Where should ti bolts be used in place of aluminium bolts?

Discuss light weight issues concerning mountain bikes & parts.

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by popawheelie

I said something helpfull I thought but you two (or is it three) are so busy throwing around terms you didn't hear what I said. If it is too easy to ask a question is the answer really what you need? Think man! Good information is precious!

Adam W
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by Adam W

Popawheelie, i di read and understnad you comment.

What do you think the items in blue should be, ali or ti?

Would a titanium axle on a mountain bike cause many problems? I would have thought it would be ok because a mtb doesn't undergo as much stress as a motocross bike?




by nikh

Adam, sorry mate I'm giving you duff information!


by Barna

7075 T6 weighs .101 lb/in^3
5AL-2.5Sn Ti weighs .162 lb/in^3
Mild steel alloys weigh .290 lb/in^3

Al is lighter than Ti.. ti has about twice the tensile strenght. Parts made out of Ti can be made with thinner walls and cross sections so the finsihed parts may be lighter than aluminum parts. Al also has one third the modulus of elasticity of steel so it deflects more with a given load and the same section so many aluminum parts are made thicker. 7075-t6 has about and equal or slightly greater tensile strenght than mild steel alloys so it can take loads but it would be more flexy.

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by DavidG

The answers to Adam's original question have gone all over the place, mostly due to either incorrect answers or unfathomable ones. Hopefully the following information will help set the record straight:

1. Aluminium ('Alu') is LIGHTER than Titanium ('Ti').
2. Ti is STRONGER than Alu.
3. Because Ti is the stronger element, certain components made from Ti can be manufactured using less material than if they had been made from Alu.
4. Therefore, even though the base material used is heavier, because less of it is used, the part will be lighter.
5. Point #4 above is NOT relevant to bolts, because you cannot manufacture a bolt using less material -- after all, the bolt must fill a pre-determined space. (The exception is if the bolt is hollow, but the size of bolts used on bikes and their required strength means hollow bolts are not relevant.)
6. Therefore, an Alu bolt will always be LIGHTER than a Ti bolt of the same specs.
7. It also means that the Ti bolt will always be STRONGER than the Alu bolt.

BTW, I am fully aware that there may be some very minor inaccuracies in the above explanation, but they are irrelevant to the gist of the discussion, so I allowed them...

Regarding which bolts should be Ti and which should be Alu, this depends on how much of a trade-off one wants between saving weight, and safety. An extreme weight-weenie may be willing to change all his bolts to Alu, whereas a safety-conscious one will prefer to use only Ti or steel. For accepted practice as to which bolts can be Alu and which should be Ti, check out Nino's bolt article at http://weightweenies.starbike.com/articles.php?ID=16 .

I hope this helps...


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by bladteth

Mantra wrote:

> Titanium is as strong as steel, but 45% lighter

... and 2 times less stiff as long as we are comparing 4130 steel with Ti3Al2.5V titanium alloys. For other materials I recommend this page: http://www.matweb.com
Best regards,
Rado bladteth Rzeznicki


by Dave

Guys, many of you are getting ti v al weights all screwed up.....I was an aerospace machinst for a decade and have been a weight weenie for longer.

Weights of pure titanium and aluminum mean nothing, since no boke components are produced from either.

Titanium as used for strength applications is typically 6Al4V2Sn. This means 6% aluminum, 4% vanadium and 2% silicon. Vanadium is a very dense element and weighs a considerable amount, but imparts much greater strength to the allow. The above titanium allow has about twice the tensile strength of pure titanium.

Aluminum used for strenght applications is typically 2024 or 7075 with various heat treatment methods. The aforementioned popular titanium allow is heavier then the listed aluminum allows. That does not always mean though that a product can be designed lighter by virtue of using a material that is lighter. Aluminum is quite susceptable to surface imperfections becoming stress risers, and aluminum has fatigue cycling issues that titanium has far less of. Designers must take this into account, as the asshole trial lawyers are always circling prey.

As to titanium used for structural bolts, it really depends on what type of tensile strength is needed and what type of strain is placed upon the bolt and in what axis. As an example, a bolt used to cross through the eye of a rear shock is under deflection strain, as opposed to a bolt used to tighten something, which is a linear tensile strain where thread form can come into play. Some threads are cut into the product, which forms stress risers, whereas some threads are rolled, sometimes hot, other times cold, both of which realigns material grain structure rather than interrupting it, with cold working being superior to hot.

I just acquired a Scott Strike Limited frame and ordered the US's last new FRM Airwave carbon suspension fork and will be getting rid of the Scott steel shock bolt and replacing it with a titanium bolt, and the pivot bolt will receive the same treatment. If the bolt were to be insufficient in strength which I highly doubt, a periodic visual inspection to look for axial deformation would be the ticket.

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