17inch wheels, offset question

[lazlokovacs]

Hi everyone,

I have a friend who is rebuilding a LM2. His plan is to go to 17inch hoops front and rear. He's also building his own custom yokes/triples. (Its a big re-build...) Anyone know the optimum offset he should be looking for?

Thanks

[LesP]

The triple clamp offset would be dictated by the intended trail measurement ? which might be in the 90 to 100mm range.

[mtiberio]

Hi everyone,

I have a friend who is rebuilding a LM2. His plan is to go to 17inch hoops front and rear. He's also building his own custom yokes/triples. (Its a big re-build...) Anyone know the optimum offset he should be looking for?

Thanks

I did this to my old Pro Twins racer. Stock, the Lemans triple clamp offset was 65mm. I had some custom triples made by Kosman. They had 45mm of offset. Interesting fact, it increases trail while at the same time shortening wheelbase. Better stability and manuverability at the same time. Win, win.

[moto-uno]

I own a Le Mans 2 also , if this is for the street , why ? Peter

[Huzo]

Hi everyone,

I have a friend who is rebuilding a LM2. His plan is to go to 17inch hoops front and rear. He's also building his own custom yokes/triples. (Its a big re-build...) Anyone know the optimum offset he should be looking for?

Thanks

When you ask for an  “offset” dimension, what are you asking to define ?
Are you referring to the wheels or yokes ?

[Huzo]

If I try to interpret the spirit of your question, I’d hazard a guess that you are referring to the following.

Imagine a line drawn between the centre points of the two top fork caps, the ones that you remove to empty out the fork oil.
The distance between the mid point of that line and the centre point of the steering stem.
Will be somewhere around zero to 2”.
But as to the optimum for your example ?
No idea.

[dxhall]

Don’t know what you mean by “offset.”  Are you talking about the difference in the angle-from-vertical between the  steering stem and the fork tubes?

[Huzo]

Hi everyone,

I have a friend who is rebuilding a LM2. His plan is to go to 17inch hoops front and rear. He's also building his own custom yokes/triples. (Its a big re-build...) Anyone know the optimum offset he should be looking for?

Thanks

Anyway, couldn’t you just measure a standard one ?

[Rough Edge racing]

I've built many bikes using wheels and fork assemblies from other bikes... When possible go with geometry of a similar bike or keep it the same as original...Rake and trail have to compliment each other...LM2 has about 27 degrees rake and perhaps 4.2 inches of trail and a 58 inch wheelbase?... Smaller diameter tires will quicken steering a bit as will increasing offset about an inch...But you have to consider the conservative rake and long wheelbase..
 And the handlebar width and even foot peg location also effects how the bike reacts

[harrytief]

I used a custom set of gsxr triples along with the gsxr fork but with the guzzi 18 inch wheels. The gsxr triples reduced the guzzi 65mm offset to 30mm. The only problem I had was with the lower triple now hitting that lovely guzzi tank. I fabbed up steering stops to solve that problem. The pros of the conversion for me was a MUCH more stable and responsive motorcycle at all speeds along with race quality radial brakes and 320 mm brake discs. Just wow! I added 18 inch spoke wheels to further lighten both ends of the bike with an additional net plus. Now that there are radial tires out there for older 18 inch rims, I think that keeping the look of vintage with 18 inch wheels is no disadvantage. I added bigger carbs feeling that my modded cx100 could now handle whatever. In my case, not a particularly high bar but...
Harry

[lazlokovacs]

err guys,

mike answered this one on response 3

Huzo - offset is what you think it is

quote from the glossary of Sportsbike suspension tuning 2008 by Andrew Trevett

"OFFSET - A dimension from the steering axis to a plane defined by the front fork tubes. Most triple clamps are machined such that the fork tubes are parallel to the steering head, in which case offset is easily measured. In rare cases that the two are not parallel, offset must be expressed in terms of a distance and and an angle."

as for the 'why would he want to build his own yokes/triples question... I've got no idea, but I think the guy likes making things

rodekyll - thanks for letting me know that this isn't something I ask the internet for opinions about. Which is in itself an opinion... I found your reply pretty strange tbh

Ride on brothers!

[Huzo]

Unsure of the question.  The context leads me to think you're asking about "trail."  In a car it would be "caster."  I'll go with that.  The term refers to the relationship between the wheel's axle and a line through the steering post axis extended to the ground.  If you make that line and draw a spot on the ground where the line meets the floor, you want another line, drawn straight down from the wheel's axle to fall somewhere behind the axis mark.  That distance is "trail."  Negative trail means that the wheel axle sits behind the mark.  Positive trail means the axle is in front of the mark.

This isn't something you ask the internet for opinions about.  It's not a belief system, and it's not a guessing game.  It's a hard question of geometry, and the wrong answer can kill you. 

The more trail you've got, the more stable the bike is on the road.  Stability is great in the straights, but it resists turns.  Too stable and it won't turn.  On the opposite side of the scale is unstable.  An unstable bike is very "flickable", as the kids say.  It likes to twitch around the curves.  It does not like to go straight.  Too unstable and you get FATAL headshake/tank slapper -- whatever you want to call it.  It's essentially the same situation you get with that shopping cart with the spinning front caster:  The wheel wants to swap ends.  When it does, there's no amount of handlebar control that will correct it.

The more negative (longer) the trail, the more stable the bike.  If you measure it you'll find that a highway cruiser might have as much as -5" trail, and a track racer might have as little as -2".  As you approach "0", the bike becomes more unstable, until the caster is so little that the wheel wants to swap ends.  The ideal point of trail is where the bike is unstable enough for nimble turns without becoming uncontrollable at any speed or attitude. 

What a particular bike needs depends on the rest of the chassis and suspension geometry.  Every change to anything else affects trail.  That includes the steering head angle, the t-tree offset, the rolling diameter of the wheels, length of front tubes, and rear shock static length.  I probably didn't mention some.

So you can't just pick a trail value randomly.  You need to be comfortable with the math and how it translates to the physics involved.  When I built my trike I learned a lot about steering geometry.  I quickly discovered that I needed far less trail than the almost 7" I started with in order to reduce the steering effort.  But as I approached -2", I realized that I could be creating a dangerous configuration.  So I went with a leading link front end (basically a swingarm pointed forward).  I could adjust the trail distance  (and some other properties) by changing the length of the swingarm and shocks until I liked it.  Properly done, I had a trike that steered straight without shaking, but had a "power steering" effect in the turns.

Actually, although it was a more in depth response than the question required, I reckon it’s a good piece if you needed instruction...

[mtiberio]

I did this to my old Pro Twins racer. Stock, the Lemans triple clamp offset was 65mm. I had some custom triples made by Kosman. They had 45mm of offset. Interesting fact, it increases trail while at the same time shortening wheelbase. Better stability and manuverability at the same time. Win, win.

FWIW, I ran this setup with 17" front wheel, and no stability issues up to 142 MPH.

[lazlokovacs]

Rodekyll - you do your thing man!!  Excepting the description of rake/trail, which I already have an understanding of, I'm still very confused by a few things in your post but I'm also grateful for the time you took and the info.

I would never ask you to hold back on anything. I'll make a deal, you be you, if it grates on me I'll PM you next time.

I feel regret to have singled you out in a thread like that.

Buds?

Winter's barely started!!!!

[acguzzi]

I went to 17s on my lemans with standard yokes and standard fork length and a 150 rear tire (larger OD), If you know Grattan there is a hump on the back section right before a right turn, so you crest the hump and then hard on the brakes, so the front is very compressed. I discovered the hard way that my geometry was unstable under those conditions. I had to lengthen the forks, the upshot is that a reduced offset may have also worked to fix the problem, but tire OD is also a factor (YMMV), be careful.