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titanium frame copy

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Posted by: arjay

Would it be possible (or feasible) to copy a simple steel dirtbike frame in titanium - exactly the same tube widths, bends etc - like the way MTB ti frames resemble the traditional steel designs. If so, how come the manafacturers haven't gone that way? Does ti have the same repetitive stress resistance as steel?


Posted by: YZ165




Posted by: Kav

cost could be a big factor. And with the "production" rule on bikes they would have to sell them to the public in order to race them. Do you want to shell out that kind of $$$?


Posted by: YZ165

That pretty much sums up what I was thinking. It would be cool though.......


Posted by: Kav

but what if it cought fire...

don't want to see that


Posted by: bikepilot

ti doesn't burn, at least not easily.

thats magnesium you are thinking of


Posted by: cr250can

I thing the problem with titanium is two things. one is cost (costly material and costly to fabricate becuase it has to be welded in the absence of air), and second is titanuim has way more flex than steel. (if you copied a steal frame and made it exactly the same only with TItanuim it would be like riding a wet noodle! )


Posted by: Rich Rohrich

Search the internet or the old magazine places for info on Joel Robert's 1972 titanium framed Suzuki RH72 works bike.


Posted by: arjay

Quote:
Originally Posted by cr250can
I thing the problem with titanium is two things. one is cost (costly material and costly to fabricate becuase it has to be welded in the absence of air), and second is titanuim has way more flex than steel. (if you copied a steal frame and made it exactly the same only with TItanuim it would be like riding a wet noodle! )


How come titanium MTB frames are the same width tubing as the steel frames?


Posted by: CK

First off, I'm a mechanical engineer. So here goes. Titanium flexes about twice as much as steel for the same size. Aluminum flexes 3 times more than steel (this is in general) - ever wonder why aluminum frames are so big? It's because the cross section has to be bigger in order to achive the desired stiffness. Titanium would need to be larger than steel (in cross sectional area) for it to be as stiff, but it would be roughly the same strength. Ti is used instead of steel in many places because it is as strong as high strength steels (Chromoly), yet weighs half as much. Ti is very dificult to machine and is not as common, so it costs a LOT more.

The main benefit of using Ti is where strength, weight and heat resistance are critical (aerospace applications). On a motorcycle, it isn't worth the extra cost for the weight savings and it won't make the bike "better" just a bit lighter.


Posted by: YZ165

I learn something new every day around here........thanks for the info guys.


Posted by: Rich Rohrich

Quote:
Originally Posted by Kav
but what if it cought fire...

don't want to see that



SFO has a great story about someone machining titanium and burning a machine to the ground because he let it get away from him.


Posted by: Kav

Rich:I remeber SFO saying something about that. We use to have a fair amount of titanium in the Harriers engines, but when the aircraft was under a high stress load the engine and aircraft would flex causing the titanium blades to rub the inside of the engine casing. Because of this every once in a while the titanium would catch fire and by-by Harrier. The titanium was replaced with a diffrent metal aloy in the next verson of the engine.


Posted by: arjay

Quote:
Originally Posted by CK
First off, I'm a mechanical engineer. So here goes. Titanium flexes about twice as much as steel for the same size. Aluminum flexes 3 times more than steel (this is in general) - ever wonder why aluminum frames are so big? It's because the cross section has to be bigger in order to achive the desired stiffness. Titanium would need to be larger than steel (in cross sectional area) for it to be as stiff, but it would be roughly the same strength. Ti is used instead of steel in many places because it is as strong as high strength steels (Chromoly), yet weighs half as much. Ti is very dificult to machine and is not as common, so it costs a LOT more.

The main benefit of using Ti is where strength, weight and heat resistance are critical (aerospace applications). On a motorcycle, it isn't worth the extra cost for the weight savings and it won't make the bike "better" just a bit lighter.


Thanks CK. I'm still interested as to why titanium MTB frames are the same width as the steel ones. Are they thicker internally? Or is it maybe you can get away with more flex in an offroad cycle?


Posted by: jake949

maybe they want the flex in a mtb


Posted by: NathanA

As far as the machining of ti goes, I milled several ti parts a few weeks ago at work. The parts were rather simple so I made them on a manual machine with a sharp 2 flute carbide endmill. The ti felt just a bit tougher than 7075. That is to say ti machines quite easily. Compared to steel, it is VERY easy to machine!


Posted by: SFO

Lots of great data posted so far.
I would venture a WAG that MTB frames like flex.
I believe BSA was the first to try a Ti frame dirtbike with a factory goldie.
Ti will flex a given amount and work harden.
this leads to cracking which the BSA team found problematic.
A mc frame sees a different duty cycle than a mtb frame.
There is a finite amount of these cycles a material can experience before changing characteristics.
Creating a composite frame or modeling said parameters could create a new generation but all of this requires large amounts of time and R&D savvy.


Posted by: mxneagle

One thing thing that you may want to look into is the work hardening charateristics of Ti. I believe that it will become brittle at the high stress (flex) points and crack after time. I read somewhere that the factories replace Ti parts on a relatively frequent schedule. In fact I saw the parts list for an outdoor season of racing in some mx rag and they replace the frame about 3 times. I'm sure it isn't bent, so I assume they are trying to avoid cracking and that is on a steel frame.


Posted by: bikepilot

On the MTB side, most ti mtb frames use larger diamater tubing than a similar steel frame, but smaller than an aluminum frame. There are a few ti frames with small diamater tubing, in these cases the whole point is to induce more flex to give a more forgining ride without the extra complexity of rear suspension.


Posted by: David Trustrum

Quote:
Originally Posted by cr250can
. . . (if you copied a steal frame and made it exactly the same only with TItanuim. . .


What’s a “steal frame”?

I’m goin’ to put another lock on my bike.


Posted by: Matt R

Rather than titanium, carbon fiber would be a better alternative material. The carbon fiber weave could be controlled to allow flex in one-direction and stiffness in another direction. It also provides a very high strength to weight ratio. The only downside (besides cost) is the longterm durability. You would have to protect the critical areas from environmental damage.


Posted by: PMK

In comparing bicycle frames of 4130 and titanium, consider that not all ti frames use a butted type tube. Last I knew to get a butted ti tube they had to do it by grinding the inside diameter. In a quality steel bike most tubes are butted some double or triple butted.

They also use several different ti alloys in bicycles. The 3/2.5 and 6/4, are common with the 6/4 being slightly less elastic. Also on the low cost ti frames many are produced overseas and these use a commercial grade ti that is supposed to be similar to 3/2.5 but who knows.

Carbon fibre is as described but also has it's own issues.

The irony is why do so few off road manufacturers not use 4130 chromoly for the frames, it seems they prefer hi-tensile steel like the low end bicycle manufacturers. Maybe cost, but it would seem in this current trend of lighter bikes it would be easier to do a slight redesign for 4130 than an aluminum frame. The Youngblood framed YZ426f comes to mind, it was like 10lbs less than a stocker.

Not sure but for current production bikes, aren't only the europeans (KTM, Husky, Husaberg, Gas Gas) the only true 4130 frames?

As for a titanium fire, I don't know if SFO's friend was the same guy I knew, I doubt it since he didn't burn it to the ground, or if he did that wasn't mentioned. Was explained by a friend that made off shore powerboats how when they were machining a titanium rudder the chips caught fire. So hot it burnt the concrete. Somehow they survived but he said it wasn't pretty, I think they used a shovel or something and got the stuff outdoors and buried it in the sand or something but it was still bad.

PK


Posted by: cr250can

Steal-Steel it is all the same. I don't believe in spell checker.


Posted by: SFO

PMK, any links to said cromo yzf frame?
I am curious about that.
Metal fires are the everything you never wanted to see, after fire school.
It's one thing to have you chemistry teacher light off some mag and quite another to see this in an uncontrolled environment.
That's all I will say on this matter.


Posted by: PMK

SFO, sorry I didn't get back sooner. The Youngblood YZ was written up in Dirt Bike several years ago, there is a guy on TT that has one of the few made "Team Bikes", last I knew he was Lightest YZ or something similar. I think his 01 426 is like 215ish. It has more than just the frame but I thought it was around a 10lb Jenny Craig for the frame alone.

PK



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