Fergo
Just got it firing!
- Joined
- Dec 22, 2008
- Messages
- 9
I am planning to fit a set of Showa upside-down forks onto my LM II.
In the process I want to steepen the rake angle and shorten the trail to quicken things a bit. I will do this via custom made triple clamps, and in the design of these I need some help.
Nowhere have I been able to find a written source for the standard steering geometry, which has to be the basic start point. Jens from guzzitech.dk tells me that period test reports quote 28* with 110mm of trail, which sounds reasonable to me. Given that the same frame and geometry is shared with the SP and T series, these relatively slow figures sound right. It's my intention to measure a standard LM II just to confirm this.
I have read everything I can find on motorcycle steering geometry; mainly Tony Foale's work, but also Phil Irving's, and come to the conclusion that there is no magic number for rake and trail; just about anything will work, it depends on how much you are prepared to compromise between stability and comfort.
The current Triumph range, for example, has rakes ranging between 23.5* and 25*, with 80 - 100mm of trail, while the Guzzi Sport 1200 quotes 25* and 110mm trail, and the V11 Le Mans 23.5* and 95mm trail. Ducati 916 is 25* and 103mm, while an ST2 is 24* and 102mm. There is no consistency or pattern, but the Lemon is longer and probably has a lower CoG than any of these.
My gut feel has been for around 26* and 95mm of trail, but that's only gut feel, with no science. 26* is about as steep as I can go without resorting to "raked" clamps, and means dropping the headstem by around 2". This, obviously, is done by sliding the forks within the clamps. The Showas that I have are already 25mm shorter than the original 35mm forks, so this is ok.
Where I am uncertain is trail, and how much to build into my clamps. Whilst I can adjust rake by fork position, trail is a function of clamp offset, and not adjustable for practical purposes. Thus I need to get it right on the first hit.
Surely I am not the first to have considered a conversion like this, so is there someone out there who has done something like this and who is happy to share that experience?
Thanks,
JFerg
In the process I want to steepen the rake angle and shorten the trail to quicken things a bit. I will do this via custom made triple clamps, and in the design of these I need some help.
Nowhere have I been able to find a written source for the standard steering geometry, which has to be the basic start point. Jens from guzzitech.dk tells me that period test reports quote 28* with 110mm of trail, which sounds reasonable to me. Given that the same frame and geometry is shared with the SP and T series, these relatively slow figures sound right. It's my intention to measure a standard LM II just to confirm this.
I have read everything I can find on motorcycle steering geometry; mainly Tony Foale's work, but also Phil Irving's, and come to the conclusion that there is no magic number for rake and trail; just about anything will work, it depends on how much you are prepared to compromise between stability and comfort.
The current Triumph range, for example, has rakes ranging between 23.5* and 25*, with 80 - 100mm of trail, while the Guzzi Sport 1200 quotes 25* and 110mm trail, and the V11 Le Mans 23.5* and 95mm trail. Ducati 916 is 25* and 103mm, while an ST2 is 24* and 102mm. There is no consistency or pattern, but the Lemon is longer and probably has a lower CoG than any of these.
My gut feel has been for around 26* and 95mm of trail, but that's only gut feel, with no science. 26* is about as steep as I can go without resorting to "raked" clamps, and means dropping the headstem by around 2". This, obviously, is done by sliding the forks within the clamps. The Showas that I have are already 25mm shorter than the original 35mm forks, so this is ok.
Where I am uncertain is trail, and how much to build into my clamps. Whilst I can adjust rake by fork position, trail is a function of clamp offset, and not adjustable for practical purposes. Thus I need to get it right on the first hit.
Surely I am not the first to have considered a conversion like this, so is there someone out there who has done something like this and who is happy to share that experience?
Thanks,
JFerg
