I have owned my M-7-235C for over six years and am based at a paved runway airport. Since buying the plane it’s always been on 8.50x6 mains and an Alaskan BushWheel ABI-3200b Baby BushWheel tailwheel. I’ve always had intermittent tailwheel shimmy’s on the pavement landings but sort of got it down to only shimmying when landing a little fast (crosswinds) or loaded with weight in the baggage.

In January, I had to replace the worn out tire and thought (Because they are so expensive) I would spend some time and money to further refine the long standing shimmy issue that I’ve had. So, as I mentioned, I have the new Baby BushWheel tire, bought the rebuild kit for the tailwheel assembly and bought the new tail spring kit-all directly from Alaskan BushWheel. We then rebuilt the tailwheel assembly using Aeroshell 22 grease (Which, Alaskan BushWheel recommended).

After putting all of this back together, to my surprise the shimmy is now every landing and is very SEVERE. So we revisited everything from replacing rudder spring chains verifying those are properly tensioned, disassembled inspected and reassembled the tailwheel again verifying the compression spring (Lockout) and swivel nut are tensioned properly. Flew it again and shimmy was still severe.

So, then I put the old tail spring back on (Because the new spring is slightly longer than the old spring) and it still shimmied but, was definitely not as bad. So, I then began looking closely at the castor angle as that’s all that I have left to examine and it seems that in fact, it does have a slightly negative castor angle.

So the next logical question is having the tail spring re-bent to provide a positive castor angle? Wondering if anyone hear has any experience with what I’ve had and specifically, has anyone had the tail spring re-bent and if so what did you have it bent to that worked?

I know several owners with Baby Bushwheels who say they tend to shimmy landing on pavement. They attribute it to several factors. The tire is larger, with thinner sidewalls, and has less inflation pressure than a standard tire, so it can wobble side-to-side more easily. Because the tire and fork are larger, they create a longer lever arm from the tail spring to the ground contact point of the tire. That makes it easier to flex the tail spring more and create a flatter angle, which is more conducive to shimmying. Greasing the tail wheel would probably increase shimmy since there would be less resistance to it.

You've probably already addressed it, but make sure that the tail spring and tail wheel are completely aligned with the longitudinal axis of the airframe and that the tire contacts the ground at its midpoint. If the airframe tubing was twisted slightly, it could put the tail wheel at an angle to the ground when viewed from behind the aircraft.

Bending the tail spring is a possible solution. Increasing the air pressure in the tire to maximum may help. If the shimmy is still there, changing to a 3224A tail wheel might be the only other alternative. That's what I've had on my MX-7-180 since 2008 and it never shimmies under any load. The downside is the smaller footprint of the tire that may cause it to sink more in some soft surfaces. The 6-ply tire that I have on the tail wheel is a lot thicker and stiffer than the bush wheel and it resists wobbling quite well.

Tightening the “kingpin” can reduce shimmy completely at the cost of making the tailwheel stick to the side a bit in the air acting as a type of rudder trim.

Bending the tailwheel leaf springs can change the angle but also makes them weaker.

If you’re running 850’s, you probably don’t need the bbw. I’d consider switching to the 400x4 glider tire from ABI that fits on the BBW fork/axle/wheel but costs much much less when it wears out. It also takes much longer to wear out. It’s about the same diameter and footprint as BBW but not as soft and has a couple ribs. Seems to shimmy less than BBW all other things equal.

-asa

I went through some of the same issues when I bought my M-5. I would get a shimmy on every 3 point, or tail first landing. If you put some forward pressure on the yoke to unload the tail spring it would go away. After some time looking at it, I came to the same conclusion that is was right at the point it would try to go into negative caster when there was a load on it. I put the tail spring in the press and put a little more arch in it. It has successfully worked for the last couple of years. A new spring is in my near future though. When loaded I sometimes get a shimmy, and again if I take Pressure off of the tail it will go away. If you re-arch the spring you do take the chance of making it weaker, and it will in time settle back to where it was. About a year after I put extended gear on it which picked the front of the plane up and helped also. For me it started as an experiment to see if it would solve the problem.... As asa said you could tighten you the king pin a little and play with that, but it comes with its own possible side effects....

Just my 2¢, but even if you are running 31’s or even 35’s you may not need the BBW. I do not run one and honestly have little desire to have one. The BBW decreases your angle of attack when on the ground which is certainly not going to hurt much but won’t help either. The stock tail wheel may sink in sand but even in light fluffy sand I have never found this to be a problem. If you are landing on gravel you should be fine until the rocks get to cantaloupe size give or take. But even then if you keep the tail up you should be just fine. Don’t get me wrong, if all you do is land in big gravel or rocks or hit the sand bars at gross weight then the BBW makes some sense. But for me, the gravel, sand and mud I fly in just doesn’t justify the downsides.

Thank you for the feedback!

I just got an email from Airframes Alaska on a new tail wheel shimmy damper that fits ABI/Scott tail wheels:
https://www.airframesalaska.com/Tailwhe ... s/2968.htm

Looks like they designed it to address the shimmy problems experienced by Baby Bush Wheels or any Scott tail wheels. At $950 it's not cheap but maybe worth it in tire and tail wheel wear.

andy wrote: ↑

Wed Feb 02, 2022 2:48 am

I just got an email from Airframes Alaska on a new tail wheel shimmy damper that fits ABI/Scott tail wheels:
https://www.airframesalaska.com/Tailwhe ... s/2968.htm

Looks like they designed it to address the shimmy problems experienced by Baby Bush Wheels or any Scott tail wheels. At $950 it's not cheap but maybe worth it in tire and tail wheel wear.

Worth every penny, I will have them in stock next week!!