History of Prop Hubs...

The original design of the Force One Prop Hub dates back to 1985. I was a member of EAA Chapter 153 in Schaumburg, Illinois at the time. Several members (more than 4) in the EAA Chapter, had 2180cc Aero-Vee conversions in their Sonerai's and had broken their 82mm crankshafts as used with their shrink fit prop hubs.

NOTE: These four crankshafts were determined to be 1045 Forged Stroker crankshafts. The crankshafts had a high amount of sulphur in the material.

At the same time, I had a friend in Texas that broke his 82mm crankshaft with a short tapered hub.

NOTE: This crankshaft was determined to be an E4340 Forged Stroker Crank.

Plus, we had a customer break one of our 82mm crankshafts with a short tapered hub.

NOTE: This crankshaft was determined to be an E4340 Forged Stroker Crank.

Upon examination of the crankshafts, all broke exactly in the same area and manner.

NOTE: Not one stock VW 1045 forged steel 69mm VW German crankshaft had broken. We cut one apart to use as a sample in the following accumulation of data.

We were very fortunate at that time to have an EAA member in our chapter that was also a metallurgist. We obtained samples from the different cranks that had broken, analyzed them and formed some conclusions that guide us yet today:

A stock VW crankshaft is made from 1045 forged steel.
A SCAT crankshaft is made from E4340 steel.
Cast cranks are made from poured steel.
Other cranks advertised as being made from 1045 steel were, but they added sulphur during the manufacturing process (sulphur speeds up the machining process = lower cost).

So what does this mean?

Yield Break
1045 Forged VW 80k 120k
E4340 Forged 122k 144k
Cast 18k 18k
(can be as low as 11k)
1045 Forged/sulphur 33k 75k

So, what does this mean?

TThe 1045 forged VW crank has a high yield and a good spread between eyelid and break, or it is flexible!

The E4340 crank is very strong, but does not have a big spread between the yield and break, or it is very strong, but not as flexible.

The cast crank is not strong to start with and has no spread. It is kind of like a piece of glass.

The 1045 forged/suplhur crank, even though it is advertised as forged and it is, has a very low yield and break point even though it has a good spread between the two.

Every single crankshaft failure we examined started at the back of the woodruff key, left side, and went counterclockwise around the crankshaft, ending up in the crankshaft timing gear woodruff key hole where the crank would finally fail.

We learned many lessons. The actual starting point of the cracks, that lead to breaks, was at the top of the threads of the 20mm x 1.5 bolt that holds the hub in place. There is only about .060" between the top of the thread and the bottom of the woodruff key hole. Every crankshaft that finally broke - quit, at the back of the threads and went in a slightly different helix around the crank to the large woodruff key hole.

Clearly the bolt hole in the crankshaft, the threads in the crankshaft and the woodruff keys all play a major role in prop hub retention and non-crankshaft failure. So by trial and error from 1985 to 1987, we played with different designs of the Force One Prop Hub and Main Bearing. The late Al Campbell of San Antonio, Texas, did most of the initial testing of the hub in his turbo powered 2180cc KR-2.

We concluded that torsional vibration and torque, are the primary culprits. The small end of the aftermarket 82mm crankshaft was simply not large enough to absorb and dissipate the propeller loading and flywheel loading or twist, not to be confused with torque. A 2180cc engine as used in an aircraft has a 52 percent increase in engine torque over a 1835cc engine. When using a shrink fit hub or std taper hub on the small end of the crankshaft where the pulley usually rides in a car, this increase in torque, combined with torsional vibration, sometimes causes the crankshaft to fail.

Note again, we are not referring to the stock forged VW 69mm crankshaft. The small number 4 front main bearing may or may not play a role in the stroker cranks breaking. We were never able to reach a firm conclusion. I suspect yes, because it doesn't support the crankshaft from side loads as does a longer bearing.

So the two purposes of the Force One Hub is to transmit the prop loading to a larger surface area of the crankshaft so that prop loading can be absorbed, transmitted and dissipated over a larger area of the crankshaft and absorb and transmit to the prop, the torque of the engine.

You may note that in our catalog we do not offer a 2180 or 2276cc engine driven off the pulley end (Front Drive) of the crankshaft without the Force One Hub. It's not a sales ploy, it's a safety issue only. We have seen enough failures, over our 29 plus years in business to dictate the use of the Force One Hub set up on stroker engines.

You may also note that our 82mm crankshaft is unlike any other crankshaft made. The thread is 1/2" fine thread and goes about 3" into the crankshaft, of which the first 1" there is no thread. The woodruff keys are flat and only .090" deep and 180 degrees apart from each other. There is no snap ring groove in the crankshaft. This is our current design of which we are not aware of any failures. Our Top Bug Type 1 Crankshaft introduced in 1994, has never suffered a known failure!

The stock forged VW 1045 steel crankshaft gets by using the standard taper hub and the shrink fit hub simply because of the generous spread between yield and breaking - and it is strong. It works very well and has no real service issues, unless there is a prop strike.

We recommend that in the event of a prop strike, that hub and crank become boat anchors (and not re-used).


Remember the crack in the back of the woodruff key? One will usually start with a prop strike. Many times the prop hub will shear the woodruff key. If this happens, the prop hub can actually unscrew on the crankshaft. While I can't remember all the names, dates and faces; just in the last year alone, we have replaced 2 crankshafts in engines that had prop strikes that didn't replace the hub and crank after the incident.

Will every aftermarket crankshaft,
forged or cast
break if using a shrink fit or short tapered hub?

Of Course Not!

You can read Email-postings that confirm this. I do think, however, that looking at it from a historical perspective (a war vs. a battle) one can clearly see that most of the failures are with short tapered hub or shrink fit hubs on aftermarket forged, forged/sulphur, or cast crankshafts.

We have also read an individual posting stating that a popular German engine, when used with a popular German constant speed prop won't ever break. This is simply not true. Again, just this week (October 6, 2000) we received another inquiry from an owner of the above set up, wanting to know if our crankshaft set up would fit in his engine. The answer is no.

Propeller weight, prop material make up, rotating mass force, transmission, absorption, dissipation etc... all play real parts in keeping the hub attached to the crankshaft. Remember, even Continentals and Lycomings occasionally break crankshafts.

Some known do's and don'ts:
1. When driving off the pulley end of the crankshaft, use wood based props only. Specifically when using an aftermarket forged crankshaft.
2. When driving off the pulley end of the crankshaft, try to keep the wood props weight to 7 lbs. or less. (That rotating force thing.)
3. Re-torque your lightweight wood prop with every change of the season. Normal torque is about 11 to 14 ft. lbs.
4. Never ever use an aluminum hub prop and carbon fiber blades when driving off the pulley end of the crankshaft. This applies to both stock 69mm forged VW cranks, as well as, aftermarket forged crankshafts.
In the last year (2010) Sensenich has developed a lightweight ground adjustable prop that has an aluminum hub. I would personally wait until someone "else" gets 500 hour on it first...

In addition to selling and making parts for use on sport aircraft and their engines, Great Plains Aircraft is a clearing house of information. We talk to hundred's of people a week and get to hear their accomplishments and heartaches. It is easier for us to spot trends simply because we are exposed to a wider group of flyers and builders and more of them.

The Force One Prop Hub kit is not just a sales gimmick as some have suggested. It is a time proven safe way to attach a propeller to a forged crankshaft, stock VW or aftermarket and have it all stay together. For someone to suggest otherwise, without the knowledge and background information that has gone into the design of this particular product is simply irresponsible.

There is no doubt that the number 4 oil galley plug has played a role in some crankshafts with shrink fit hubs breaking. If the oil supply to the bearing is blocked and the bearing wears out, it is only X number of occilations until the crankshaft will eventually break.

Many of you may remember a very popular Arizona based VW conversion company that was a true leader in this field for a number of years. Do you recall what in part lead to their sale? It had to do with the replacement of 25 short tapered prop hubs on stroker forged crankshaft in a new Magnum series of engines. The crankshafts were breaking. History is a great teacher if you take time to study it.

Steve Bennett

Hit Back to go back to where you were - or

Great Plains Aircraft Supply Co., Inc.
7011 N 160 Avenue
Bennington, NE 68007

Orders: 402-493-6507
or Fax 402-493-3846