First evaluation on my new 35mm Ceriani GP fork

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Hi, I had thought that with more offset on the fork yokes affect inversely the trail , so reduce stability ..........less offset on the yokes increase trail ?
 
marinatlas said:
Hi, I had thought that with more offset on the fork yokes affect inversely the trail , so reduce stability ..........less offset on the yokes increase trail ?

Yes, less offset increases trail. You are moving the wheel to the rear with less offset, but the line through the steering head doesn't change, so intersects the ground at the same point. Trail is the distance from that point to the tire contact patch, so moving the wheel back increases trail.

Ken
 
The two extremes are stability or self-steering in corners. With a rake of 27 degrees does increasing the trail theoretically increase stability or decrease it ? What happens when the rake is 24.5 degrees as with a featherbed frame, and you increase the trail ? I can only tell you what happens with a Seeley MK3 when you have too much offset on the fork yokes. The rest simply tangles my brains. Perhaps there is no continuity of trend when you alter rake and trail ? Sometimes you might end up with lead instead of trail ?\
All I do is consider what the steering geometry is on certain benchmark bikes. e.g. a 70s TZ has neutral steering with 18 inch wheels and 26 degree rake, a manx slightly self-steers with 19 inch wheels and 24.5 degree rake but becomes too stable with 18 inch wheels. Our Australian replica manxes often have 18 inch wheels and 26 degree rake and handle like Suzukis. When I fixed the Seeley, I knew about the TZs and figured one degree difference in rake might be tolerable - ARSE BEATS CLASS every time !
If you can make sense out of the figures, you are better than I am. I don't believe there is a formula which fits the evidence. I've read a bit of Tony Foale's stuff, as well as a paper on motorcycle handling from the Society of Automotive Engineers. The conclusion from the latter was that they did not know what they were talking about. Tony Foale is quite a bit smarter than the SAE engineers..
The problem is that with this steering geometry stuff is there are plenty of ways to get it wrong and if you get it wrong the result can be dangerous. When you consider the wheelbase of a Seeley is about 54 inch and mine self-steers when you gas it, what fraction of a degree do you think the rake changes as the rear end squats about 3 inches ?
 
Yves,

Nice looking forks sir!

Do you know if the internals are specially made, or are they 'off the shelf' items from another bike?

You must surely be running out of things to do to that Seeley??
 
Matchless said:
Yves,

As you have done plenty of miles on your beautiful (& very shiny) Seeley, how well does the frame stand up to the vibes. Have you experienced any cracking? Also, what balance factor have you used & are the vibes tolerable on a long ride. Sorry if this has been covered in a previous post.

Martyn.

Hy Martin,
With the long rod and special pistons from Jim (JSMotorsport) the vibes disapears, the crank I use is also coming from JSMotorsport, a piece of mecanical art, it is a fully billet one piece crank with a BF at 65, the balancing was done by Jim.
A few years ago i did the trip to from Brussel Belgium to Dijon in France, almost 700 kms only on B roads, and no problems with vibes, but I sleep well that night.
Of course if you wish to avoid vibes, the BF is the most important, but you must have equal weights on the rods and pistons, the combustion chambers must have the same volume and so on, I am using A Fullauto head and the volumes are spot on.
Do you know that some original crank can have a difference up to 5 degrees between the two piston at TDC....
Your carbs must be sinchronised also, I d'ont have this problem any more sins I use the Keihin FCR 35 from CNW, they are linked.
About Cracking:
It happens once, at a drag meeting, but it was becouse of the power and the brutality of the starts, I broke one of the gusset that hold the engine plates on the transmission side, a good welding
and forget it.
Yves
 
Fast Eddie said:
Yves,

Nice looking forks sir!

Do you know if the internals are specially made, or are they 'off the shelf' items from another bike?

You must surely be running out of things to do to that Seeley??

I am not sure, but I think that the internals are specially made, I can not reconise something from another bike.
My bike is a eternal "work in progress" the next step will be the yam TD rear drum brake and after I wish to have a fuel tank with the same shape as the MV Agusta 750S
Don't believe that I am a ritch man, I am retired but I work a lot on other bikes to pay my stuff, I do also a lot of polishing for other customers
 
acotrel said:
The two extremes are stability or self-steering in corners. With a rake of 27 degrees does increasing the trail theoretically increase stability or decrease it ? What happens when the rake is 24.5 degrees as with a featherbed frame, and you increase the trail ? I can only tell you what happens with a Seeley MK3 when you have too much offset on the fork yokes. The rest simply tangles my brains. Perhaps there is no continuity of trend when you alter rake and trail ? Sometimes you might end up with lead instead of trail ?\
All I do is consider what the steering geometry is on certain benchmark bikes. e.g. a 70s TZ has neutral steering with 18 inch wheels and 26 degree rake, a manx slightly self-steers with 19 inch wheels and 24.5 degree rake but becomes too stable with 18 inch wheels. Our Australian replica manxes often have 18 inch wheels and 26 degree rake and handle like Suzukis. When I fixed the Seeley, I knew about the TZs and figured one degree difference in rake might be tolerable - ARSE BEATS CLASS every time !
If you can make sense out of the figures, you are better than I am. I don't believe there is a formula which fits the evidence. I've read a bit of Tony Foale's stuff, as well as a paper on motorcycle handling from the Society of Automotive Engineers. The conclusion from the latter was that they did not know what they were talking about. Tony Foale is quite a bit smarter than the SAE engineers..
The problem is that with this steering geometry stuff is there are plenty of ways to get it wrong and if you get it wrong the result can be dangerous. When you consider the wheelbase of a Seeley is about 54 inch and mine self-steers when you gas it, what fraction of a degree do you think the rake changes as the rear end squats about 3 inches ?

Hi Acrotel,
My yokes are 57 mm offset, the yokes made by the Hungarian are 45 mm offset, and your Yams are 35mm
Minnovation make replicas from the Yams, but they dont give any information about thr offset in their catalogue, so I mail them to know and keep you posted
Maybe I should try the 45 mm from Hungary, something between 57 and 45 mm
but again a lot of money to spend
Yves
 
Yves, I was not trying to be pedantic when I commented on your fork yokes. When I got caught out, my mate had ridden the bike the year previously and warned me that it had mishandled. ( I didn't believe him). I think my incident happened in exactly the same place on the circuit as his. I was coming off a fast left hand sweeping bend and braking as I turned into a slight right hand bend. The bike stood up and turned the wrong way, throwing me off-balance. I know I was gone so decided to crash the bike onto the grass. As I turned it on again to get there, the bike came back under control. I'm not joking, it is a serious problem and can come from nowhere and bite you.
I remember a similar thing happening to a friend many years ago. He built a Tribsa using a Bonneville motor and an A10 (Gold Flash) BSA frame. His bike stood up and scared the shit out of him. He was using the standard A10 fork yokes which have a lot of offset, the rake was 26 degrees. The fork yokes out of a sixties 650cc unit Triumph solved that problem. The weight of the motor seems to have a bit to do with this. MY Seeley was originally fitted with a 750SFC Laverda motor which was very heavy. The handling was not good, but nothing dangerous happened.
If you buy the shorter offset fork yokes, you will have to get used to the bike tightening it's line in corners. However I love the way my Seeley handles. I can brake about a third of the way into corners then get straight back onto the gas - the bike does the rest with minimal help. It is a major advantage with an under-powered bike.
 
When I fitted the TZ fork yokes to my Seeley, they had been lying in my backyard on a spare frame. I'd got a quote for about $500 to make a set, but had simply no idea what offset to use. I knew the TZ has 26 degree rake, so I figured I was in with a chance. All I had to do was machine the bottom yoke to fit the spindle, all the rest was perfect. I suggest you should send a message to Kenny Cummins and ask his opinion about what the best offset would be for your purposes. I know some Seeleys feel very stiff as they come out of corners - tend to run wide. You should not have a problem unless you road-race the bike or get silly on a public road.
I raced for years and never twigged to this steering geometry bullshit.
 
acotrel said:
I suggest you should send a message to Kenny Cummins and ask his opinion about what the best offset would be for your purposes. I know some Seeleys feel very stiff as they come out of corners - tend to run wide. You should not have a problem unless you road-race the bike or get silly on a public road.
I raced for years and never twigged to this steering geometry bullshit.

Hi Acotrel,

I appreciate your posts and intensity of having correct offset on your yokes, and your desire to instill this important factor into our psyches. However, I detect a running intensity in your writings that you're at the verge of death every time you go out on your bike!

I have ridden and raced a good many different bikes - Manxes, Seeleys, Commandos, SV650, KTM, Guzzi, Aermachis, etc - each with a considerably different rake and trail than the others, and I have never felt like I've been on the edge of death with the steering. Yes, each of the bikes mentioned have their idiosyncrasies, but after a lap or two you lean a bit about how the bike handles and (hopefully) ride into it. Yes, we all fall down sometimes. But in my time I've not felt like I was racing a bike that was going to kill me. I have been out on poorly setup bikes, but I bring them right back in and put them away!

As for Yves - He needs no consultation. His bike is superb, and makes me smile every time I see it. He is using the same yoke offset as all our NYC Norton bikes run at every different kind of track you can imagine. It is a beautiful bike and very well put together. And I expect Mr. Yves knows how to ride a solo quite well. :wink:

There have been various posts over time containing a lot of mis-information about offset and handling. Ken Canaga has a way of setting everyone straight in a subtle way (see above), and I will impart a bit. At the risk of over simplifying - the MORE offset you have (decreased trail) the quicker the steering will be on the bike, and turn-in will be easier. The less offset (increased trail) the slower the steering but increased stability in a straight line. Bike makers alter their offset to compliment their rake. For example, a Super Motard bike I raced once had the steepest rake I'd ever ridden, but also very little offset in the yokes. This by design, not accident.

Lots of fun here: http://gotagteam.com/KTM_Days/Story_Pag ... _RC8R.html


Respectfully,


-Kenny (my last name is) Cummings
 
Kenny, A while back I read the specs of your Seeley and I'm pretty sure it said the yoke offset you use is 35mm. I only know what happened when I raced with too much offset and what fixed the problem. I suggest that generalising about the effects of increasing or reducing trail might be misleading. Perhaps it is the COMBINATION of rake AND trail which determines the handling ? I don't know about you, however when I race I am usually aware of what the guys around me are riding.
That stuff about facing death every time I ride - old habits die hard. I did not have to even blink and my old short stroke Triton would deck me. I raced it for 12 years. Even though the Seeley is a world apart, I still have the same mentality.
Incidentally, I watched the video of you racing at Phillip Island a couple of years ago - did you enjoy Turn #1 ? ( Nice video ! - and I am ashamed about not being there.)
Best Regards,
Al
 
Holmeslice said:
acotrel said:
I suggest you should send a message to Kenny Cummins and ask his opinion about what the best offset would be for your purposes. I know some Seeleys feel very stiff as they come out of corners - tend to run wide. You should not have a problem unless you road-race the bike or get silly on a public road.
I raced for years and never twigged to this steering geometry bullshit.

Hi Acotrel,

I appreciate your posts and intensity of having correct offset on your yokes, and your desire to instill this important factor into our psyches. However, I detect a running intensity in your writings that you're at the verge of death every time you go out on your bike!

I have ridden and raced a good many different bikes - Manxes, Seeleys, Commandos, SV650, KTM, Guzzi, Aermachis, etc - each with a considerably different rake and trail than the others, and I have never felt like I've been on the edge of death with the steering. Yes, each of the bikes mentioned have their idiosyncrasies, but after a lap or two you lean a bit about how the bike handles and (hopefully) ride into it. Yes, we all fall down sometimes. But in my time I've not felt like I was racing a bike that was going to kill me. I have been out on poorly setup bikes, but I bring them right back in and put them away!

As for Yves - He needs no consultation. His bike is superb, and makes me smile every time I see it. He is using the same yoke offset as all our NYC Norton bikes run at every different kind of track you can imagine. It is a beautiful bike and very well put together. And I expect Mr. Yves knows how to ride a solo quite well. :wink:

There have been various posts over time containing a lot of mis-information about offset and handling. Ken Canaga has a way of setting everyone straight in a subtle way (see above), and I will impart a bit. At the risk of over simplifying - the MORE offset you have (decreased trail) the quicker the steering will be on the bike, and turn-in will be easier. The less offset (increased trail) the slower the steering but increased stability in a straight line. Bike makers alter their offset to compliment their rake. For example, a Super Motard bike I raced once had the steepest rake I'd ever ridden, but also very little offset in the yokes. This by design, not accident.

Lots of fun here: http://gotagteam.com/KTM_Days/Story_Pag ... _RC8R.html


Respectfully,


-Kenny (my last name is) Cummings

Dear M. Cummings

I am so proud about the words you use for my bike,
coming from a legend as you, it's better as wining a World Championship
I follow your story from the begining and I love it.
You don't know, but I am also the interviewer for the "Guest of honnour" at the retro bike show of Wieze here in Belgium
Few years a go, the Guest of honnour" was Jim Redman and I spend a lot of chat with him, what a live story, what a man!
Guest of honnour for 2017 will be Peter Williams, again a fantastic WE for me
Maybe one day you will be able to come to Belgium and be our Guest of Honnour one day.

Thanks,
Yves
 
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