake and over 6” trail and his bike is rock solid. I’m very new to this so any help is appreciated.

[55chevr]

If you run production you cant change the rake.   Most purpose built bike run 35 to 40 degrees of rake.

[Koncretekid]

3.25" (82mm) of offset at the triple clamps is a lot, especially with only 26* of rake and a short wheelbase, which usually makes a bike want to steer quickly (twitchy), not exactly what you want at high speed (not even at 72mph).  The easiest way to change this is to try to find triple clamps from another bike with less offset to fit your forks and wheel, or just use the whole front end from another bike, including the wheel. Small Hondas (CB350, 360, for sure) have 60mm offset and 33mm forks, and these should be easy to source.  
If you intend to run "Production," and you can't change the front end, you'll at least want to add a good steering damper.  The cheaper ones have bit of play (0 damping) at neutral.  An old friction type damper might be the best thing.

[ThumbBiker]

Appreciate the input - now time to go to work. This bike will be in the modified class, not production. Thanks again

[wobblywalrus]

We monkeyed a lot with the Triumph to cure its wobbles.  Someone on this forum in an ancient post recommended the trail be 8% of the wheelbase.  This is the best combination for our bike, too.  It gives steady handling on the salt and works well on the street.

[JimL]

M class...add the 10% to the swingarm and keep yourself forward.  Dont get too much weight over the rear wheel.  Consider fitting front forks from something like a KX80 Big Wheel to get more trail and stiffer front end.  Those had a 19" front wheel, so your tire is easy (they were not wide tire, just larger diameter than standard 80cc motocrossers.)  I've ridden one, with 105 kit, faster than you need to go and it was very stable.  Youll find old ones from the 1980s with shot or no engine for cheap.

It will be easy to get lower, also, and the tubes/sliders are stiffer.

See you there!
JimL

[grumm441]

You have to run a steering dampner and that may stop any "wobbly" any way

G

[debgeo]

Well I am jumping onto someones thread. But my question fits with the title so I hope I offend one. I have a sidecar on the drawing board and before any screams I have searched this info here and on the web. I presently chosen to go with 40 degrees and 7.5 inched of trail. This tends to meet with the 8% of wheelbase rule. I will be a a class that runs 170 mph. I hope to get close to this. Your thoughts positive or negative. Understand I am a salt rookie still wet behind the ears No salt yet but I have a room and will be at SpeedWeek this year.Your guiadance please. 

[wfojohn]

Also, pay close attention to the swingarm mounting/pivot points. The bushings/bearings may have loose tolerences from new and you might be able to gain stability there, its worth checking. A loose arm will allow the tail to wag the dog a bit and have you play catch up steering.

[bak189]

Regarding....rake and trail on a sidecar, depends what you plan to do with the sidecar outfit........straight line racing, leave in the normal rake and trail as a solo bike......if you plan to go around corners a major change has to be made in the rake and trail in order to keep it  "between the hedges".........................................................

[wobblywalrus]

There are some production class bikes that run fast speeds without a lot of trail.  They pay a lot of attention to the things mentioned in the previous post.  They also spend a lot of time on the basics like making sure the steering head bearings are not tight, notchy, or loose, the wheels are in near perfect alignment, and the wheel bearings are in good condition.  The salt is not always smooth.  Rear dampers, if suspension is used, should give adequate damping.

One thing to pay attention to are the rims and tires.  The rims should be true.  Then mount the tires on the rims and make sure the beads are properly seated and the tires are inflated to the correct pressure.  Check the tires to make sure they are true.  If not, put on another tire until you get some that are true.  Also, if a tire takes a lot of weight to balance it, it is not good for LSR.  Sometimes, with certain brands, two or the three tires need to be fitted to get a good one.  The Metzelers we run are almost always true and often require no weights to be in balance.  This is common for premium tires from some of the other major brands, too.

"True" is a subjective term.  Usually I try for less than 5 mm tire runout.       

[manta22]

..."5 mm tire runout."


5mm = 0.2"  that seems like an awful lot of runout.

Regards, Neil  Tucson, AZ

[maj]

I took the raked frame off my busa this season and went back to a stock frame,
also made a few other changes, but it handled much better than it had previously
decided to do this after the stock frame 750 always handled good ,when the busa always had a weave

imo spend the time you would have done raking the frame , checking the straightness , making sure your wheels are dead true to the frame
your headstem and swingarm pivot is in perfect condition and your steering damper is not too tight