Everything You Wanted To Know About Titanium
If steel and titanium are both equal in strength, then aluminum is half the strength. When you get into the 7000 series of aluminum they get to about two thirds the strength of titanium. The problem is that the ductility of aluminum is less than steel. This is what comes into play when designing parts, especially the axles and some bolts, like cylinder head bolts, the stretchiness becomes an issue with titanium. And the technical term for that is modulus of elasticity. That's why a when looking at a titanium spring, it has less coils in it. With the modulus number of titanium being half of steel it's going to flex twice as much for the same amount of load.
So let's give an example. Let's say it's a steel bolt and you give it 1,000 pounds of force on the bolt, the bolt is going to stretch a certain amount. A titanium bolt with that same 1000 pounds of force is going to stretch twice as much. Because of stretching it will still carry the load as long as you are within the elastic limit. That's why people say they have a problem with the bolt stretching, but they both stretch until they come up to the clamp load that holds the assembly together. In motorcycle terms we see this in the triple clamps. There have only been a few riders we know that have the ability to tell the difference between steel bolts and titanium bolts. That same rider can tell the difference between 15 foot-pounds and 18 foot-pounds of torque. There are only a few riders that are that sensitive to tell the tiny differences in the triple clamps. Across the board right now we pretty much have everybody running titanium triple clamp bolts and titanium front axle pinch bolts.
The precursor to it, everyone talks about the factory teams having the advantages. Back in the ‘60s and ‘70s, with the titanium coming out of the aerospace industry, a lot of the factory teams had access and the budget to be able to build titanium fasteners. Back in the ‘70s, going into the early eighties, the machining of titanium was quite tricky. The CNC machines and the quality of the cutting tools to cut the material were still relatively expensive and hard to handle. If you talk to anyone that has been around motocross for a long time like Roger DeCoster, back in the ‘70s he had bikes totally decked out with titanium fasteners with the factory programs. But for privateers it was really hard to get their hands on it. Then CNC machines were more available, but this is where a lot of the bad wrap started with titanium axles and fasteners. CNC machines were coming along and people could take titanium bar and machine it and make a fastener by just purely machining it out of bar stock. The problem with that is if you want to make an aircraft quality fastener, it needs to be hot forged, meaning it needs a forged head to get the proper grain structure, followed by the proper heat treating and then you need to have a rolled thread.
When you put a piece of bar stock into a CNC machine and machine it, you build up stress risers in the surface and they look like little Grand Canyon fishers on the microscopic level and then they would get fatigue cracks under the head or they get fatigue cracks in the threads and the bolts would break. Titanium seems to be more sensitive to the finish of the surface. This is where people were having random breakages with titanium and they were calling it brittle or saying it didn't have the strength of steel. But this was a processing problem, not a problem with the actual material.
If steel and titanium are both equal in strength, then aluminum is half the strength. When you get into the 7000 series of aluminum they get to about two thirds the strength of titanium. The problem is that the ductility of aluminum is less than steel. This is what comes into play when designing parts, especially the axles and some bolts, like cylinder head bolts, the stretchiness becomes an issue with titanium. And the technical term for that is modulus of elasticity. That's why a when looking at a titanium spring, it has less coils in it. With the modulus number of titanium being half of steel it's going to flex twice as much for the same amount of load.
So let's give an example. Let's say it's a steel bolt and you give it 1,000 pounds of force on the bolt, the bolt is going to stretch a certain amount. A titanium bolt with that same 1000 pounds of force is going to stretch twice as much. Because of stretching it will still carry the load as long as you are within the elastic limit. That's why people say they have a problem with the bolt stretching, but they both stretch until they come up to the clamp load that holds the assembly together. In motorcycle terms we see this in the triple clamps. There have only been a few riders we know that have the ability to tell the difference between steel bolts and titanium bolts. That same rider can tell the difference between 15 foot-pounds and 18 foot-pounds of torque. There are only a few riders that are that sensitive to tell the tiny differences in the triple clamps. Across the board right now we pretty much have everybody running titanium triple clamp bolts and titanium front axle pinch bolts.
The precursor to it, everyone talks about the factory teams having the advantages. Back in the ‘60s and ‘70s, with the titanium coming out of the aerospace industry, a lot of the factory teams had access and the budget to be able to build titanium fasteners. Back in the ‘70s, going into the early eighties, the machining of titanium was quite tricky. The CNC machines and the quality of the cutting tools to cut the material were still relatively expensive and hard to handle. If you talk to anyone that has been around motocross for a long time like Roger DeCoster, back in the ‘70s he had bikes totally decked out with titanium fasteners with the factory programs. But for privateers it was really hard to get their hands on it. Then CNC machines were more available, but this is where a lot of the bad wrap started with titanium axles and fasteners. CNC machines were coming along and people could take titanium bar and machine it and make a fastener by just purely machining it out of bar stock. The problem with that is if you want to make an aircraft quality fastener, it needs to be hot forged, meaning it needs a forged head to get the proper grain structure, followed by the proper heat treating and then you need to have a rolled thread.
When you put a piece of bar stock into a CNC machine and machine it, you build up stress risers in the surface and they look like little Grand Canyon fishers on the microscopic level and then they would get fatigue cracks under the head or they get fatigue cracks in the threads and the bolts would break. Titanium seems to be more sensitive to the finish of the surface. This is where people were having random breakages with titanium and they were calling it brittle or saying it didn't have the strength of steel. But this was a processing problem, not a problem with the actual material.