# Stihl fs55r



## buy*david (Nov 7, 2011)

I have a Stihl fs55r that seems to have a vibration in the shaft when accelerating. That is not the best description but it feels kind of like the flex shaft is flexing then bouncing back. But I'm not sure. If I accelerate slowly it wont do it. It's kind of a big sudden shake for a moment. Does that sound like anything you know of ?


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## geogrubb (Jul 28, 2006)

buy*david said:


> I have a Stihl fs55r that seems to have a vibration in the shaft when accelerating. That is not the best description but it feels kind of like the flex shaft is flexing then bouncing back. But I'm not sure. If I accelerate slowly it wont do it. It's kind of a big sudden shake for a moment. Does that sound like anything you know of ?


I think your trimmer has a solid shaft and it is bowing when you throttle up, this would normally happen if you have a cutter blade attached or a restriction at the trimmer head. Have a good one. Geo


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## buy*david (Nov 7, 2011)

*fs55r*

Hi Geo, it has a flex drive shaft in it. I took it apart last night. The sides of the flex shaft are polished and shiny like it has been rubbing on the sides a lot. So I think it is flexing and rubbing on the sides. The head is a string bump head, typical Stihl type. It seems to be free, turns well by hand. The unit show a lot of use. Maybe the flex shaft has been strained, maybe it did have a cutter blade at one time. I still want to check and make sure that the plastic insert in the tube that the shaft rides in is okay and goes the full length of the tube. Also, I have a solid shaft from another unit I may be able to fit into it. Have you seen a flex shaft go bad and cause this?


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## paulr44 (Oct 14, 2008)

All FS55R units have flex cables (piano wire), even though they're straight-tube units.

Whenever I did a service/tune-up on a flex-shaft (but only curved shaft) trimmers, I'd take the flexshaft out, grease it, and flip it end-to-end so as to change the "flex" area in an effort to extend it's life. Flex shaft units, both straight or curved can produce a vibration (inherent) due to torque wind-up. Physics...can't get around 'em.

As Geo says, it's more noticable with a blade, that's because a blade is slower to spin up and the torque factor on the flexshaft therefore increases. Ever do a burn-out in a front-wheel drive car? Pulls to one side, may even hop a bit...torque wind-up on the longer axle is the cause.

Physics at it again!

*BUT*, a worn shaft liner (plastic support tube within the metal tube) allows the flexcable to thrash about even more from the wind-up, so it becomes more noticable or even downright annoying. You could replace parts, but between the cost of the unit and how much it must have been used to get to that state, I'd say it has probably come to the end of it's lifecycle.


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## buy*david (Nov 7, 2011)

*fs55r stihl*

Hi Paul. It is nice to hear from you, knowing what the problem is. I kind of guessed it was about what you said, but without your insight it was speculative.
The solid shaft I had didn't fit. I did have another flex shaft I used. It measured about .040 bigger die. then the old one. So it is a different one, bigger one, and it fit in to the plastic liner tighter. It works a lot better. I can still notice the torque storing in the shaft a little but doubt anyone else will. The engine winds up very quickly. Anyhow thanks for taking the guessing out of it. And now I know about the torque and flipping the flex shaft I'll add that to the abundance of knowledge stored in the vault. Thanks again.


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## paulr44 (Oct 14, 2008)

David, be aware that Stihl trimmer driveshafts come in 3 flavors...solid steel, flex, and steel-tipped aluminum (RX units). All 3 use *different liners*, as the shafts are different diameters. You were talking about switching to a solid steel shaft, you'd have to change the liner to have proper support.

You should note though, that Stihl driveshaft tips are all the same for most common models, so it would be feasible to be able to swap drive tubes from one unit to another with some modifications possibly needed, as long as the O.D. of the tube is the same. I'm pretty certain most flex shaft units have a larger O.D. tube.


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