This is mostly a 'thought experiment', and a companion piece to the clutch worm experiment:
http://www.xs650.com/forum/showthread.php?t=31554
The XS clutch has the notorious feature of slackening-up when the engine is hot, and the conventional wisdom blames this on differential thermal expansion rates between the aluminum engine case and clutch pushrod(s). This exercise explores the potential relief if an alternate material is used for a 1-piece pushrod. I have never handled, much less seen, a 2-piece setup, which I understand utilized aluminum in the longer of the 2-piece pushrod setup to overcome this expansion problem.
The computations here are rough ballpark, to determine the feasability of using alternate clutch pushrod materials. These alternate material candidates were selected based on availability, strength and thermal expansion coefficients.
Pic #1 - I have a few 5/16" rods of these materials, which are 304 Stainless, Navy brass, 6061 aluminum and 7075 structural aluminum.

Considering whether it's feasable to conduct this experiment. Others, especially ME's, are encouraged to jump-in here and comment on this project. I have this in a spreadsheet, but decided to expand it here to improve understanding.
1-piece stock clutch pushrod:
Length = 9.570" (243 mm), we'll use 10" for our rough sample calculations
Diameter = 8mm (.315"), can use 5/16" (.3125)
Material = unknown Carbon steel
Depending on type of clutch worm and stock/aftermarket clutch levers, total leverage multiplier at clutch lever is about 30-50.
Pushrod stress:
At a tested mid-range compression of 370 lbs (stock XS1B) , a 5/16" rod (1/13 sq-in cross-sectional area) is experiencing about 5000 psi of compressive stress.
Normal compressive load distribution can be used when length/diameter ratios are below 6. Special column loading considerations and limits must be employed when the length/diameter ratios are 6 to 60, and especially at 60-120 (severe buckling phenomena). The clutch pushrod length/diameter ratio is about 30. Without going into all the engineering math here, all the selected materials here (within this stress value) qualify for normal compressive stress/strain calculations, without worrying about buckling.
Decided to drop the 6061 aluminum since it would be a slightly weaker and redundant version of the 7075 aluminum.
Coefficients of thermal expansion (within normal engine temperature ranges). Units are MicroInch per °F (.000001" / °F) per inch of material
Engine case Aluminum = 13.1
Carbon Steel = 7.8
304 Stainless steel = 9.6
Navy Brass = 11.8
7075 Aluminum = 13.1
Compute expansion of 10" of material based on 200°F temperature rise (70°F - 270°F)
Engine case Aluminum = 0.026"
Carbon Steel = 0.016"
304 Stainless steel = 0.019"
Navy Brass = 0.024"
7075 Aluminum = 0.026"
However, these candidate clutch pushrods undergo compression during clutch disengagement, and deform (shorten) under stress. The amount of length change (strain) can be predicted using the modulus of elasticity (Young's Modulus) for each material, when stressed below the material yield strength. The higher the modulus, the stiffer the material. The length change (strain) is: stress (psi) / Modulus, then times the overall length.
Rough values of Young's Modulus (in millions of psi):
Carbon Steel = 30
304 Stainless steel = 30
Navy Brass = 15
7075 Aluminum = 10
Compute length change (strain) for 5000 psi stress on 10" rod
Carbon Steel = -0.002"
304 Stainless steel = -0.002"
Navy Brass = -0.003"
7075 Aluminum = -0.005"
Now, compute overall length change (expansion -strain).
Carbon Steel = 0.016" - 0.002" = 0.014"
304 Stainless steel = 0.019" - 0.002" = 0.017"
Navy Brass = 0.024" - 0.003" = 0.021"
7075 Aluminum = 0.026" - 0.005" = 0.021"
Interestingly, the Navy brass and 7075 aluminum are equal.
Now, compute overall length differential to aluminum engine case (0.026" - rod expansion).
The smaller, the better.
Include 30-50 leverage multiplier to show slackening value at clutch lever.
Carbon Steel = 0.026" - 0.014" = 0.012" (0.36" - 0.60" at clutch lever)
304 Stainless steel = 0.026" - 0.017" = 0.009" (0.27" - 0.45" at clutch lever)
Navy Brass = 0.026" - 0.021" = 0.005" (0.15" - 0.25" at clutch lever)
7075 Aluminum = 0.026" - 0.021" = 0.005" (0.15" - 0.25" at clutch lever)
It's winter, too cold to experiment with this now, just throwin' this out there. Anybody else try any of this? I see no issues in at least trying the 304 stainless, but would appreciate any insight.
http://www.xs650.com/forum/showthread.php?t=31554
The XS clutch has the notorious feature of slackening-up when the engine is hot, and the conventional wisdom blames this on differential thermal expansion rates between the aluminum engine case and clutch pushrod(s). This exercise explores the potential relief if an alternate material is used for a 1-piece pushrod. I have never handled, much less seen, a 2-piece setup, which I understand utilized aluminum in the longer of the 2-piece pushrod setup to overcome this expansion problem.
The computations here are rough ballpark, to determine the feasability of using alternate clutch pushrod materials. These alternate material candidates were selected based on availability, strength and thermal expansion coefficients.
Pic #1 - I have a few 5/16" rods of these materials, which are 304 Stainless, Navy brass, 6061 aluminum and 7075 structural aluminum.

Considering whether it's feasable to conduct this experiment. Others, especially ME's, are encouraged to jump-in here and comment on this project. I have this in a spreadsheet, but decided to expand it here to improve understanding.
1-piece stock clutch pushrod:
Length = 9.570" (243 mm), we'll use 10" for our rough sample calculations
Diameter = 8mm (.315"), can use 5/16" (.3125)
Material = unknown Carbon steel
Depending on type of clutch worm and stock/aftermarket clutch levers, total leverage multiplier at clutch lever is about 30-50.
Pushrod stress:
At a tested mid-range compression of 370 lbs (stock XS1B) , a 5/16" rod (1/13 sq-in cross-sectional area) is experiencing about 5000 psi of compressive stress.
Normal compressive load distribution can be used when length/diameter ratios are below 6. Special column loading considerations and limits must be employed when the length/diameter ratios are 6 to 60, and especially at 60-120 (severe buckling phenomena). The clutch pushrod length/diameter ratio is about 30. Without going into all the engineering math here, all the selected materials here (within this stress value) qualify for normal compressive stress/strain calculations, without worrying about buckling.
Decided to drop the 6061 aluminum since it would be a slightly weaker and redundant version of the 7075 aluminum.
Coefficients of thermal expansion (within normal engine temperature ranges). Units are MicroInch per °F (.000001" / °F) per inch of material
Engine case Aluminum = 13.1
Carbon Steel = 7.8
304 Stainless steel = 9.6
Navy Brass = 11.8
7075 Aluminum = 13.1
Compute expansion of 10" of material based on 200°F temperature rise (70°F - 270°F)
Engine case Aluminum = 0.026"
Carbon Steel = 0.016"
304 Stainless steel = 0.019"
Navy Brass = 0.024"
7075 Aluminum = 0.026"
However, these candidate clutch pushrods undergo compression during clutch disengagement, and deform (shorten) under stress. The amount of length change (strain) can be predicted using the modulus of elasticity (Young's Modulus) for each material, when stressed below the material yield strength. The higher the modulus, the stiffer the material. The length change (strain) is: stress (psi) / Modulus, then times the overall length.
Rough values of Young's Modulus (in millions of psi):
Carbon Steel = 30
304 Stainless steel = 30
Navy Brass = 15
7075 Aluminum = 10
Compute length change (strain) for 5000 psi stress on 10" rod
Carbon Steel = -0.002"
304 Stainless steel = -0.002"
Navy Brass = -0.003"
7075 Aluminum = -0.005"
Now, compute overall length change (expansion -strain).
Carbon Steel = 0.016" - 0.002" = 0.014"
304 Stainless steel = 0.019" - 0.002" = 0.017"
Navy Brass = 0.024" - 0.003" = 0.021"
7075 Aluminum = 0.026" - 0.005" = 0.021"
Interestingly, the Navy brass and 7075 aluminum are equal.
Now, compute overall length differential to aluminum engine case (0.026" - rod expansion).
The smaller, the better.
Include 30-50 leverage multiplier to show slackening value at clutch lever.
Carbon Steel = 0.026" - 0.014" = 0.012" (0.36" - 0.60" at clutch lever)
304 Stainless steel = 0.026" - 0.017" = 0.009" (0.27" - 0.45" at clutch lever)
Navy Brass = 0.026" - 0.021" = 0.005" (0.15" - 0.25" at clutch lever)
7075 Aluminum = 0.026" - 0.021" = 0.005" (0.15" - 0.25" at clutch lever)
It's winter, too cold to experiment with this now, just throwin' this out there. Anybody else try any of this? I see no issues in at least trying the 304 stainless, but would appreciate any insight.
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