fork offset for tonti frame...ideas?

[thomas]

I fitted an USD fork from a 2005  Yamaha R1 to my 1981 SP 1000. The forks are the same length as the stock forks. This being an SP, it was staying a Sports Tourer and I stuck with the stock offset on custom clamps. So while it handles better due to the better forks but it is as predictable as it always was.  I'm using the recommended Ikon shocks for the SP (which are pretty close to stock) so my geometry hasn,t changed much at all.

As an aside, I stayed with the stock front wheel and the stock discs but plumbed the R1 callipers into the original linked brake system. By the seat of my pants the brakes are now better than with the original Brembo 2 piston callipers.

Regards
Thomas

[Groover]

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

Agreed, So in a nutshell, the Rake Angle did not change because that would only change if the headstock were cutoff the frame and reattached at a different "rake angle", so that variable is out of this equation (I used the link Rodekyll posted to understand this). The only variables here coming into play now are offset and trail as a result. Offset we know and is fixed and makes that initial trail number set (current setup), but the trail can change depending on the height of the front or rear (that's why as you said, to measure the stock front as reference) and the pivot point would be the front wheel axle. So in this case, the rear went up, so the trail went down. To change the trail now since the offset is set, play with heights of the front and rear shocks. So either raise the fork tubes (if you have the room) or shorten the rear, and tweak from there.

We got this now, lets start a new quiz.


Edit: I read back at some of the replies, and it's all there. Basically I've summarized a collection of correct replies. I'm also assuming the triples have equal offset in this case not messing with the rake angle, a variable mentioned by a few earlier in this thread.

That said, I would like to share the gold stickers with the rest of the class since this is definitely a group project 

[oldbike54]

Agreed, So in a nutshell, the Rake Angle did not change because that would only change if the headstock were cutoff the frame and reattached at a different "rake angle", so that variable is out of this equation (I used the link Rodekyll posted to understand this). The only variables here coming into play now are offset and trail as a result. Offset we know and is fixed and makes that initial trail number set (current setup), but the trail can change depending on the height of the front or rear (that's why as you said, to measure the stock front as reference) and the pivot point would be the front wheel axle. So in this case, the rear went up, so the trail went down. To change the trail now since the offset is set, play with heights of the front and rear shocks. So either raise the fork tubes (if you have the room) or shorten the rear, and tweak from there.


We got this now, lets start a new quiz.

 The rake angle has changed slightly by raising the rear , what we don't yet know is effective fork length , which if shorter or longer has also changed the rake , and altered weight distribution . A few months back we had the discussion about raising forks in the trees changing rake , and we put some carpenter math to that . Turns out the rake is altered very little even with a 26 MM change , but weight distribution does change by a noticeable amount .

 Thomas posted something interesting about his fork swap , keeping everything else near to factory settings did NOT upset the bike . Will put some math to how much the effective fork length might be changed by the change in triple trees , the clamping point being nearer to the level of the top of the steering neck , thus shortening the overall effective length , although I doubt it makes much difference . However , if the forks now in use are just a bit shorter than stock , and in combination with the longer shocks , the geometry could be a bit weird .

 Dusty

[groundhog105]

I fitted an USD fork from a 2005  Yamaha R1 to my 1981 SP 1000. The forks are the same length as the stock forks. This being an SP, it was staying a Sports Tourer and I stuck with the stock offset on custom clamps. So while it handles better due to the better forks but it is as predictable as it always was.  I'm using the recommended Ikon shocks for the SP (which are pretty close to stock) so my geometry hasn,t changed much at all.

As an aside, I stayed with the stock front wheel and the stock discs but plumbed the R1 callipers into the original linked brake system. By the seat of my pants the brakes are now better than with the original Brembo 2 piston callipers.

Thomas, who made your custom triples for you.

Regards
Thomas

[thomas]

Thomas, who made your custom triples for you.

My bike lives in Durban on the East coast of South Africa, (I live in South Africa and I work in Germany). The new triple clamps were made from 30mm billet alloy by a local machine shop. (Ophie Howards').

To measure up the required fork spacing I hung the front wheel with discs and axle attached from the roof rafters, attached the calipers to the new set of forks and then fitted the forks to each end of the axle. Once the whole plot had come to rest I simply measured what needed measuring.



 

I also took the opportunity to rewire the bike and mounted 2 headlight relays (dim & bright) as well as the indicator relay and a relay to protect the ignition switch on top of the upper clamp. The fuse box I mounted to the underside of the upper clamp. In the next photo you can see part of the electrical stuff as well as the double-jointed 'Gooseneck' clip-ons I made.

 











Cheers
Thomas

[Diploman]

Thomas, Congratulations on your successful adaptation of the Yamaha forks and calipers to your SP.  I like your bike very much.  I am about to start a modernization/refresh on my 83 SPNT.  This will include fitting Brembo 4-pot calipers and probably new rotors.  (I want to use HH pads, which wear down cast iron rotors quickly.)  I was interested in your comment that the linked system works even better with the Yamaha caliper:  I am very much a fan of Guzzi's linked brakes and plan to retain the system on my updated SP.  Question:  Are you still using the stock F-09
Brembo caliper on the rear?  There is some odd round disc on your bike (license/tax tag?) that obscures the caliper.

I think you were wise to keep the stock geometry intact with your fork adaptation.   As this thread illustrates, introducing variables - with forks or shocks - into the bike's known stock geometry is a step into the unknown, and an invitation to mysterious phenomena.   

[thomas]

Hi
Yes the rear caliper is the stock F09 and it is a licence disc :-)

One more thing: I made the upper fairing mountings such that they rest on the upper yoke and engage with the outer corners of the upper yoke. The lower fairing mounts to the lower yoke in a stock manner. I also drilled underside of the lower yoke to accept a mounting for a steering damoer but for the past couple of years I've been riding without a damper. 

Thomas, Congratulations on your successful adaptation of the Yamaha forks and calipers to your SP.  I like your bike very much.  I am about to start a modernization/refresh on my 83 SPNT.  This will include fitting Brembo 4-pot calipers and probably new rotors.  (I want to use HH pads, which wear down cast iron rotors quickly.)  I was interested in your comment that the linked system works even better with the Yamaha caliper:  I am very much a fan of Guzzi's linked brakes and plan to retain the system on my updated SP.  Question:  Are you still using the stock F-09
Brembo caliper on the rear?  There is some odd round disc on your bike (license/tax tag?) that obscures the caliper.

I think you were wise to keep the stock geometry intact with your fork adaptation.   As this thread illustrates, introducing variables - with forks or shocks - into the bike's known stock geometry is a step into the unknown, and an invitation to mysterious phenomena.

[fotoguzzi]

Please post pics of a Mini Griso   

[mtiberio]

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

Ran the stock rake angle, however, I routinely needed ground clearance, and would run the longest rear shocks that my u-joint would tolerate (typically Lemans IV shocks on a Lemans I swingarm). This would steepen rake. I'm not sure how I decided on 40, I wanted to reduce the wheelbase as much as possible, and Kosman (who made the clamps) probably had some 180 width, 40mm offset clamp blanks on the shelf, and that may have pushed me in that direction. Didn't seem worth the money to have a set made for much less reduction.

[rocker59]

Late to the party, here, but it sure sounds like some stacking of changes has reduced the rake (and probably the trail) because of the lower front and higher rear the bike now has.

- shorter forks
- wheel/tire changes
- longer shocks


You may have reduced the rake by several degrees with these changes, which is probably where your instability at speed is coming from.

Some measuring, drawing, and calculating are in order.

[Rough Edge racing]

  I've built a few bikes using suspensions from various makes.....The build has to be mocked several times to get an idea of the various angles affecting how it'll handle...
 This is the 81 Honda 750 turd I'm trying to make into less of a turd. I adapted the front end from a 96 750 Interceptor with a 17 inch front wheel and 18 inch back wheel. The bike is on the stand with angle level on the fork tube ,level on the table and frame just to see what it looks like... I have new tires coming and will set the bike on it's wheels to check ride height and fork geometry..Then adjust the front and ass end up or down to get it where I think it should be..... A 96 Interceptor is supposed to have 26 degrees of rake....I used the complete front end and same sized tires so in theory all I have to do is set it up for 26 degrees rake  ...But in reality the 81 Honda has a few inches more wheelbase and different weight distribution that will affect handing...So it may take a few tries to get it right...