919 Track Day Chassis Setup Notes - Wrist Twisters
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post #1 of 1 Old 09-26-2010, 12:08 PM Thread Starter
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919 Track Day Chassis Setup Notes

Today's changes as must made.
Supposed to be at a Track Afternoon on Wednesday.
Below is intended as Grist For The Mill as well as offer some insights.
Keep in mind my Rear is a Penske, so I can make shock length changes.

9-25-2010 Chassis Set-Up Changes

Rear Extension Length was 485 mm. (from measuring SS fitted pin on body work down to swingarm axle slot block)

Free Sag was 12 mm. (always fully back off the rebound and low speed compression for this check)
Add 1/4 Turn of Preload.
Not enough Free Sag.
Back off 1/8, so net is now actually 1/8 more.
Free Sag 10.5 mm. (based on average from 3 mm range of measurements) (“Up & Release” measurements are more random than the “Down & Release” measurements.)

Increase Rear Extension Length.
Went with an increase of 5 mm (non linked swingarm means major axle centerline changes with each half turn of the adjusting eye at the bottom of the shock) (a half turn less was not enough change and another half turn was too much change so settled on 5 mm )

Raise Front Ride Height to compensate for increased rear ride height.
Need to lift front 5 mm vertical.
Use the Cosine rule for 25 degrees steering head angle.
Therefore need 5 / 0.9 = 5.6 mm of linear change along fork length to raise front 5 mm vertically.
There was only 4 mm of ride height adjustment left in the adjusters and used it.
Lowered Fork Tubes 1.6 mm to get the rest.

If this overall setup is kept, then off season:
• Make up new fork tube spacers that are 10 mm longer.
• Raise tubes back up to where they were before. (1.6 mm higher) (top of blue flange to upper surface of top triple clamp = 4.5 mm)
• Back out adjusters 4 mm

Now to Swingarm Angle check.
• Zero the angle gauge to the floor by resting it on a 4 ft long edge laying on floor beside the bike (I used a 4 ft aluminum level that is machined on both sides as the “edge”.
• Bike resting on rear race stand, to hold it square and steady.
• Front wheel blocked up so bike is level in space as it would be on the road
• “Free” Swingarm Angle is now approximately 10.7 degrees.
• “”Rider” Swingarm Angle is now approximately 9.9 degrees.

Overall :
• Front and rear of chassis both raised by 5 mm
• Rear "Rider" Swingarm angle increased by a bit less than 2 full degrees, so Anti-Squat Force will be greater in relation to Squat Force, than it was before. (nice would be a force diagram program to figure out actual values)
• Centre of Gravity has been raised, so “Roll Transitions” should be easier, but, for any given combination of radius and corner speed, some more lean angle will be required.
• 5 mm more ground clearance has been gained.


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