To hoop or not to hoop?

Kev Nemo

Honda Hacker
So the rear section of my frame has been hacked off right after the shocks for awhile now and I have debated:

-putting in a rear hoop
-welding in slugs
-making support for aluminum cobra head seat that would support a passenger.

thoughts?
 
i threw some bmx bar ends in the ends of my rails. easy fix. but if you want more of a stock look I would say weld a hoop on. just my opinion.
 
It needs something to stop the shock tops moving independant of one another.
The rear frame moves about a bit when stock, chopping bits off isnt going to improve handling
You loose a lot of structural integrity when back end is chopped off.
I would weld something in.

PJ
 
crazypj said:
Yep, and they flex all over the place, but, they are 'designed' to flex.
Its not such a good thing on street bike

PJ

You know that flat track frames are meant to flex all over the place from where?

That's news to me. We were always wanting them to track where we wanted to go, not to flex around wherever it pleases. Most flat track frames have chromemoly frames and swingarms that flex very little. The swingarm bushing/bearings are close tolerance to eliminate flex.

That piddling little bit of frame loop on the back of any flat track frame means very little in the line of ridgitidy, especially since most frames have a cross tube very near the upper shock mount anyway. The swingarm will usually flex before the shock support area of the frame will. The biggest reason for any need for a loop willl be what it will be needed to support. The early Trackmaster frames were chopped off because all that was needed behind was a little support for the fender and bounce pad. The later frames had full loops to support the Champion style tail sections after Champion made their design and it became popular.
 
So by these standards a good bit of the bikes on the forum are death traps? There are quite a few bikes just lopped off behind the rear shocks. I think we should warn all of them. I'll go post in the cafe and bob sections so they won't get hurt
 
Not death traps, just unsafe if ridden properly.
The handling wasn't fantastic when new, cutting large chunks off for 'style' doesn't improve things.
Its possible the make frame stronger where it needs to be and hide bracing, but, as you said in rear hoop posting, step back and think about what your doing.

PJ
 
crazypj said:
I dunno why everyone thinks chromoly is some magical material, it isnt.
Its just lighter than 'ordinary' mild steel of same mechanical strength, flex it enough and it will break exactly the same as cheap stuff. (you'll have to ask an enginer about modulus of elasticity)
Most flat track frames I've looked (including some of Kenny Roberts factory Yamaha) at have relatively small diameter tube.
The wheels should stay in vertical plane relative to each other and rear shouldn't twist in frame under power. (unlike some early HD where you had to set bike up on a 'curve' because it straightened out under power)
It's easy to make a very lightweight sub-frame if your using a monoshock set up, with twin shocks, even very expensive matched pairs, the twisting loads on mountings are very high under full bump, (the paint falls off at flex points)
If you happen to be sliding through a corner and catch edge of groove, you know what happens, the wheel twists and frame moves, your relying on a very small diameter axle to hold ends of swing arm in line.
I guess you have never cracked a frame around top of seat tube area? (just behind/level with rear tank mount)
If you have a nickel plated frame, the nickel flakes off on the stressed area's as it doesnt flex very well
The Rickman brothers, when developing frames, used to have them hard nickel plated, where it fell off was a flex area so that's where it got braced, rinse and repeat as necessary.
Some bikes are much worse than others, Kawasaki KH 400, you can flex wheel to touch shock on either side, 250~550 Honda's, twins or fours, rear flexes and paint falls off around top shock mounts and at front where seat tube meets top 'tube' (the pressed tin part)
CB350 twin usually has 'C' section of rear frame boxed in to help stiffen it.
To put it another way, get a 4x2 about 4 ft long. put one end under rear frame 'horn (after end is cut off its no longer a loop), press on other end, see how far it moves.
You put way more stress than that when your riding.

PJ

I studied engineering along with statics and strengths of materials in my background, so I have a bit of understanding about the subject.

Modulus of elasticity has little to do with the frame strength, It has to do with a material’s ability to return to original shape when deflected. It is the ultimate strength of materials, resistance to stress (loading per unit area) and strain (deformation/unit length). Chrome moly is not remotely equal to “ordinary” mild steel. 4130 chrome moly has 100-200% higher ultimate strength, along with over 300% higher tension yield stress capacity, than 1020 carbon steel (ordinary steel). It has well over twice the compression, tension, and shear strengths for the weight – and that isn’t inconsequential. All steel has extremely high fatigue cycle ability, nearly infinite in most cases, where aluminum and composites don’t. That allows frame makers to follow several possibilities when building frames using chrome moly.

Frame builders can go thinner wall and smaller diameter tubing while maintaining strength, increase strength while maintaining tube size, or any combination of the two. A chrome moly frame of equal tube sizes in a bicycle can save as much as 20% in weight, due to tubing design options. A bicycle frame is considered to have a virtually infinite fatigue cycle on material versus aluminum or carbon fiber. So your evaluation on tube diameter alone is meaningless. It is how thick the wall for the tubing may be. You’d find the gusseting is thinner material than the mild steel used in the stock street bike frames too. But all of it will be higher strength and/or lower weight. Ron Woods built the big tube Norton flat tracker to take advantage of the chrome moly strength using the large diameter, but very thin wall tubing. He had to weld extra pads on the frame for mounting points to spread the load over more area.

Experimentation is part of why some frames crack or break. Frequently those experimenting with strength vs weight or in highly critical areas, smaller thinner wall tubes are welded in and will break before the surrounding stronger tubes, indicating high stress areas that may need re-engineering or inspection for possible approach of the end of material fatigue cycle. The production Trackmaster, Champion, and Star Racer frames that have survived literally decades give witness to how well chrome moly stands up to the demands of racing.

If your examples of broken carbon steel frames had been built of chromemoly the chances are the frames wouldn’t have broken. Cracking is shear, chrome moly shear strength is higher by nearly 3 times over that of mild steel. Sure, plating might crack, as might paint, but that’s because the material is more brittle than the steel plus at joints, plating can have issues with adhering to the material due to gassing. I might also add that any plating can and will alter the steel alloy characteristics. Plating can harden the underlying steel. For best strength chrome moly frames are not plated, but painted. In addition, if there was no advantage to chrome moly, it wouldn’t be used by the makers. One key point though is that chrome moly requires special care when welding, otherwise it does become brittle.

When you mentioned the Kawasaki 400 you lost much credibility with me. I rode a Kawasaki S3 400 triple (pre-KH designation) very very hard as a 22 year old chasing those Z1s of my buddies around the roads of eastern Ohio. You have to watch the exaggerations when some of us actually rode those bikes and actually have uncolored memory of them. Yes it did flex, courtesy of undersize swingarm tubing without enough bracing plus plastic bushings and thru holes with excessively large clearance tolerancing too. It never flexed enough to hit the shocks. A friend told me he could see it flex when following me. All of the early Japanese bikes suffered from under-engineering in the frame areas. The fixes ranged from tightened swing arm pivot holes and bushings to bracing. On the high horsepower 900s the frame required some added bracing too. The 400 used the similar design frames, based on the Norton featherbed, but made from carbon steel. Still the 42 hp of the little triple probably did little to the frame itself, mainly the swingarm. Oddly enough, the chrome moly Rickman frames were also featherbed style too.

One last comment, unless the frame loop is bridged up overtop the frame like a brace, it will be of minimal consequence when it comes to the flex you speak of. Most are in the same plane as the rear shocks and are easily flexed due to the nature of the mild steel and the light tubing thickness. There is tubing with some X axis triangulation effect from either the head set or the backbone tube back to the shocks, depending on the frame design, and there is the Y axis triangulation from the swingarm to the shock to those tubes exteneded back. Finally there are several cross tubes and/or stamped steel gussets between those side tube sets. Those are quite strong, compared to that bit of strengthening from the back loop. That’s why for a street ride, it doesn’t make that huge a difference whether the loop is there. Now if you cut out the cross bracing ahead of the shocks that would make a bigger difference.
 
Wow-lot's of good info in response! The part of the frame that was cut is past the shock mount (red circle) NOT the hoop under the seat (red arrow). It is truly excess material and even not having an engineering background, I knew it was safe to cut:
MyBike7-08002-1.jpg


however, in putting on the new smaller tank, I did remove the triangle brace beneath the tank (red arrow). I haven't noticed any flex at highspeed or stress cracks. That being said, I'd rather be safe than sorry. My work around would be a "T" brace (roughed out in red):

seat-tank6-5002-1.jpg
 
Here's a Trackmaster. It stops right after the shock mount, but notice the "U" shaped brace right past and between the shocks:
10Oct05TrackmasterFrame.jpg

Champion:
championframe.jpg

Boss Rotax:
10Oct19BossRotaxFrame.jpg

Kenny Roberts (has the top brace, but it looks pretty thin):
09Sep23KR-TT-frame.jpg
 
Hey Kev:

That track master frame looks under braced to me. The bent tube is already preloaded and by sight, doesn't add much to the rigidity of the bike. That wasn't the best idea to chop out the chop out the frame brace to make the tank fit. See if you can get a 3/4" DOM X-shaped brace in there.

klx678:

You bring up some very good points, specifically with the 4130/1020 comparison. Yeah, 4130 is significantly stronger then DOM. The nice thing about it is that it comes in so many sizes. Carroll Smith shoved a lot of things down peoples throat and is pretty much considered the Jesus of motorsport engineering. One of the things he demanded was that 4130 be stress-relieved after welding. Yet even in aerospace, this is sometimes not done. To me--and feel free to correct me if I'm wrong--the trick is to get in and out of the weld as fast as possible; e.g. don't site there and hold the arc on the pool unnecessarily. I suppose the same is true for mild as well.

Building a frame from scratch is something I'd like to do--perhaps even this winter. As much fan as it would be to build it out of 4130, I'll likely just import some T-45 from France or the UK. The nice thing about it is that it approaches the strength of 4130 but since it is technically a mild steel, it does not need the stress-relieving. I've read a lot about it, but I honestly need to research it some more because I'm getting conflicted info--I suppose because it's so new. Some sources claim T-45 is just a euro-spec 4130 while others claim it's a next gen mild steel--significantly stronger and yet lighter. Who knows? It will be fun to play with it one way or the other.

CrazyPJ:

Similarly, you bring up some great points, specifically: just chopping off the rear cross-bracing is precarious. I welded a straight tube of 3/4" 16ga DOM across the shock bolts. The tire has rubbed it once when I crested a hill at speed. I too think a dampner mounting brace is necessary. The nice think about my RD400 frame is there is a factory cross brace just ahead of the dampner mounts. This allowed me to chop off the rear of the frame and TIG in a rounded piece. I believe this was done safely.

You've developed quite the reputation around here :) I for one appreciate the blunt honesty and the wealth of information you bring to the table. Keep it up! It's all in good fun gents.

--Chris
 
so you think a "T" shaped brace (in my crappy illustration above) wouldn't work, Chris? If I did an "X", it'd be rather narrow at the top.
 
I figured since I had the data, I'd throw it up here. I'm not that familiar with steels since most of my work deals with aluminum and composites, but I understand the mechanics.

From MMPDS-01 (FAA released material properties)

I could only find 1025 Carbon steel so that will have to be close enough

Property (1025 / 4130)

Ultimate Tension Strength (55ksi / 95ksi)
Tension Yield Strength (36ksi / 75ksi)
Ultimate shear Strength (35ksi / 57ksi)
Modulus of elasticity (29msi / 29msi)
density (0.284 / 0.283) pounds per cubic inch

So, to summarize, they are nearly the same density, but since chromoly is 73% stronger, you can use thinner walled tube to get the same strength, making the structure lighter. The MMPDS doesn't even provide Fatigue data for 1025 which leads me to believe it has very poor fatigue characteristics, evidenced by the low tension yield allowable. The stiffness (modulus of elasticity) of the two materials is identical. Therefore, if you were to build two frames of equal strength (thinner chromoly tubing=lighter frame), the chromoly frame would be MORE flexible. If you built two frames with the same size tubing, they would have equal weight and stiffness, but the chromoly frame would be stronger. Obviously, a chromoly frame would be better, but the driving factor to use one or the other would be cost.

Now, I think the original discussion was about frame flexibility. Again, I am by no means a motorsports frame expert. My expertise is in aircraft frames which are designed to different criteria. But a motorcycle frame must maintain a certain amount of rigidity in order to maintain predictable handling characteristics. The factory spends a great deal of time perfecting the stiffness of a frame such that it can handle numerous riding conditions. As with aircraft, there would also be some safety factors built in. Although there are some areas of a frame which can be lightened because they are overdesigned (usually in order to keep manufacturing costs down) additional bracing added to the frame should be maintained unless it is obviously for something that is no longer needed (i.e. frame rails that extend back to support fender/luggage rack). If key parts of a frame are removed, the load distribution within the frame changes, increasing the loads in some locations above what they were designed for.

Unless you are doing trick jumps off ramps or offroading your CB, you will probably never experience an ultimate failure of the frame material. The most likely failure to occur would be due to fatigue cracking. Cracks form near stress concentrations (welds, holes) where the stress is higher than the surrounding material (like when someone puts square holes in their brake rotor. :) ) Fatigue damage builds up over time at the grain level as the material is loaded past a critical spot on the stress-strain curve (see Miner's Rule) until a crack forms. If the frame is adequately stiff, the material can not be stretched to the point where it will reach this critical strain, and the fatigue damage will not accumulate.

Alright, I know I'm not explaining this well (I'm not a teacher) so I'll just say that flexible frames will crack before stiff frames, given the same loads.

BTW, material will not fail in fatigue due to shear, cracks form in tension fields. If something in shear fails due to fatigue it is because of local bending that induces tension on one side, like prying of a bolt head.

I plan on cutting off an arch just behind the shock mounts on my bike so my seat will fit, but I'm going to try and replace it with something.

Ride safe.
 
klx678 said:
I studied engineering along with statics and strengths of materials in my background, so I have a bit of understanding about the subject.

When you mentioned the Kawasaki 400 you lost much credibility with me. I rode a Kawasaki S3 400 triple (pre-KH designation) very very hard as a 22 year old chasing those Z1s of my buddies around the roads of eastern Ohio. You have to watch the exaggerations when some of us actually rode those bikes and actually have uncolored memory of them. Yes it did flex, courtesy of undersize swingarm tubing without enough bracing plus plastic bushings and thru holes with excessively large clearance tolerancing too. It never flexed enough to hit the shocks. A friend told me he could see it flex when following me. All of the early Japanese bikes suffered from under-engineering in the frame areas. The fixes ranged from tightened swing arm pivot holes and bushings to bracing. On the high horsepower 900s the frame required some added bracing too. The 400 used the similar design frames, based on the Norton featherbed, but made from carbon steel. Still the 42 hp of the little triple probably did little to the frame itself, mainly the swingarm. Oddly enough, the chrome moly Rickman frames were also featherbed style too.


Cool,
glad you understand.
I know chromoly is used in thinner section even if tube size is same diameter to make lighter frame. (that doesnt sound right but I think you know what I mean?)
As for KH 400, I've seen them do it as well, tyre marks on inside of shocks on more than a few (although two of my friends raced the Kawasaki 400 series in early 80's so I guess they put extra sttress on bike?)
My sister had a 1973 Z1B until about 5 yrs ago, it had braced frame which made it a completely different animal to stock one (and big bore kit)
It was going to get a full restoration until her (ex) husband crashed bike with her on it and messed her back up real good.
It had been stored for about 15 yrs before she sold it. Tank and motor were stored in spare bedroom, she didn't know garage was leaking :'()
Heres pic showing frame bracing around steering head ( almost completely boxed to seat tube) and above swing arm pivot, it was state of the art in 1974~75 :D (the bit between sat tube and top tube cant be seen, but it is there)
PJ
Kawasakiframe1.jpg
 
I'm not an engineer.....or a frame builder.....but....

I've known a few frame builders over the years who only use DOM steel tubing as their preferred material (this is hardtail / chopper frames). They will NOT use chromoly under any circumstances as they say it is brittle and will fail in these applications.

And they DEFINITELY don't use pipe...
 
Yeah, but those guys are using .125" wall tube--way heavier than what we use. That said, I build a car from scratch out of DOM, so it is good stuff.

--Chris
 
hillsy said:
I'm not an engineer.....or a frame builder.....but....

I've known a few frame builders over the years who only use DOM steel tubing as their preferred material (this is hardtail / chopper frames). They will NOT use chromoly under any circumstances as they say it is brittle and will fail in these applications.

And they DEFINITELY don't use pipe...

First off chrome moly isn't more brittle unless it isn't welded and heat treated properly, so I have a good guess why they use carbon steel... the same reason I would - not overly stressed in their applicaton and no need for special welding and heat treating. When properly welded, chrome moly is stronger than equivalent dimension carbon steel. The fact is when properly welded and heat treated it will be no more brittle than carbon steel. That's the key point, special care is needed.

Second, I take it your buddies classify "pipe" as like plumbing stuff. By definition a pipe is cylindrical in nature, regardless of material. Of course the term used by most now for what they see as better quality pipe is "tubing" Still the same, a pipe. You need to specify what material alloy it is to be made of to truly differenciate it from one to another.
 
klx678 said:
First off chrome moly isn't more brittle unless it isn't welded and heat treated properly, so I have a good guess why they use carbon steel... the same reason I would - not overly stressed in their applicaton and no need for special welding and heat treating. When properly welded, chrome moly is stronger than equivalent dimension carbon steel. The fact is when properly welded and heat treated it will be no more brittle than carbon steel. That's the key point, special care is needed.

Second, I take it your buddies classify "pipe" as like plumbing stuff. By definition a pipe is cylindrical in nature, regardless of material. Of course the term used by most now for what they see as better quality pipe is "tubing" Still the same, a pipe. You need to specify what material alloy it is to be made of to truly differenciate it from one to another.

So, unless you have an oven big enough to fit a frame in and know how to properly heat treat it, chromoly isn't the best choice for the average Joe to use for a rear hoop.

And the pipe referred to is the plumbing type (or ordinary black pipe). Pipe is designed to hold liquids / gases under pressure, tubing is designed to be used for structural applications.
 
hillsy said:
So, unless you have an oven big enough to fit a frame in and know how to properly heat treat it, chromoly isn't the best choice for the average Joe to use for a rear hoop.

And the pipe referred to is the plumbing type (or ordinary black pipe). Pipe is designed to hold liquids / gases under pressure, tubing is designed to be used for structural applications.

Like I said, proper treatement and welding. Thus your buddies' preference... as I said.

Pipe is not exclusive to galvanized or black pipe by definition. Proof of that is if you simply do a search on chrome moly pipe. You will find both terms used to describe it, pipe and tubing. That was the point - generic terms like "pipe" means nothing. Besides, if the black pipe has the same chemical make up, it's the same. In fact, when we were doing problems on thin wall pressure vessels, the cylindric portion was frequently referred to as thin wall tubing, not pipe. Of course it's all semantics ...
 
Back
Top Bottom