Why big tires?

Contents Why big tires? Nose fork Tires Weight Conclusion Relevant STCs

After a long period of time researching options for increasing the usability of my 1956 172 in the backcountry, I decided that there were a few crucial items for backcountry operations, so I've tried to address them in an order that makes sense for my budget and my mission. The first thing that I did was have my prop re-pitched for climb performance. This is about the best bang for your buck on an underpowered airplane. My 172 already had a Horton STOL kit installed, so I decided that additional wing modifications could be put off until a later date. One of my big concerns with the 172 on unimproved strips was the nose gear, so I set out to address that problem. No amount of power or gadgets would do anything for me on a rough strip.

My initial temptation was to purchase the STC from Stoots aviation to convert my 172 to a tailwheel. At a cost of $15,000, I came to the same realization that many others had warned me of: I would be better off buying a 170. My 172 has a special place in my heart, as it was my grandpa's and I had spent my entire life in that airplane, so selling it for a 170 was out of the question. I began to research the larger nose fork and tires for my existing configuration. Here are some things I learned along the way. Please remember, this information is purely based off of my experience and opinions.

Nose fork

The first thing that has to be done if you want to increase the tire size on your tricycle-equipped Cessna is to replace the nose fork with either an Airglas Fork or a heavy-duty Cessna fork that will accept a 6 inch wheel and larger tire. Many people have gone to the effort of finding a used heavy duty Cessna fork. These forks were commonly found on the C-206, C-207, C-310, and likely many others. This upgrade seems to be very common in Alaska but there is no STC for it. The only way to get legal approval for adding the heavy duty Cessna fork is by field approval. Several years ago, the field approval process became a bit more complicated, requiring a larger amount of documentation, engineering approvals, and expense. For these reasons, I elected to purchase the Airglas Nose fork from Hitchcock Aviation. Purchasing the fork with the STC allowed me a quick installation with no additional approval needed for the larger tire sizes. The Airglas STC allows you to run up to an 8.50 tire on all three wheels. That being said, as per the STC, you must run the same size tire on all three. In my research, many people do not care for this. Some like to run a size smaller on the nose to prevent wheel barrowing, or hitting nose first, as the Airglas Fork extends the nose gear by approximately 4 inches. I have not found this to be a problem so far.

The Airglas heavy duty nose fork on the left, stock fork on the right.

The installation of the fork itself is very straight forward. Some models, such as mine, require that you cut a length off the bottom of the strut tube. As scary as this may sound, it is no big deal. The idea is to install the new Airglas block in the same position as the Cessna block, where the torque links attach. This is outlined very well in the installation instructions and includes several pictures of proper and improper installations. Installing the block requires a heat gun and little attention to detail when tapping it onto the strut tube, in order to keep the two parts lined up and the block straight. Once you have the block on, it's as simple as bolting the fork to it with four bolts and reassembling the nose gear. I won't go into much more detail about the installation, as Airglas has a great website with installation documentation. One thing that I learned during this process was that the early C-172 came equipped with what Cessna calls a ‘Heavy Duty Landing Gear'. As part of the heavy gear, the torque links consist of 3/8" (AN6) attach bolts. This is a problem because the Airglas block that the links attach to are machined for and AN4 ( ¼") bolts. Obviously a ¼" bolt won't work in a 3/8" hole. After a couple of phone calls, I was advised to install a later model torque link bushing in my lower torque link. This bushing reduces the size to ¼". The bushing is readily available from McFarlane and was the perfect solution.

Tires

This was the fun part! When deciding on tire size, I quickly chose the largest approved by the STC, 8.50x6. The first reason was it looks bad ass. I know that sounds lame, but here's the deal, I do this for fun! If I'm going to end up spending $5,000 on my airplane, I want to walk up to it and think damn that looks cool!

On the practical side, there is a weight and speed penalty. I almost went with the 8.00x6 tires, but after I talked to Todd at Hitchcock, I went for the 8.50s. This was mainly due to the footprint. The 8.50 has a substantially larger footprint than the 8.00. This means better floatation and more shock absorption. These were both key items for reducing the stresses on the nose and firewall. Also, the Goodyear 8.50x6 Rib tire, which has a fairly stiff sidewall that allows you run lower pressures, is desired. Goodyear Rib seems to be the overwhelming tire of choice in this size.

The only problem regarding the tire installation is that there is very little clearance between the brake pad and the tire sidewall. Alaskan Bushwheel sells a kit to install a spacer between the brake rotor and the wheel. This provides enough clearance to avoid your pads rubbing the sidewalls. Others have installed washers as spacers and some have found replacement brake disks with slightly larger offset to gain the needed clearance.

8.50x6 tires all around

Weight

There is no doubt that a substantial weight penalty is incurred when installing the Airglas Fork and 8.50 Tires. During this mod, I removed wheel pants so the weight penalty wasn't that big. Here are the weights of what I removed and what replaced it:

Removed Weight (lbs) Nose Wheel Fairing -4.5 Old Nose Fork -3.0 5.00x5 Wheel and Tire -9.5 Main Wheel Fairing -13.0 6.00x6 Tires and tubes -14.0 Wheel Fairing Brackets -2.5 Total Removed -46.5 lbs

Added Weight (lbs) Airglas Fork +13.5 6.00x6 Nose Wheel +4.0 8.50x6 Nose Tire/Tube +15.8 8.50x6 Main Tire/Tube +15.8 8.50x6 Main Tire/Tube +15.8 Total Added +64.9 lbs Net Gain +18.4 lbs

Although the change isn't huge, remember that most of the weight you are adding in this mod, is on the nose. Just the fork is an increase of 9.5 lbs at -7 inches. For my particular airplane, I will be looking for ways to move the C.G. aft. It seem as though most early Cessna 172s push the forward side of the C.G. envelope in a stock configuration. It is still quite easy for me to stay within the envelope, but I like the flight characteristics of it much better with the C.G. further aft.

The aircraft takes on a decidely more capable appearance with the larger tires.

Conclusion

I'm very happy with this installation. I believe that this fork/tire combination will allow me to access strips that I would not be comfortable with in a stock configuration. I think it's important to understand that there is still a fair amount of risk in damaging the firewall and nose gear on unimproved strips but this modification will greatly decrease that risk and add a huge safety factor to off airport operations. I believe that good pilotage and good judgment will make this a very capable and economical backcountry aircraft.

Prop clearance before Prop clearance after

Relevant STCs