Topic: Chro-Moly frame?

[GroovieGhoulie]

I recall reading that the factory racing CR750s used frames that were, (in keeping with the rules), dimensionally identical to the CB frame, but instead of being made of mild steel, they were made of Chro-Moly steel, (which was bending the rules).

What are the advantages of this in a motorcycle frame?  I am familiar with Chro-Moly since it's used in gun barrels, but when used in motorcycle frames, do the same advantages apply, ie: rust resistance, increased tensile strength and stiffness/hardness?

I'm asking because a buddy of mine turned me onto a guy who builds custom motorcycle frames out of whatever material you want.  He said that I could take him my CB frame and he would build a Chro-Moly copy that was so exact, even Honda engineers couldn't tell it apart from stock at firstt glance if I so desired.  I can have him add bracing, gusseting, etc if I want it stiffer for racing.

My buddy told me that it would likely be about $800, but I'm gonna call him and get a actual quote.  Do y'all think it would be worth it?  I mean the frame I got seems solid, but I don't know how rusted out and degraded the frame tubes are on the inside.  This would be a NEW frame, and made of better materials and the same stuff the racers used.  They must've done it for a reason, right?

[Slapguts]

Chromoly is a lot lighter than regular steel.

[bwaller]

I would be really surprised if you could have a complete frame built of chrome moly steel for $800. It would be trick I'll give you that! If you were building a full balls out road racer then absolutely go for it, but if not........ that decision is your own. You can remove unwanted brackets, add some bracing and pretty up the original yourself.

I recently bought some thin wall 1" CM tubing, which wasn't bad, but the bending cost set me on my ass! 

[bwaller]

Hey GG,

I sent a response, went back to my shop and decided I must sound like an old poop.

On second thought, Go BOY GO, treat yourself to a one off frame. It'll give you bragging rights to be sure!

Brent

[GroovieGhoulie]

Hey GG,

I sent a response, went back to my shop and decided I must sound like an old poop.

On second thought, Go BOY GO, treat yourself to a one off frame. It'll give you bragging rights to be sure!

Brent

Well if the price is cheap enough, I surely will.  Too much $$$$ and I don't think I will.  See, I like the dimensions and such of the stock frame, and the handling and appearance that result, so I want an IDENTICAL frame, but brand-new and made of better material. 

Sure, I could change the rake or swingarm location, but why do that?  I think Honda got it right the first time and I like the quirks and "faults" of the stock frame and its design dimensions.

[MRieck]

800.00 does sound cheap IMO.

[Jim F]

Chro-Moly steel is lighter than conventional steel but it is also much stronger
But be carfull if you have someone weld it, you have to anneal the weld or it will be very brittle.
if you have a frame made, that is about as trick as you can get other than aluminum.
800.00 is too cheap
the material alone would cost that

Good luck
Jim

[Geeto67]

I'm curious where you heard the factory racing CR750s were chromoly. The CR750s were built out of a 970 kit which used a stock cb750 as the base for the bike. Most of the CR750s in racing were privateer entries. The only factory backed effort I heard of was the Daytona 200 in 1970. From what I have heard the daytona 200 bikes were built in the states out of parts shipped to Honda North America.

I have heard this rumor too but I have never seen any evidence to back it up. Dick Mann's CR750 currently resides in paris but nobody wants to fly me out there to investigate it, and most I have spoken to about that particular bike tell me it is a standard frame.

Personally $800 is nothing for a custom frame and if you read the race reports about the CR750s they were notoriously flexi espically with the 96 to 82 hp motors in them. Chromoly would go a long way to improving the handeling. Considering stupid-simple HD chopper frames start at over $1000 it would be a bargain if you are planning to make crazy hp with the honda motor. 

[Seyser]

Chromoly is a lot lighter than regular steel.

Actually thats not true. A common misconception is that chromoly is lighter than mild steel, which isnt true; however it is true that a thinner-wall chromoly has some of the same strength characteristics as thicker mild steel.

Steel has a yield strength of around 30,000 to 45,000 psi where as chromoly has a yield strength in the 80,000 to 85,000 psi range

-Jeremy-

[Steve F]

Chromoly is a lot lighter than regular steel.

Actually thats not true. A common misconception is that chromoly is lighter than mild steel, which isnt true; however it is true that a thinner-wall chromoly has some of the same strength characteristics as thicker mild steel.

Steel has a yield strength of around 30,000 to 45,000 psi where as chromoly has a yield strength in the 80,000 to 85,000 psi range

-Jeremy-

What he said.  Beat me to it.

[MRieck]

Chromoly is a lot lighter than regular steel.

Actually thats not true. A common misconception is that chromoly is lighter than mild steel, which isnt true; however it is true that a thinner-wall chromoly has some of the same strength characteristics as thicker mild steel.

Steel has a yield strength of around 30,000 to 45,000 psi where as chromoly has a yield strength in the 80,000 to 85,000 psi range

-Jeremy-

What he said.  Beat me to it.

Yes...I agree.

[tsflstb]

I keep posting this link hoping somebody has the money to use them...anything you can imagine made from titanium.  No personal experience, but the pictures of their work look pretty nice.

www.arcfabrication.com

I sometimes think of these frames and swingarms when I'm alone and touch myself.

[Geeto67]

I keep posting this link hoping somebody has the money to use them...anything you can imagine made from titanium.  No personal experience, but the pictures of their work look pretty nice.

www.arcfabrication.com

I sometimes think of these frames and swingarms when I'm alone and touch myself.

With a wet weight of 265lbs for their titanium framed rat bike I don't wait to be alone to touch myself when looking at those pics.

+4 on the chromoly strength

[chung]

Isn't RENOLDS 531 very close to the 4130 Chromoly we use here in the States?

Most of my experience is with A36, A500 and various grades of rebar. But I believe the following is, for the most part, correct.

Chromoly is NOT a good first choice of alloyed steel from a welding standpoint. That's why it always costs more; not just the name, material cost or strength. It's the weld and post weld process. And if the welder is skilled, he knows it

Extreme Caution should be used if you actually "weld" Chromoly. When you bring it to a near melting point, it changes the metal and makes it brittle.

I believe that most failures occur well away from the weld in the "heat affected zone" rather than along the toe of the weld where the metal was actually melted? And it's the difference between the two areas that cause the failure.

I don't recall Preheat but Post Weld Heat-treatment (stress relief) is required to restore it. That's why they used to BRAZE all the bike frames, the temps aren't high enough to embrittle the base metal.

It's this post weld treatment that burns up the time, gas and $$$$.

[Steve F]

Isn't RENOLDS 531 very close to the 4130 Chromoly we use here in the States?

Most of my experience is with A36, A500 and various grades of rebar. But I believe the following is, for the most part, correct.

Chromoly is NOT a good first choice of alloyed steel from a welding standpoint. That's why it always costs more; not just the name, material cost or strength. It's the weld and post weld process. And if the welder is skilled, he knows it

Extreme Caution should be used if you actually "weld" Chromoly. When you bring it to a near melting point, it changes the metal and makes it brittle.

I believe that most failures occur well away from the weld in the "heat affected zone" rather than along the toe of the weld where the metal was actually melted? And it's the difference between the two areas that cause the failure.

I don't recall Preheat but Post Weld Heat-treatment (stress relief) is required to restore it. That's why they used to BRAZE all the bike frames, the temps aren't high enough to embrittle the base metal.

It's this post weld treatment that burns up the time, gas and $$$$.

I agree.  I know of a guy that built an aircraft frame using 4130, and he told me that the weld joints had  to be "stress relieved" afterwards.  He used a torch.  I found this article on the web this morning that covers the heat treating of 4130 after TIG welding.  Using the proper filler material is also a must.
http://www.netwelding.com/Welding%204130.htm

[Sam Green Racing]

I'm curious where you heard the factory racing CR750s were chromoly. The CR750s were built out of a 970 kit which used a stock cb750 as the base for the bike. Most of the CR750s in racing were privateer entries. The only factory backed effort I heard of was the Daytona 200 in 1970. From what I have heard the daytona 200 bikes were built in the states out of parts shipped to Honda North America.

Hi Geeto, you are correct about the privateer frames, the kit came with drawings of the modifications to the stock frame, needed to accommodate the factory supplied race parts.

The factory frames made for the 1970 Daytona race were made in chromoly tubing.
If the bikes frame in Paris was to be weighed and compaired to a modded stock frame, it might help to prove it's authenticity.

Sam.

[Sam Green Racing]

Geeto, did you get the PM ?

Sam.

[chung]

THANX Steve, very informative article. Yea...... what he said. LOL

I did not know that the alloy was that old although I did know that it was an aircraft grade of metal like 6160 that has gained fame and mention amongst we commoners of late. (do people really talk like that?)

[Geeto67]

Geeto, did you get the PM ?

Sam.

yes I did. My only concern is that I had never seen anywhere where honda or the riders could verify that the frames were chromoly. I'll take your word for it.

[Sam Green Racing]

Not sure who I heard it from, perhaps Steve Murray or Tommy Robb, I will check tonight, it's early morning now and I am off home to bed shortley.

Sam.

[Rookster]

A note on the 1970 CR750.  They were built in England in cooperation with Honda.  There is very little about those CR750 that is stock cb750.  The frame and forks may look similar but they are definitly special pieces.  You can see this in an article in Classic Bike from about 3-4 years ago.  There is a nice big picture of the Mann bike being serviced without the bodywork.  Those bikes were made to look stock to cheat the AMA Class C homologation rules.  Honda can't be singled out for this as BSA/Triumph were doing many questionable things too.
On another note the handling of a stock CB750 is horrendous.  Those frames flex like crazy.  I understand wanting to make an improved handling "stock" frame (isn't that what the Seeley frame was all about), but don't use the handling of the stocker as justification.  Ride a British bike from the same era and you will immediatly understand why Japanes handling was laughed at until the late 70's.  The Trident, Commando or Bonneville will eat CB750s for breakfast in the twisties.
Scott

[eldar]

And yet all those bikes almost completely disappeared didnt they.  Are you sure they are so much better or is that your experience?  It all depends on rider and how they set up their bike.

[Geeto67]

And yet all those bikes almost completely disappeared didnt they.  Are you sure they are so much better or is that your experience?  It all depends on rider and how they set up their bike.

To a point. I had a 1971 commando and as far as how the bike handled it was a lot better than my cb750. Unfortunatly the bike was fragile and no where near as tough as my cb750. It was also less forgiving. Rode my buddy's 71 trumpet and that bike handles well (and it is one of the poorer designed OIF bikes) and feels more confidence inspiring than my K5.  If you look at brit frame design they are purpose built and incorporate as few tubes as possible, unlike the japanese way of thinking at the time which was "not sturdy enough? throw more tubes at it". The brits really tie everything together (look at the rear z-plates on a commando for instance) a lot better and you don't get nearly the lateral flex a cb750 or a kawi triple does, yet when you look at the bare frames side by side the jap frames look stronger. 

[Rookster]

Wow, the British bike industry didn't disapear because of questionable handling.  It is my experience.  My first bike was a 1976 CB750.  I rode it all over the place in rain or shine, even in winter (sometimes) in Buffalo, NY.  I experienced the death wobble first hand many times on the freeways.  I did 115 mph indicated on the speedo down a 2 lane backroad on it.  I spilled it twice in low speed parking lot manuvers.  I had the back end slip out on me while doing a u-turn, not enough to go down but enough to scare the he!! out of me.  I have owned a Triumph Bonneville for the past 10 years.  It has never wobbled on me.  It has never stepped out on me.  It leans so low into corners that I back off before the bike does.  I don't ride the Bonneville quite the same as the CB750.  No more top speed runs on two way streets lined with driveways.  The Bonneville has made me feel like a better rider.  The CB750 made me feel like it was on the edge of going down or over.  I have ridden many British bikes since the K6 and all of them have given me the same feeling as the Bonneville.  If you think I'm full of Sh!t make friends with some britbike riders and see for yourself.  Understand I'm not knocking the CB750.  I'm currently going broke trying to build a Dunstall CB750.  Understanding the limits of the bike and its strengths and shortcomings is all part of the experiece.  Blindly loving the CB750 and denying its faults is foolishness.  The CB750 is not produced anymore either, does that mean it wasn't a great bike?  The brits didn't develop new products using new technologies.  That's why they aren't around anymore.
Scott

[MRieck]

 I rode my friends 1975 850 Commando in 1976...it handled  a lot better than my 1975.CB750F. Over the years though you can guess which bike was more reliable and easier to soup up.