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The only reason to go the high strength alloy route is to avoid the bent axle syndrome when chasing grams. I remember an article in Motorcyclist many years ago when they built an SRX600 Yamaha racer and had "an exact duplicate" of the OEM steel swing arm made using 4130. It was claimed to be sooo much stiffer, but of course it doesn't work that way.

Titanium is less stiff than steel, and flex in the front fork (such as at the axle) has been demonstrated to increase vehicle instability, so a Ti replacement for the front axle seems unlikely to be a good thing once you get past the lower weight.

Unfortunately, when you are having to try and improve the OEM part without changing any dimensions, it can be difficult to make any worthwhile improvements. A redesign is often needed to take advantage of other materials, and once you start down that path you may as well just throw the stock chassis out and build what you'd really rather have. :) You may even save time by building the new chassis instead of fiddling around with the old one.
 

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Titanium is less stiff than steel, and ....
That is like stating "Motorcycles are faster than cars", not necessarily true.
What alloys of steel and titanium are we talking about?
I don't know the stock axle alloy we are dealing with, nor do I know the titanium alloys used for a potential replacement.
https://en.wikipedia.org/wiki/Titanium
 

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Right there in the Wiki link it shows the Youngs Modulus of Ti as 116GPa


shows it as 114 GPa, and carbon/low alloy steels about 200 GPa. The modulus of elasticity will vary a bit depending on alloying elements, but not a great deal, and it is the number you want to look at for stiffness.

"Modulus of elasticity (or also referred to as Young’s modulus) is the ratio of stress to strain in elastic range of deformation. For typical metals, modulus of elasticity is in the range between 45 GPa (6.5 x 106 psi) to 407 GPa (59 x 106 psi). Modulus of elasticity is also a measure of material's stiffness or resistance to elastic deformation. If the Young modulus of metal is greater, it's stiffer. Modulus of elasticity is an important design factor for metals for calculations of elastic deflections. "

So Ti is roughly 50-60% as stiff as the steels are. Don't confuse Ti's high strength to weight ratio with stiffness. If you can increase the section of the part to get a higher second moment of area then you can end up with a strong/light/stiff part from titanium (which also goes for aluminum which is about 75% the stiffness of Ti). But size for size, Ti is much less stiff than steel (ask the engineers at BSA who messed that up and lost the world open MX championship in the 1960s when they duplicated the steel frame of the works bike in Ti tube of the same size. It was lighter, but flexed enough to throw the chain 1-2X a lap.)

When substituting different materials in a part you need to look at a variety of physical properties to see which one Mr. Murphy is going to use to mess up your plans. :)

cheers,
Michael
 

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Discussion Starter · #28 ·
I have purchased R3 parts from ebay for comparison purposes. Will report back when they arrive.

I really enjoy trying to learn about these metal properties and how to understand the practical application of that knowledge. Thanks to all that contribute!
 

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Right there in the Wiki link it shows the Youngs Modulus of Ti as 116GPa


shows it as 114 GPa, and carbon/low alloy steels about 200 GPa. The modulus of elasticity will vary a bit depending on alloying elements, but not a great deal, and it is the number you want to look at for stiffness.

"Modulus of elasticity (or also referred to as Young’s modulus) is the ratio of stress to strain in elastic range of deformation. For typical metals, modulus of elasticity is in the range between 45 GPa (6.5 x 106 psi) to 407 GPa (59 x 106 psi). Modulus of elasticity is also a measure of material's stiffness or resistance to elastic deformation. If the Young modulus of metal is greater, it's stiffer. Modulus of elasticity is an important design factor for metals for calculations of elastic deflections. "

So Ti is roughly 50-60% as stiff as the steels are. Don't confuse Ti's high strength to weight ratio with stiffness. If you can increase the section of the part to get a higher second moment of area then you can end up with a strong/light/stiff part from titanium (which also goes for aluminum which is about 75% the stiffness of Ti). But size for size, Ti is much less stiff than steel (ask the engineers at BSA who messed that up and lost the world open MX championship in the 1960s when they duplicated the steel frame of the works bike in Ti tube of the same size. It was lighter, but flexed enough to throw the chain 1-2X a lap.)

When substituting different materials in a part you need to look at a variety of physical properties to see which one Mr. Murphy is going to use to mess up your plans. :)

cheers,
Michael
I was dead wrong I guess, so I apologize Mr. Moore. I was thinking strength to weight.
How does shear Modulus come into play for axles?
 

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I was dead wrong I guess, so I apologize Mr. Moore. I was thinking strength to weight.
How does shear Modulus come into play for axles?
No need to apologize to me, I've been wrong plenty of times and I expect I'll be wrong in the future. It is easy to be wrong. :) The difficult part is recognizing it and learning what you need to do to be right (or at least less wrong). The older I get, the more I've started hedging my statements in case I've forgotten what little "right knowledge" that I've learned.

I suppose (not being any kind of engineer) that the shear modulus will vary similarly to Youngs Modulus per


Ti has a shear modulus a bit under 1/2 that of steel (which kind of tracks the modulus of elasticity differences) so I'd guess that it will need roughly twice the amount of cross section area at the shear point to have a resistance similar to a steel part. But that is something that was never discussed when I was getting my business degree while at university.

I suppose there's a hazard in learning a little bit, like about modulus of elasticity, second moment of area, etc, and then trying to apply it to everything whether it is appropriate or not. That would be the "when you have a hammer, everything looks like a nail" syndrome. It is nice to be able to do a quick web search and often find a website where someone can give an explanation understandable by the non-engineer, though I'll admit that sometimes I've got to go through several links before I find one that dumbs things down enough for me to think that I understand it. :)

Duckman, nice job on the safety wiring! That's the kind of axle they have in Lake Woebegone, where all motorcycle parts are above average. LOL
 

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Lol! That axel was an extravagance for sure. But I was doing a make over on the bike snd stumbled across it and couldn’t pass it up. It’s a 2006 Ducati Monster S2R1000. I got it down to 330 lb wet and 110 rear wheel hp. It’s a blast to ride. :)

I have to correct myself. I checked my notes. The bike now weighs 360 wet. I thought that sounded too light. :)
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Tire Wheel Automotive tire Helmet Sports equipment

Wheel Tire Land vehicle Vehicle Automotive tire
 

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Lol! That axel was an extravagance for sure. But I was doing a make over on the bike snd stumbled across it and couldn’t pass it up. It’s a 2006 Ducati Monster S2R1000. I got it down to 330 lb wet and 110 rear wheel hp. It’s a blast to ride. :)

I have to correct myself. I checked my notes. The bike now weighs 360 wet. I thought that sounded too light. :)
Love those old Monsters! I've always been a fan of new stuff and always lived by the notion that "newer is better" when it comes to cars, motorcycles, and any sort of automotive or tech stuff, but those S2 and S4 Monsters are still badass even by today's standards! If I was to buy an older bike now, it would be one of those.
 

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Discussion Starter · #39 ·
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