Rake, Trail, Offset and Steering Geometry
There are engineers around the world who spend their lives developing frames and suspension to perform exactly the way they want, with virtually limitless computing power for analysis and prototyping facilities for testing. No cafe racer builder has those resources and I imagine few have the qualifications and experience to understand the complicated dynamics that go into suspension design.
There are a few really good resources for those who want to understand more about the way your bikes steering and suspension geometry influences the way it rides. Motorcycle Chassis Tuning by John Robinson is a really well written book that helped me understand the dynamics of motorcycles and how the changes I am making will effect the bikes ride quality. However, he does use the term 'Centrifugal force' instead of 'Centripetal force'; my dynamics lecturer would be apoplectic.
Full disclosure... I knew what I wanted to do before I did the analysis on it, so it was more a case of checking that I wasn't creating an unwieldy monster than tuning it to be as fast as a modern race bike. Lets be frank, this is an XV; it'll never compare to a sports bike of the early 80s let alone a modern sports bike. If I wanted a focused race replica, I'd spend my money on a 5 year old litre bike, not a 35 year old cruiser.
On to the analysis of the bike.
As a basic summary, there are a number of things I'm doing to change the dynamics of the bike:
2014 R6 Suspension Geometry Calculations
The R6 suspension geometry calculations are pretty easy, most of the information is readily available online, except for fork length - so I measured it. This will be a little inaccurate due to the pre-load effecting the total free length, but for my purposes its close enough.
1981 XV750 Suspension Geometry Calculations
There is quite a lot of information on forums and the web for the XV, though Yamaha did some really strange things with the triple tree and front axle that make the rake and trail calculations a bit of a hassle. The axle offset in particular makes the trail calculation difficult, plus there is additional angle built into the triple tree that alters the rake.
Interestingly, the data for suspension travel vs ride height indicates that the bike can bottom out when the suspension is fully compressed. I suspect that this is the center stand (which is near the rear wheel and impossible to bottom out on) and the clearance with the center stand removed is quite a bit more than that.
Argentum XV Suspension Geometry Calculations
Putting it all together and the following is the result. Lowering the steering head by putting shorter forks in decreases the rake angle; anecdotally by 1 degree for every 50 mm of lowering. Given the original forks are 62.8mm longer than the R6 forks there is a 1.3 degree change to rake.
My biggest concern with this change was the resulting ground clearance vs suspension travel. Based on this analysis, the forks will be at the limit of acceptability, but with the center stand removed it should have enough clearance to be manageable.
The handling characteristics won't be too extreme based on the trail number indicated; it is nowhere near the aggressive geometry on a sports bike, but will be significantly sharpened over the stock XV. The wheelbase will be reduced by 90mm making it a significantly more maneuverable. These changes will be balanced by the rear tyre selection which will resist turn in.
One thing to note is that if I was using a larger diameter rear tyre, the trail numbers would be more aggressive due to the change in angle of the frame (and hence the steering head). By using a 140/70-15 I've maintained the bikes original stance at the expense of ride height. Tuning of the rake and trail will be managed by setting the pre-load on the front and rear suspension and adjusting the fork position in the triple-clamp (from 5mm recess to 10mm protrusion).
There are a few really good resources for those who want to understand more about the way your bikes steering and suspension geometry influences the way it rides. Motorcycle Chassis Tuning by John Robinson is a really well written book that helped me understand the dynamics of motorcycles and how the changes I am making will effect the bikes ride quality. However, he does use the term 'Centrifugal force' instead of 'Centripetal force'; my dynamics lecturer would be apoplectic.
Full disclosure... I knew what I wanted to do before I did the analysis on it, so it was more a case of checking that I wasn't creating an unwieldy monster than tuning it to be as fast as a modern race bike. Lets be frank, this is an XV; it'll never compare to a sports bike of the early 80s let alone a modern sports bike. If I wanted a focused race replica, I'd spend my money on a 5 year old litre bike, not a 35 year old cruiser.
On to the analysis of the bike.
As a basic summary, there are a number of things I'm doing to change the dynamics of the bike:
- Decreasing weight by ~40kg
- Reduce rear wheel rolling diameter by 63mm
- Reduce front wheel rolling diameter by 63mm
- Increase relative rear spring and damper rates (stock spring rate with lighter bike)
- Increase ride height at rear (greater adjustment of preload to raise ride height)
- Center of gravity moved forward
- Reduce length of forks by ~60mm, effectively reducing rake
- Reduce trail
- Reduce unloaded ride height at front by 25mm
- Reduce sag (stiffer suspension front and rear)
- Shorten wheelbase
If you imagine the bikes wheels being static with the engine and front of the frame rotating to accommodate these changes you would see the engine lowered towards the road and rotated forward, with the steering head becoming more vertical and moving downwards. If you've got that in your head, you will see the first problem with the set-up is that the ground clearance at the front has the potential to get very small.
Fork Selection
If there isn't enough ground clearance at the front no amount of calculation of rake and trail is going to make the bike more rideable in the real world. One truism for USD forks is that they are always much shorter than traditional forks. This makes ground clearance a difficult balance and one that is hard to overcome if the wrong forks have been selected. It is possible to machine (or buy) spacers to extend the forks, but that is really an afterthought that should be designed out if at all possible. Of the USD forks that are most suitable for this bike, the 08- R6 forks are the longest of the R6/R1 forks. This provides the most flexibility for tuning ride height, rake and trail without costly workarounds. There are probably other options that would work...
2014 R6 Suspension Geometry Calculations
The R6 suspension geometry calculations are pretty easy, most of the information is readily available online, except for fork length - so I measured it. This will be a little inaccurate due to the pre-load effecting the total free length, but for my purposes its close enough.
There is quite a lot of information on forums and the web for the XV, though Yamaha did some really strange things with the triple tree and front axle that make the rake and trail calculations a bit of a hassle. The axle offset in particular makes the trail calculation difficult, plus there is additional angle built into the triple tree that alters the rake.
Interestingly, the data for suspension travel vs ride height indicates that the bike can bottom out when the suspension is fully compressed. I suspect that this is the center stand (which is near the rear wheel and impossible to bottom out on) and the clearance with the center stand removed is quite a bit more than that.
Argentum XV Suspension Geometry Calculations
Putting it all together and the following is the result. Lowering the steering head by putting shorter forks in decreases the rake angle; anecdotally by 1 degree for every 50 mm of lowering. Given the original forks are 62.8mm longer than the R6 forks there is a 1.3 degree change to rake.
My biggest concern with this change was the resulting ground clearance vs suspension travel. Based on this analysis, the forks will be at the limit of acceptability, but with the center stand removed it should have enough clearance to be manageable.
The handling characteristics won't be too extreme based on the trail number indicated; it is nowhere near the aggressive geometry on a sports bike, but will be significantly sharpened over the stock XV. The wheelbase will be reduced by 90mm making it a significantly more maneuverable. These changes will be balanced by the rear tyre selection which will resist turn in.
One thing to note is that if I was using a larger diameter rear tyre, the trail numbers would be more aggressive due to the change in angle of the frame (and hence the steering head). By using a 140/70-15 I've maintained the bikes original stance at the expense of ride height. Tuning of the rake and trail will be managed by setting the pre-load on the front and rear suspension and adjusting the fork position in the triple-clamp (from 5mm recess to 10mm protrusion).