fork offset for tonti frame...ideas?

[harrytief]

I built a front end for my "mini griso" project using gsxr forks in triple clamps with 30mm offsets. The stock guzzi forks have 65mm offsets. I certainly got great suspension and superb brakes but handling that I did not expect. The decrease in offset of the gsxr clamps gave me the INCREASED trail I expected that resulted in heavy and awkward steering at parking lot speed. However, I also got some unexpected instability at speed in gusty crosswind. In those conditions, the front wheel wants to tuck under and swap front to rear. Without cross wind, the bike can be twitchy at speed but no drama. I have added a steering damper that helps. This behaviour is the opposite of what I expected form the increase in trail. In the build, I also added ohlins rear shocks at 13.25 inches where stock is 12.5 inches and the bike runs 18 in borranni wire wheels with stock sizes in sport demons.
I am thinking of going to triple clamps with increased offset (45mm is recommended for 18 inch wheels) to help with low speed handling by reducing trail but that may INCREASE the high speed twitchiness. Is there something about the guzzi rake angle, frame, or even the oem triple clamps that I am overlooking that gave the bike its stability and agility at 65mm fork offset? Should I just make triples for the gsxr forks at 65mm offset. Will the recommended 45mm offset give me the magic trail number. Or just buy a new Griso!
Thoughts?
Harry

[Groover]

Interesting trigonometry project... Have you tried how it rides with the 12.5 rear shocks instead of the 13.5? I'm thinking, off the top of my head, that should stabilize it some.

[rodekyll]

I'm not sure what you mean by "offset".  "Rake angle" means something to me though.  Offset is relative to the angle of the steering head.  Rake angle is absolute, measured from vertical.  Given the same initial rake (headstock angle), the same added rake (offset) to the trees will reduce trail.  That's just geometry.

I notice you don't give your trail number.  How did you discover the numbers/do the math for the trail figure?  Without an actual measurement you've no idea what you've got there, and the actual number is actually important.

Yes, increasing the trail distance (larger negative number) should increase stability (reluctance to turn) and reducing it (as you approach zero from negative numbers) should decrease stability.  In no case do you want to reach or exceed zero trail.  That gives the "end around" or "bad shopping cart" experience, in which the front wheel swaps ends so quickly and violently that maintaining a hold on the grips is humanly impossible.  This has been fatal to some.  So measuring that trail distance BEFORE riding is a good idea.

In addition to the rake angle, the length of the forks and the offset of the axle v centerline of the forks have an effect on trail distance and therefore stability.  15mm of change to the fork length should be noticeable.

If you increased trail and got more unstable there's something else going on.  Especially with the parking lot speed symptoms you describe, I'd check the headstock bearings for proper preload first.  I've found increased "flopover" at parking lot speeds and sloppy highway handling with loose steering bearings, especially when loaded heavy to the rear.

That's all I've got.

[oldbike54]

 Offset is the measurement between a line drawn thru the steering head and a line drawn thru the fork tubes .

 Harry , I think what you are experiencing is the wheelbarrow effect . By decreasing the offset so dramatically you are pushing the front wheel instead of pulling it . Have a headache at the moment, so my brain isn't clear , let me think on this a bit .

 Dusty

[oldbike54]

 A couple of questions . Is the overall length of the gixxer front end identical to the overall length of the stock front end . If it is shorter , in combination with the longer shocks you may have altered the weight bias and effective rake angle enough to be causing some issues . Also , what brakes are you running , are the calipers mounted in front of or behind the forks ?

 Dusty

[Lumpy Idle]

i am on the edge of my seat for this one. i have been toying with the idea of a gxr front end for my t3 rebuild but i am so completely out of my depth with this that i didn't even approach it. there are so many variables here...any one of which could launch you over the bars.

any and all explanations by others will aid in my education.

[fotoguzzi]

Please post pics of a Mini Griso   

[rodekyll]

Offset is the measurement between a line drawn thru the steering head and a line drawn thru the fork tubes .

 Harry , I think what you are experiencing is the wheelbarrow effect . By decreasing the offset so dramatically you are pushing the front wheel instead of pulling it . Have a headache at the moment, so my brain isn't clear , let me think on this a bit .

 Dusty

That's what I'm getting at.  If you have a 22º head angle on one bike and a 27º angle on another, the same tree offset (fork length being equal) gives a different trail number.  So what you see on one configuration means nothing in another.  Rake and trail though are more absolute.  Those are the numbers you want to pay attention to.

You are right about the wheelbarrow effect.  Going over a curb with a wheelbarrow becomes easier the more horizontal (greater offset) the relationship between the wheelbarrow handles and the wheel axle.

Where's Greg F when you need him?  He's done the upside-down UJM fork conversion on a loop.  Not the same frame, but certainly the same considerations.  He'd be a big help.

[oldbike54]

That's what I'm getting at.  If you have a 22� head angle on one bike and a 27� angle on another, the same tree offset (fork length being equal) gives a different trail number.  So what you see on one configuration means nothing in another.  Rake and trail though are more absolute.  Those are the numbers you want to pay attention to.

You are right about the wheelbarrow effect.  Going over a curb with a wheelbarrow becomes easier the more horizontal (greater offset) the relationship between the wheelbarrow handles and the wheel axle.

Where's Greg F when you need him?  He's done the upside-down UJM fork conversion on a loop.  Not the same frame, but certainly the same considerations.  He'd be a big help.

 That was why my next post started asking questions .

 Dusty

[harrytief]

Just so we are all on the same page with terminology, I will use
- "offset"  to mean the distance between parallel centre lines through the steering stem and the fork tubes in a plane lying on the top surface of the top triple clamp."
  A zero offset means that the top of the steering stem is in line with the top of the fork on either side of it. Offset increases as the fork tubes move forward ahead of the steering stem creating an       
  increasingly "v" shaped top clamp. Tonti guzzi triple clamps have a 65mm offset and the gsxr clamps I used have a 30mm offset.
 - "rake" to mean the degrees of clockwise rotation that a vertical line dropped through the centre of the front wheel will rotate at the centre of the wheel before aligning with the  front fork. It is as
  if the fork is a hand of a clock rotating clockwise at the wheel hub coming to rest at the required "rake".
- "trail" to mean the distance along the ground between two points A and B.  A is the point on the ground where a vertical drawn through the centre of the front hub intersects the ground plane. B         
  is the point on the ground where a line drawn along the centreline of the steering stem and extended to the ground touches the ground. The trail is the length of the distance A to B. As fork offset
  increases, trail will decrease and as offset decreases, trail increases. A deeply "v" shaped set of triple clamps will have a low trail number and be unstable at speed while a flatter set will increase trail
  yielding better stability at speed. These flatter clamps require more effort to turn and feel clumsy at parking lot speeds.
 

[oldbike54]

 Harry , we understand all of that , what we are curious about are the questions we asked re overall fork length , chassis attitude , changes in rake due to any changes to chassis attitude , etc .

 Dusty

[harrytief]

Now that I've gotten the retired math prof me out of the way, does anyone know where I can find tonti frame specs using OEM forks and shocks. I can then calculate (or be given) OEM guzzi trail spec and then find an offset that will come close using the gsxr forks and custom triples. The rule of thumb I've remembered is that a 30mm offset works best with a 17 inch front wheel while an 18 inch wheel suits a 45mm offset. The guzzi 65mm offset would seem to call for a 19 or even 20mm front wheel all other factors being equal.

[harrytief]

I used gsxr forks as I recall that they seemed to be close in length. I did increase the length of the rear shocks using a set of 13.25 inch ohlins instead of the 12.5 inch stock.

[oldbike54]

I used gsxr forks as I recall that they seemed to be close in length. I did increase the length of the rear shocks using a set of 13.25 inch ohlins instead of the 12.5 inch stock.

 How close , if they are say 1" shorter , and you have raised the rear by 3/4" , and by chance the sag up front is more than stock , and the rear is now stiffer than original , then the chassis could be seriously tilted . You are looking at this in a binary fashion , it seems maybe this issue might be due to a series of changes .

 Dusty

[guzzisteve]

Frame specs you look for are in the service manual for V7Sport/850T

[RinkRat II]

 If this helps, from an article on LeMans 1000 "The frame geometry had a 28-degree fork rake of 28 degrees and trail of 98 mm. "

     Paul B
   

[rodekyll]

These notes are based on this article and other sources I studied while I was designing the trike.  Scroll a little down the first page to start the good stuff:

http://chopperhandbook.com/rake.htm


I think you're using the wrong references for your trail calculations.  You're right about the vertical line drawn from the axle to the ground, but I think the reference from a line drawn from the fork tube is incorrect.  The reference should be from a line drawn through the axial center of the steering tube to the ground.  Try that instead.

You confuse me with "rake".  Clockwise is relative.  Initial rake is the natural angle of the steering tube, measured forward from vertical, and measured from the stem, not the wheel.  Some trees have additional rake built in (explained below).

"Offset" is the distance between the triple tree's center (the stem) and the fork tubes.  In a parallel tree configuration, no offset means the three points, left fork, steering tube, and right fork, are on the same plane.  2" offset would mean the forks are on a plane 2" ahead or behind the stem.  Any offset affects rake angle because it moves the axle nearer or farther from the rake point (as you draw the line to the ground)

Some trees include an additional rake.  That means that the offset at the top clamp might be zero, and the offset at the bottom clamp might be 1" (to toss out random numbers), and the fork tube bores in the trees are cut at angle to take advantage of the offset.  I have a set of trees with an additional 7 degrees of rake.  7* is enough to see the increased angle through the trees -- the tree bores are angled and the angle is clearly visible where the fork tubes protrude. 

Another way to achieve offset, either parallel or angled, is to piggyback a second set of fork clamps to the existing trees and moving the forks forward.

With this clarification, you can easily see how changing rear shock length, front fork length, and wheel diameter affect the trail distance.  If you squint, you can see how the geometry works even for springers extended to silly lengths.


On a guzzi you can measure the trail by removing the front wheel and fender and blocking the forks at ride height (wheel diameter).

The steering tube is hollow.  Wrap tape around a laser pointer to make a secure, centered fit in the hollow tube.  Be sure the laser is centered in the pointer by rolling it on a flat surface.  If there is wobble in the light, play with the tip until it's true.

Put the laser into the tube and mark the point on the ground.  If you still have wobble, rotate the pointer and make several marks.  Your target is then the center of those points.  You might have to fiddle with the laser to get it trued up in the tube.

Put the axle through the forks and drop a weighted line to the floor.  I make a loop in a line and hang it from the axle.  This puts the drop on center.  Measure the distance from your laser point to the drop point.  You want the drop point to be behind the laser point.

With my trike's modified trees I reduced my trail to -3.5"  I could slide the forks up or down in the trees and change that.  With the leading link I'm down to about -2.25".  This is bordering on twitchy, but still stable.  On three-wheel rigs, which don't like to turn, severe reductions of trail are desirable because it reduces steering effort.  Remember -- you STEER these rigs, you don't lean them and there is no countersteering.  So hacks and trikes can benefit from instability.  2-wheelers don't steer and suffer from the same amount of instability.  You want much more trail -- inches more -- than a 3-wheeler.  Where the rigs like 2" - 5", bikes like 4" or better (more negative), and I've heard of as much as 11"


Hmmm.  I *think* that reads right . . . .

[harrytief]

Hey RK
You are right and I mispoke. The trail is the distance along the ground between the intersection on the ground of the centre line through the steering stem and the point vertically below the front axle centre. I corrected my post above. As penance for my mea culpa I worked out a bit of trigonometry that would give me rake angle and trail by taking two height measurements on the fork at a fixed interval along a horizontal. I'll sit on the bike holding it upright with my weight compressing the suspension while Wendy uses a large T-square to measure height at the front axle and height a known horizontal distance back from the vertical through the axle. The offset in my triples is fixed at 30mm.  After coffee.

[Groover]

Is the offset measured at the top clamp?

[Rough Edge racing]

Is the offset measured at the top clamp?

  Yes, and to find out what really going on ....With the rider on the bike on level ground put an angle level on the front fork and the frame neck if possible...There are some bikes having different offsets on top and bottom fork yokes...Modern sport bikes have about 24-25 degrees fork rake with about 4.2 degrees trail using 17 inch front wheels...I believe stock Tonti frames are in the range of 28-30 degrees fork rake?

[mtiberio]

When I raced, I had some special triple clamps made for my tonti racer. they had ~40mm of offset. I used them with some 38mm marzocchi forks. I ran that front end with 18" rims and with 17" rims. Much more stable and maneuverable than the stock 65mm offset clamps. If you are having stability issues, I'd look into the rear shocks, and also look into the ride height of your front end.

[mtiberio]

...There are some bikes having different offsets on top and bottom fork yokes...

while true, it requires that the holes be bored at an angle. rare, but guzzi did do this in the 1400 eldo.

[harrytief]

Well this is interesting.
Using the measurements taken as I described above, I calculate a rake angle of 28.1 degrees and a trail of 139.7 mm. With that much trail, the bike should be on rails at speed and clumsy in a parking lot. I am getting clumsy in slow turns but not on rails in cross winds.

[harrytief]

Mike,
My thoughts follow yours. I'm thinking of having a set of custom clamps made at 45mm offset to reduce trail but I don't want to worsen issues at speed in winds.
Harry

[swooshdave]

Serious lack of pictures of said bike. 

[harrytief]

Mike
How did you decide on a 40mm offset and what rake angle if you remember did you run with those clamps? I will look at ride height and rear shocks tho' the shocks are newish and with new bushings.
Pics to come
Harry

[Rough Edge racing]

while true, it requires that the holes be bored at an angle. rare, but guzzi did do this in the 1400 eldo.

  I just ran into this on an 81 Honda 750C....it's an easy way to get more rake using the same frame...I believe some Harleys had the same...

  OP, yes look at the rear suspension...Althou gh hydraulic steering dampers are thought of to "stabilize " steering, they can cause the bike to fall into turns .I had this happen on a Tonti with a damper.......I suppose this can also cause the bike to react funny to side winds as the rider tries to correct and has to put more effort into the bars and perhaps "over corrects"   

[geodoc]

Off-set - ref: lower right (Sport 1100 Non-USD).

[Groover]

I think I had the right on first try. I worked this problem out again, and by raising the rear you likely decreased the trail even though the offset at the triples added some, just not enough to make the increase in trail as you may be thinking.

I want an A+ on this, and a gold star, and some sort of other sticker of some sort 

[oldbike54]

I think I had the right on first try. I worked this problem out again, and by raising the rear you likely decreased the trail even though the offset at the triples added some, just not enough to make the increase in trail as you may be thinking.

I want an A+ on this, and a gold star, and some sort of other sticker of some sort 

 How about an

                                                      Attaboy

 Actually I brought this up earlier . We still need to know what the effective length of the forks are compared to stock , and some other things as well . My suggestion would be to take the entire setup back as close as possible to the original factory settings , and make minor adjustments from there . Drastic changes create issues that are hard to predict , and W/O a baseline there is no way to measure all of the changes .

 Dusty