This text is copyrighted and duplication, re-distribution or publication is prohibited without consent of the author.
Article: What Exactly is a
"Bowden" Tube? (clutch stuff)
Last updated: 10/26/01
Symptom: You have excessive shudder/clutch
chatter when starting off. Some one told you to adjust your "bowden tube".
You thanked them for the advice, but you have no idea what a bowden tube
A testament to the design of the VW Beetle is the amount of bone jarring abuse it can take and still drive away as if nothing happened. None the least of this is the excessive "clutch chatter" that can be felt in a poorly maintained/improperly repaired or otherwise neglected VW. I very often get inquiries on this symptom and/or reply to posts about it on the newsgroup and reply with the standard, "Make sure you have 1 inch of sag in you bowden tube" and in almost minutes receive the reply, "Thanks. But what is a 'bowden' tube and how do I adjust it's 'sag'?".
So finally, I have decided to write
an article about it.
Twisting and Flexing
The whole principle of the bowden tube revolves around the engine/transmission movement relative to the body (keep reading, I'll tell you exactly what the bowden tube is after we get through some conceptual basics). Remember that the engine is bolted to the tranny and the tranny (and clutch) are mounted the chassis via rubber mounts. This is to reduce the engine vibration felt in the car and allow the shock of engine torque to partially be absorbed somewhere. The engine torque is oriented around a longitudinal axis down the center of the car. Imagine a straight line which passes through the crankshaft, end to end. So from a point right between the tailpipes to the dead center of the front apron. Your engine, when you rev it up, wants to "twist" around that axis. To see this, stand behind the car while someone revs it hard and look at those tailpipes. You will see them "rock" a bit. That is engine torque. It is the tranny's job to convert that torque to a lateral torque, send it down the axles and to the tires, and it does a pretty good job at this. This engine movement (relative to the body) is actually more pronounced when the engine is under greater load, such as when starting off.
Know where your clutch cable goes? It goes down a hard steel tube inside the tunnel. A tube which ends just under and behind the back seat, to the side of the tranny that the driver sits on (to make this a truly "global" article). From there it is carried by a flexible tube, about a foot long, to a bracket attached to the transmission side. This is your bowden tube. At that point the clutch cable emerges free and makes the last 8" or so run to the clutch lever on the tranny. And it is presumed that you know that as you push the clutch pedal down, the cable is pulled through the tube and hence pulls on the clutch lever down back.
Now the pedal is fixed to the chassis
as is the hard steel tube inside the tunnel. But the bracket that holds
the end of the bowden tube is attached to the tranny which, as described
above, moves relative to the car. And remember that the path of this cable,
end to end, is off center of that "torque axis" of the drive train. This
means that the actual linear distance between the end of that hard
steel tube under the back seat and that bracket actually changes slightly
as the engine and tranny "twist".
Ok class, let's do little a demonstration. (Tommy there in the back still doesn't "get it") Hold your thumb and index fingers on each hand apart like you are holding an AA battery at its ends. Let your hands face each other and touch the tips of your two index fingers together. They are torque axis. Let your thumbs stay apart just a bit (if you are normal, they should be just a tad shorter than you index fingers anyway). They are the clutch cable path. Now twist your hands relative to each other just a bit, pivoting on your index fingertips. See how the distance between the tips of your two thumbs changes? Aaahhh, there. (I'm a sucker for a good visual)
Yes, in your car, this distance differential is very small, but think about how a little change in your clutch position at the pedal with your foot makes a big difference in how your clutch is engaging. And given the design of the clutch pedal, a tenth of an inch in cable movement corresponds to over an inch of pedal movement.
So here's where it all will make sense. Imagine that the bowden tube is not there, but the cable just shoots out of the back of the hard steel tube and goes straight, bare, to the clutch lever. Now you start the engine, put it in gear, and slowly start to let out the clutch. You hold the pedal and don't move it when it starts to engage and the car starts moving. The clutch engages, but immediately the engine torque's (rotates) on it's mounts. The distance between the end of the hard steel tube and the clutch lever increases slightly. That slight increases pulls your clutch lever forward some, the clutch disengages. The load on the engine drops now, the engine comes back to normal resting position on its mounts. The linear distance that the cable has to make is now back to normal, so the clutch starts to engage again. Engine torque's, distance increases, clutch disengages, distance goes back to normal, ....... It's a vicious cycle. This produces a sometimes violent shudder or what is often called "clutch chatter".
Enter the bowden tube. How can you
allow the engine/tranny to move relative to the body (and clutch pedal)
yet keep the linear distance that the clutch cable has to travel from the
pedal to the tranny constant?? With the bowden tube. If a flexible tube
is used to carry the clutch cable between the end of the hard steel tube
and the bracket on the tranny, and it has enough "extra length" to allow
for the engine movement, the linear distance end to end will stay the same.
Think about it. The distance through a flexible tube is the same no matter
if it is bent or straight. Ingenious.
But the trick to making this work is
to insure that the bowden tube has an adequate "bend" to it when the engine
is at rest. This is so at maximum engine "twist" on its mounts the tube
is still connected at its end points (just barely straight). This resting
bend is what is referred to as "sag" in many manuals. The bowden tube has
a male and a female end. The female end has a sleeve that slips over the
exposed end of the steel clutch cable tube. The male end has a narrowed
shaft that slips through a hole in the end of the bracket attached to the
tranny. The "sag" is achieved by slipping washers over the male "snout"
of the tube prior to inserting it into the bracket attached to the tranny.
More washers makes the linear distance available for the tube less, thus
increasing the "sag" in the tube. The sag is measured by placing a straight
edge across the ends of the tube while it is in place and measuring the
deflection of the tube at it's midpoint. Actually using a straight edge
is usually not necessary, and "eyeball" measurement is usually adequate.
Removing the Bowden tube
You do not need to remove the clutch cable or tranny bracket to remove and/or adjust the bowden tube. You can simply remove the nut fully from the end of the clutch cable and free it from the clutch lever. Once you have done this, grab the bowden tube at it's mid point and pull down with one hand while helping the male end of it slip out of the tranny bracket with your other hand by keeping it straight with the alignment of the hole. It can be bit of a struggle, you have to pull the tube down pretty hard and it is definitely a two hand operation. Once the male end has cleared the bracket, work the remaining length of the clutch cable through the bracket then slip the whole bowden tube off of the cable.
The bowden tube is show on the left lightened
on the far side of the transmission
Re-installing the tube
Sometimes the male end of the bowden tube can get a little corroded up, making it difficult to slip out of the bracket. If it was a real struggle to remove it, clean up the male end (snout) of the tube prior to re-installing it. A wire brush wheel on a drill works well for this. And take some time to clean the inside of the tube up a bit too. Spray some carb cleaner through it or run some gas through it to flush out any old grease and gunk. Then take some time to get some grease in there. If you have a grease gun a few pumps of the old handle in the female end will do. At the very least, make sure that you smear a lot of grease on the exposed clutch cable before re-inserting it. Remember that the tube bends here and the tension on the cable is pretty great; grease is a necessity.
Replace or add any washers that you need to, there is really nothing special about them, a hardware store variety will do just fine. A thin film of grease on the male and female ends will help re-installation go a bit smoother too. You may have to install it and remove it a few times if you are unsure of how many washers that you will need to get the proper sag.
Thread the clutch cable all the way through
(greased) the tube and slip the female end over the end of the steel clutch
cable tube under the back seat. Then thread the threaded end of the clutch
cable through the tranny bracket as much as you can. Now, with the same
two handed contortions that you used to remove it pull a big bend in the
tube and slip the male snout end into the tranny bracket making sure that
the loose cable is feed through as you go. The trick is to have a strong
hand on that male end to "point" it into the tranny bracket hole whilst
pulling down hard on the midsection of the tube to get a big bend in it.
I can be done.
Other Causes of Clutch Chatter
While in my experience a non-sagging bowden tube will cause the most severe clutch chatter, there are other things that can create a similar symptom. But before I list those, there is one other thing that can cause you to fail at adjusting the sag in the tube no matter how many washers you use. The steel line for the clutch cable in the tunnel is very prone to breaking itself loose from its welds, especially in an older, well used Beetle (see "Pedal Stuff and Clutch Tube Re-attachment"). If it has broken free at its rear-most weld (usually it will only do this after the forward two welds have broken loose first) the exposed end of this tube under the backseat that the female end of the bowden tube slips over will "push in" when sufficient pressure is applied. This will prohibit you from ever being able to get sufficient sag in the bowden tube. Verify that yours is solid by pressing very hard on the end of the steel line from underneath the car (with the bowden tube removed) if you suspect this problem. While you can re-weld the tube from the top of the tunnel under the back seat, it is very likely that if it is broken off there, it will be broken at the mid and forward attachment point as well. The "Pedal Stuff and Clutch Tube Re-attachment" article for more details.
Other internal clutch things that can
cause clutch chatter are:
As I said in another article, make the best use of your efforts by replacing everything that is even partially suspected of being worn or broken when undertaking a major repair. This is especially true for the clutch. If you are yanking the engine (or replacing it) on a tired old Beetle, replace the clutch pressure plate, disk, throw out bearing, throw out bearing retaining springs and throw out fork. It is well worth the piece of mind. At the time of this writing, you can get all of these things for around $130. At the very minimum I would always replace the disk and bearing, but only if the pressure place and fork looked perfect.
I hope that I have dispelled some of the
mysteries behind the "Bowden Tube". It is really not all that complicated.
And it helps to under stand the dynamics that require it to be there and
properly adjusted. I have always said that a well adjusted Beetle is a
happy Beetle and is a joy to drive.
Copyright© 2001; John S. Henry
[back to FAQ Index]