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I was doing a Tyrol Sport sub-frame deadset kit this weekend and ran into the age old question on the sub-frame bolts. Here’s what I've discovered, and I thought I'd share it.
Food for thought or fuel for the discussion
“When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region, since the fastener has suffered permanent plastic deformation.”
It appears VW engineers have chosen graded (grade 4) medium carbon cold drawn steel bolts to attach many parts together intentionally, rather than using high strength steel bolts.
So why not replace all those mild steel bolts with grade 5 or grade 8 bolts? Everything will be stronger right?
Not so fast!
Steel and aluminum have very different properties when it comes to how they react to heat and cold. Steel can be a very elastic metal, but the more alloy the less elastic. VW engineers probably had two things in mind when they chose the mild steel bolts they chose.
1) Using a material that can deal with some elongation without receiving a “permanent set or deformation” is a good way for mating aluminum and steel parts together. VW Sub-frames require long attachment bolts thay may be subject to tens of thousands of heating a cooling cycles over their service lives.
2) By preloading a fastener to a certain percentage of its tensile strength you can predict how much force it will take to cause it to shear. These mild steel bolts are an important part of how VW meets their safety standards, the engineered energy diffusion in a front end collision.
So if you were to change out the OEM bolts for high strength steel bolts, the front will transfer a lot more energy to the passenger compartment in the event of a front end collision.
Why don’t they just give me a torque setting rather than that 70Nm + ¼ turn.?
The length of a mild steel 100mm bolt can possibly vary as much a couple mm depending on the ambient temperature. Don’t believe me take one and throw it in the freezer for an hour, take it out and measure it. Throw it in boiling water for a couple minutes and measure again, what’s the difference? I have done fencing projects where the fence sags in the summer and is tight as a drum in winter.
If you used the base setting of 52Flbs on a cold winter day, on a hot summer day it could be something like 44Flb, but by using a base torque setting and adding a ¼ turn you’ve put the fastener into the elastic zone ,and the fastening strength becomes relatively constant year round.
Your base setting is just that it’s the minimum VW thinks is required to fasten the fastener to the frame. You could torque all of your sub-frame fasteners to their base torque settings on a really hot day and never have to worry about it again. If you in Las Vegas in August and torque to the base settings you should be good for January in Winnipeg.
What about all those recommendations to replace the sub frame bolts every time?
The only bolts (fasteners) VAG recommends replacing every time they’re removed are the motor mounting bolts(per the VAG shop specification guide). All others can be reusable with the following caveats. If you were torqueing a bolt and it felt like it slipped, it stretched, replace it. If the threads are damaged, or there is visual stretching replace it. If its length measurements are off e.g. 100mm bolts is 105mm (1-2% variance is normal depending on the temperature) replace it.
If you were to switch to hardened steel fasteners use the base settings only (no ¼ turn stuff), these bolts are not as elastic as the mild steel bolts.
Food for thought or fuel for the discussion
“When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region, since the fastener has suffered permanent plastic deformation.”
It appears VW engineers have chosen graded (grade 4) medium carbon cold drawn steel bolts to attach many parts together intentionally, rather than using high strength steel bolts.
So why not replace all those mild steel bolts with grade 5 or grade 8 bolts? Everything will be stronger right?
Not so fast!
Steel and aluminum have very different properties when it comes to how they react to heat and cold. Steel can be a very elastic metal, but the more alloy the less elastic. VW engineers probably had two things in mind when they chose the mild steel bolts they chose.
1) Using a material that can deal with some elongation without receiving a “permanent set or deformation” is a good way for mating aluminum and steel parts together. VW Sub-frames require long attachment bolts thay may be subject to tens of thousands of heating a cooling cycles over their service lives.
2) By preloading a fastener to a certain percentage of its tensile strength you can predict how much force it will take to cause it to shear. These mild steel bolts are an important part of how VW meets their safety standards, the engineered energy diffusion in a front end collision.
So if you were to change out the OEM bolts for high strength steel bolts, the front will transfer a lot more energy to the passenger compartment in the event of a front end collision.
Why don’t they just give me a torque setting rather than that 70Nm + ¼ turn.?
The length of a mild steel 100mm bolt can possibly vary as much a couple mm depending on the ambient temperature. Don’t believe me take one and throw it in the freezer for an hour, take it out and measure it. Throw it in boiling water for a couple minutes and measure again, what’s the difference? I have done fencing projects where the fence sags in the summer and is tight as a drum in winter.
If you used the base setting of 52Flbs on a cold winter day, on a hot summer day it could be something like 44Flb, but by using a base torque setting and adding a ¼ turn you’ve put the fastener into the elastic zone ,and the fastening strength becomes relatively constant year round.
Your base setting is just that it’s the minimum VW thinks is required to fasten the fastener to the frame. You could torque all of your sub-frame fasteners to their base torque settings on a really hot day and never have to worry about it again. If you in Las Vegas in August and torque to the base settings you should be good for January in Winnipeg.
What about all those recommendations to replace the sub frame bolts every time?
The only bolts (fasteners) VAG recommends replacing every time they’re removed are the motor mounting bolts(per the VAG shop specification guide). All others can be reusable with the following caveats. If you were torqueing a bolt and it felt like it slipped, it stretched, replace it. If the threads are damaged, or there is visual stretching replace it. If its length measurements are off e.g. 100mm bolts is 105mm (1-2% variance is normal depending on the temperature) replace it.
If you were to switch to hardened steel fasteners use the base settings only (no ¼ turn stuff), these bolts are not as elastic as the mild steel bolts.
