Topic

For a while I have been wondering how to torque the rear bolts for the front, lower suspension arms.

The bottom suspension arm bolts need to be torqued to 85Nm, but they also need to be torqued under load. This is called preloading and must be done with the weight of the car on the suspension. If you torque up the bushes while the car is up on axle stands, when the weight is lowered onto the suspension, the bush will be under excessive load and can fail fairly quickly. Also, if you torque up the bushes without preloading the suspension, it can cause the handling to be quite bad.

This poses a problem that when the sub frame is fitted, you can’t get a socket in due to the closeness of the bulkhead.

The only thing I could think of was a tool called a crows foot.

This fits to the end of a ratchet or a torque wrench and extends it. It essentially turns your torque wrench into a torque spanner, but also, in this case, used with a lower profile torque wrench it allows access to the bolt in the limited space.

As all the engineers will tell you though, torque is the product of the force applied and the length of the lever used to apply it. As you are extending the overall length of the “lever” (torque wrench) by adding the crows foot on the end, you will be increasing the torque for the same applied load.

Hope I haven’t lost you. Basically it means that if you set the torque on a torque wrench and add extra length, the torque applied to the bolt will be greater than you want.

There are some nice complicated formulae out there if you like your maths, but all you need to remember is that for every percentage increase in length you need to decrease the torque setting proportionally.

We will take my torque wrench as an example.

If we measure from the middle of the handle (where you apply the force) to the centre of where the socket goes (where the force is applied to the bolt) we get 270mm.

I know on the picture it looks like they don’t line up with the figure, but it is a close up shot and the angles make it look different.

Now, if we do the same measurement with the crows foot fitted, we can see it is 300mm, or 30mm longer.

All I do now is divide 270mm by 300mm which gives us the reduction value of 0.9

Now multiply your listed torque value of 85 Nm by 0.9

This gives us 76.5 Nm, which is the new figure that I need to set my torque wrench to.

Now I can tighten the bolts.

With the weight of the car on the wheels and lock applied you can just about get access to the bolt. Obviously the wheel arch liners aren’t in place at this point.

Now tighten the bolts, keeping the crows foot in line with the torque wrench.

Interestingly (if you’re interested in this sort of thing) if the crows foot is at 90 degrees to the torque wrench and the distance from centre of handle to centre of bolt then you don’t need to apply any correction factor. i.e. If it was like this:

The front bolts are easier as you can access them through the oval cut outs on the front of the sub frame arms

So, I now have correctly torqued lower front arms.

Epic work!

Fank you.

Also, you might notice sunshine in some of the pics.

Had to do something when it wasn't raining.

Clever stuff indeed