XS650: Caster/Rake and Trail   1 comment

Motorcycle manouverability and stability is a product of many factors: …eg…

  • Front wheel alignment
  • Wheelbase
  • Weight distribution
  • Centre of gravity
  • Position, type and performance of the suspension
  • Size and performance of tyres
  • Riding position

2 of the most critical are Caster or Rake and Trail.

 

Caster/Rake

 

Caster/Rake:

The angle created by the vertical through the front wheel centre and the line through the centre of the steering head tube.

 

changing caster/rake

 

trail

 

Trail:

Take that vertical line through the centre of the front wheel. And follow the Caster/Rake through to the ground. This distance is the trail – how far the wheel trails the rake geometry.

Large Caster/Rake and Trail give better high speed stability-tracking-longer wheel base and poorer low speed handling. Great for straights.

Small Caster/Rake and Trail has the opposite effect. Reducing wheel base. Allowing you to carve through the corners.

In practcal terms it means you can optimise your riding by understanding where you are going to ride. 2 or 3 triple trees drilled to different Casters/Rakes allow you a certain flexibility.

Understand your frame and steering geometry. Know your suspension dynamics. And how to change them for a desired effect..

However. When we talk wheel base, it makes sense to me that not all wheel base is the same. I see 3 types:

  • before the steering head..trail
  • between the steering head and the centre of gravity..rake and stretch
  • after the centre of gravity..traction..probably be able to divide into pre and post suspension points

Easy variables:

  • Fork Length
  • Fork preload
  • Fork Spring Rate
  • Caster/Rake
  • Swingarm Length
  • Rear Shock Length
  • Rear Shock Preload
  • Rear Sock Angle
  • Rear Shock Length
  • Wheel Size and Weight Distribution
  • Tyre Characteristics

And don’t forget the Fork Brace.

It’s simple physics at low cost. And a lot of feeling.

Large return on investment.

 

XS650: Frame Dimensions

 

Early:256, 306
Mid:447, except 78-9 french
Late:specials
Rake 27° 27° 27°
Length: Steeringhead bottom to swing arm pivot 685 688.7 688.7
Swingarm pivot to upper shock mount, rear 302 311.5 300
Height Steeringhead  bottom to Swingarm pivot 434.6 409.5 409.5
Steeringhead top to frame bottom 816 811.1
Swing arm pivot to upper shock mount, rear 311 311.5 291.5
Swingarm length: To centre of adjuster
Rear-shock length: On bench, spring fitted
On bike,at rest:
On bike, with rider:
Sag: rear
Rear shock: Spring rating
Angle
Front Forks: Rake
Trail
Fork length: At rest
With rider
Sag: front
Wheel and tyre height: Front
Rear
Weight: Frame
Bike, dry
Bike, wet
Rider and gear

 

 

 

Somewhere in the back of my mind comes a voice telling me that european models after ’75 had a reduced rake, 26.5°. Trail being reduced from 115mm to 108mm. Is this correct? I don’t know.

What I do know is that frame modifications meant drastic weight gains. Ride improved.

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One response to “XS650: Caster/Rake and Trail

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  1. Hi Vernon,
    the late model frame weighs 22.4 Kg, without the swing arm, I have a 256 and 447 frames as well which I will weigh soon. As for the swing arm length the 256 is 435 mm, 447 and the later swing arms are the same length at 480 mm. You can swap swing arms between frames as they are the same width across the front pivot but the early swing arm is narrower across the rear 200 mm compared to the 447 and later specials of 215 mm. Across the front pivot all swing arms are 197 mm. I hope this helps everyone.
    Daryl

    Daryl Hutcheon of Professional Motorcycle Tuning, Melbourne, Australia

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