Tire Changes & Geometry Affects on Rocket Roadster

1olbull

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For those interested in why tire changes have a positive handling affect on your Roadster . . .
The OEM set of the rear axle lower than the front axle sux HUGE!
Getting the front axle lower than the rear is paramount to good handling.

I finally finished up calculations and scale CAD drawing depicting the geometry of the the Stock Roadster with 240/50R16 rear & 150/80R17 front tires. And also for it with the larger Exedra Max 240/55R16 rear & 140/75R17 front tires installed.

Changes made by raising the rear axle and lowering the front axle are calculated.
The resultant rake & trail produce amazing improvements in handling performance.

Black are the stock dims - Red are the adjusted dims.

OEM Trig_2.jpg
 
Last edited:

barbagris

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I finally finished up calculations and scale CAD drawing depicting the geometry of the the Stock Roadster with 240/50R16 rear & 150/80R17 front tires. And also for it with the larger Exedra Max 240/55R16 rear & 140/75R17 front tires installed.
Excellent. Now then work out the differences in CoG change due to disparity in tyre width, at differing angles of lean. Also the weight transfer increase due to the lack of over centre locking the OEM high front axle affords. Thusly one can identify teh spring/damping changes necessary. :whitstling:

I tried, but life is too short.
 

rng3

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Cool, thanks for taking the time to plot that out. Care to attach it as a DXF file?

Just trying to sort this all out in my head.
So the rake and angle are changed, more agile, less effort to turn but probably bit less stable?
CoG moved to more weight on front?
Shorter front tire so front ground clearance decreases .69+ inch? Maybe more because of CoG change?

What would results be by staying with stock tire sizes and raising rear of bike with taller shocks say 1/2"?
Would that also change the rake and angle in the same direction? Although this would probably also reduce wheelbase.
Or shorter front springs.
Is there a difference in changing the frame height / angle versus changing axle height?

Thanks,
Rick
 

1olbull

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Responses are posted inside your comments:

QUOTE="rng3, post: 581179, member: 7701"]Cool, thanks for taking the time to plot that out. Care to attach it as a DXF file?
Send me an email 1olbull@gmail.com

Just trying to sort this all out in my head.
So the rake and angle are changed, more agile, less effort to turn but probably bit less stable?
CoG moved to more weight on front?
Thing triangle formed from rear axle up to the top center of the steer head and then down to the front axle.
As the rear axle lifts and front axle lowers the triangle rotates downward decreasing the rake angle.

Shorter front tire so front ground clearance decreases .69+ inch? Maybe more because of CoG change?
The front axle lowers less than a half inch and the rear raises less than a half inch. The resultant change in the center, peg/board area, is pretty much a wash.

What would results be by staying with stock tire sizes and raising rear of bike with taller shocks say 1/2"?
Would that also change the rake and angle in the same direction? Although this would probably also reduce wheelbase.
Axle heights are unchanged.

Or shorter front springs.
Is there a difference in changing the frame height / angle versus changing axle height?
Dropping forks would also reduce rake & trail as well as ground clearance. I tried a 130 front tire and would drag the front of the peg rails - not good!

Thanks,
Rick[/QUOTE
 

1olbull

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Excellent. Now then work out the differences in CoG change due to disparity in tyre width, at differing angles of lean. Also the weight transfer increase due to the lack of over centre locking the OEM high front axle affords. Thusly one can identify teh spring/damping changes necessary. :whitstling:

I tried, but life is too short.
I doubt any changes would be significant
Excellent. Now then work out the differences in CoG change due to disparity in tyre width, at differing angles of lean. Also the weight transfer increase due to the lack of over centre locking the OEM high front axle affords. Thusly one can identify teh spring/damping changes necessary. :whitstling:

I tried, but life is too short.

I reckon, lean angle is lean angle, regardless the aspect ratio of the tires.
The COG change rate would be affected by the tire profiles.
Profile & size diffs between tires would likely be absorbed by rear and front suspensions.
 

rng3

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So... Changing axle height is different from changing the frame angle in relationship to the road?
PM sent.
Thanks,
Rick
 

1olbull

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So... Changing axle height is different from changing the frame angle in relationship to the road?
PM sent.
Thanks,
Rick
Indeed. The axle height is controlled by the wheel radius, period.
The seat height and steer head angle can be changed by different wheel sizes, rear shock length & setting and front fork length & setting.

Probably a good opportunity to "qualify" my previous cad drawing. Although close enough for our purposes, it is not completely accurate.
Using the rear axle as the pivot for lowering of the front axle, the numbers change slightly because is moves down on an arc, not in a straight line perpendicular to the ground.

OEM Trig_2.jpg
 

warp9.9

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Indeed. The axle height is controlled by the wheel radius, period.
The seat height and steer head angle can be changed by different wheel sizes, rear shock length & setting and front fork length & setting.

Probably a good opportunity to "qualify" my previous cad drawing. Although close enough for our purposes, it is not completely accurate.
Using the rear axle as the pivot for lowering of the front axle, the numbers change slightly because is moves down on an arc, not in a straight line perpendicular to the ground.

View attachment 69829
Hey @1olbull , where did you get the fork leg measurement and is that the fork length to center line of axle without sag of a body being on the bike? Just curious as since I'm switching to the 18 inch 240/40 rear which is smaller then the 240/55 yet larger the the 240/50 . Anyway will first be pairing the rear with a 140 matching 140/75 front and trying to think ahead on fork length adjustment.
 

1olbull

Riding Motor Since 1950
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2012 R3R "Kong" - 2011 R3R "YurMama"
Hey @1olbull , where did you get the fork leg measurement and is that the fork length to center line of axle without sag of a body being on the bike? Just curious as since I'm switching to the 18 inch 240/40 rear which is smaller then the 240/55 yet larger the the 240/50 . Anyway will first be pairing the rear with a 140 matching 140/75 front and trying to think ahead on fork length adjustment.
Measured mine from top center to axle center as it sat with no load.
10 thousandths one way or the other difference from resting on the kick stand or at vertical will not produce an appreciable affect. :rolleyes:

Give me your wheel & tire sizes, wheelbase and fork length and I'll knock one our for you.
 
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