Sorry for my terminology. I've got a project that I am currently turning into a cafe racer. I have leafsprings going from the frame where the battery box is, to the rear wheel. I still am using the rear wheel section of the frame which is completely normal. I am wondering if I will get any type of wobble from my back wheel. By bouncing on the back of the bike with all my weight, I am able to get the suspension the travel about 0.5" up and 0.5" down giving it about an inch of play. Just trying to get some thoughts. Thanks
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1980 CB750K Cafe with leafspring suspension
- Thread starter Czargar
- Start date
amc49
CB750 Guru
Dude, nothing personal at all intended but I see so many potential safety related things wrong with that pic I for one am not going to comment on it other than to say yes, it will have a probability of wobbling. And no, I would NOT ride it.
Dave
CB750 Junkie
Without going into detail why, just welding on a spring ruins it and the welds will break pulling material out of the spring, most likely while riding. Which after looking more closely at that picture will most likely flip your bike in such a sudden and violent manner that you will earn a Darwin award.
The spring must be attached like this including the link that allows for the length change ....
The spring must be attached like this including the link that allows for the length change ....
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Not so sure you guys understand...or maybe just me ha. There is no pressure from spring, the bend in the leafspring is in its natural position the way it is currently. If I were to take it off, the metal would still hold the exact form.
Dave
CB750 Junkie
Ride that and you will die.
I don't know how to make it any clearer.
I don't know how to make it any clearer.
I don't understand why though. Everything that needs to be there is there. It's not like I don't have a stable swing arm. The leafsprings are suspension and nothing more. They completely and only take the role of the shock absorbers.
madmtnmotors
CB750 Addict
The fact that you don't understand is why you should not be "engineering" motorcycle "suspensions" (a term used loosely here)...
Some of the most glaring deficiencies have been pretty clearly explained (with pitchurs!) in responses given so far. Even I understand some of the issues you have created and I'm just a dumb-ole' construction worker...
I did not customize this frame. I purchased it this way because of the style. I am a mechanical engineer and I know exactly what I am talking about here. I will be rewelding lots, especially where the leafspring joins to the frame and where the leafspring joins the swing arm to increase integrity. I was wondering if you all thought I would get sideways wobble. Not a matter of if I was going to die or any other thoughts.
madmtnmotors
CB750 Addict
Well then, by all means, carry on...
Welds on a spring will break, at the weld/spring joint, you need a to mechanically clamp them. Yes leaf springs can and have worked on motorcycles but you cant weld them for attachment. As far as rewelding frame joints, you need to grind out the old welds and reweld the, dont just pile more weld on top of the old crappy weld. Like everyone has said...its your bike and your life so continue on if you must, people just trying to point out some safety concerns.
I have already stated that I will be doing the work again to make sure it is safe. Like I've also already stated, not looking for any opinions on safety. Simply looking for thoughts on whether the idea would create any type of horizontal wobble. Thanks for the super helpful information gents.
madmtnmotors
CB750 Addict
You're super-duper welcome!
Thanks for being so grateful without being condescending...
Would horizontal wobble not be a safety concern?
madmtnmotors
CB750 Addict
You never said anything about redoing the work again. The only reference I can find is this:
My only takeaway from that reply is that you are going to continue to weld on spring steel which is typically a last resort and requires some advanced methods to maintain the structural integrity of the weld as well as the intended properties of the spring steel. Until you outed yourself as a master of all things mechanical we have no idea who we're dealing with. No need to get all bowed up, just trying to look out for the well being of a brother rider.
Not sure why you would think a leaf spring would introduce "wobble". The stock rear suspension is a simple coil spring over shock that is rubber mounted on both ends. Side-to-side tractability (IMHO) is dictated primarily by:
>Swing arm bushings
>Swingarm true and undamaged (not bent)
>Rim trueness
>Rear wheel alignment
>Frame straightness
>Overall structural integrity of the frame (i.e. is the frame straight and true and rigid enough for the anticipated load)
>Might as well lump steering stem bearings and front suspension integrity in there too.
>Road conditions
>Riding side-saddle?
Any or all of these things (and quite possibly others not mentioned) could potentially induce "wobble".
Break out the slide rule and start running some calcs.
madmtnmotors
CB750 Addict
No big deal. It can be very difficult to convey inflection with a keyboard. For anyone to try and answer your simple request regarding wobble would be doing you (or anyone else for that matter) an egregious disservice, not to mention irresponsible, if they failed to address the glaring deficiencies with your current setup.
I try to approach online forums with a sense of humor. Emoticons don't always do a sarcastic wit justice...
As for correcting the integrity of the design, besides the issues discussed regarding welding spring steel, you need to look closely at the arc of travel as the rear wheel pivots around the swingarm. The way the rear of the spring is mounted to what appears to be the lower shock mount will travel in a different arc around the fulcrum of the spring mount than the arc that the rear wheel is trying to move around the swingarm pivot. As the spring "compresses" it also straightens out changing it's overall length creating more of an elliptical arc. I would venture to say, that the current setup may very well induce some wobble and could likely be added to the list in the previous post, due to the fact that there are several opposing forces working against each other in its current state.
amc49
CB750 Guru
You are NOT a mechanical engineer or if you are well, I will stop there. I'm not and listen up, I worked building various drag cars some of which ran the 1/4 mile at over 200 mph. Motorcycles? let's just say I pretty much know what I'm talking about there too. Mostly anyway.
Your lack of connection between safety and wobble could get you killed. Why? For one a massive failure to realize how quick a wobble can turn into accident.
(1) You CANNOT weld on spring steel used in a leaf spring, the heat treat is what gives the springiness to the part, welding kills that since the heat induction and cooling process overthrow the original which is what made the steel part springy. NOBODY welds on spring steel unless they have not a clue. You will lose either the spring tension or worse leaf will break close to a weld point. I've seen it before. NO mechanical engineer on the planet would approve that!
(2) You have cut the leaf it appears, that GREATLY increases the spring rate just like cutting a coil spring. It will be much stiffer than the entire spring was because it is shorter, the same forces going into it have less leverage because the working arm there is shorter. Witness your one inch total of up and down travel, not enough, too stiff. Normal rear on one of these bikes uses around three inches normally. The one inch makes it much easier for spring to break since it cannot work as a spring nearly so easy. Shortening it made it much closer to a solid arm or bar.
(3) The puny bolts you have at the back end of the leaves are extended as compared to the OEM ones and they are also not centered and pinched over the arm both of which made them much much stronger. Your bolt is long enough that with no suspension because of (3) the stress will be heavily on that bolt and break it or swing arm boss in a matter of minutes. Worse because bolt not supported on both ends, it is now a lever worked up and down in the hole by the leaf. Weakest part of the whole thing, take a look at how leafsprings are mounted on the ends, ALWAYS from BOTH SIDES of the eyelet NOT ONE. One side mounting is guaranteed twist to wobble.
(4) You don't have enough leaves there at all, too thin. Too much force has to be absorbed by the two there, they will break. The loss of heat treat from the weld will add to that. So will essentially being a hardtail.
(5) The difference in lengths producing interference arcs in suspension travel. Already called out. Spring and arm are fighting each other, another reason why you only have one inch in travel, go further and parts will likely break.
(6) With assumed rubber bushings in the leaf eyelets, since eyelets are outside of the swingarm instead of centered over it and pinched on both sides the swingarm will wobble in its' play at frame since what is supposed to damp that out is missing, your leaves not only flex up and down but can flex twist as well, the rubber does nothing to damp that out, rather giving space to twist. Think about why the OEM swingarm is not rubber mounted to frame, it has bearings to help positively stop that twist. You with the leaves though have added forces to let more twist in the 'suspension' if you could call it that, I certainly don't. More like mounted solid as a rock with all forces going into those puny little rear bolts. BIG mistake my friend.............................
Leaf springs are NEVER considered suspension only, they cross the line to be part of the structure (frame) guaranteeing driving wheel(s) stay straight, look how they are used in cars and trailers. They ceased being 'suspension only' parts when you began to ask them to stop wobble which is what this thread seems to be about! Suspension alone does NOT stop directional integrity yet clearly you have asked the leaves to hold that integrity.
Again, no insult intended, if you were my best friend I would be telling you even more.
DON'T, your basic idea is dangerously flawed. I just got a free Nighthawk 750 because someone simply extended swingarm and the frame runners at top to mount standard OEM shocks further back by 5 inches. The frame extensions were 3/8" steel plate but extended like that they flexed and then tops of shocks moved side to side to make for an uncontrollable wobble over 30 mph.
Your lack of connection between safety and wobble could get you killed. Why? For one a massive failure to realize how quick a wobble can turn into accident.
(1) You CANNOT weld on spring steel used in a leaf spring, the heat treat is what gives the springiness to the part, welding kills that since the heat induction and cooling process overthrow the original which is what made the steel part springy. NOBODY welds on spring steel unless they have not a clue. You will lose either the spring tension or worse leaf will break close to a weld point. I've seen it before. NO mechanical engineer on the planet would approve that!
(2) You have cut the leaf it appears, that GREATLY increases the spring rate just like cutting a coil spring. It will be much stiffer than the entire spring was because it is shorter, the same forces going into it have less leverage because the working arm there is shorter. Witness your one inch total of up and down travel, not enough, too stiff. Normal rear on one of these bikes uses around three inches normally. The one inch makes it much easier for spring to break since it cannot work as a spring nearly so easy. Shortening it made it much closer to a solid arm or bar.
(3) The puny bolts you have at the back end of the leaves are extended as compared to the OEM ones and they are also not centered and pinched over the arm both of which made them much much stronger. Your bolt is long enough that with no suspension because of (3) the stress will be heavily on that bolt and break it or swing arm boss in a matter of minutes. Worse because bolt not supported on both ends, it is now a lever worked up and down in the hole by the leaf. Weakest part of the whole thing, take a look at how leafsprings are mounted on the ends, ALWAYS from BOTH SIDES of the eyelet NOT ONE. One side mounting is guaranteed twist to wobble.
(4) You don't have enough leaves there at all, too thin. Too much force has to be absorbed by the two there, they will break. The loss of heat treat from the weld will add to that. So will essentially being a hardtail.
(5) The difference in lengths producing interference arcs in suspension travel. Already called out. Spring and arm are fighting each other, another reason why you only have one inch in travel, go further and parts will likely break.
(6) With assumed rubber bushings in the leaf eyelets, since eyelets are outside of the swingarm instead of centered over it and pinched on both sides the swingarm will wobble in its' play at frame since what is supposed to damp that out is missing, your leaves not only flex up and down but can flex twist as well, the rubber does nothing to damp that out, rather giving space to twist. Think about why the OEM swingarm is not rubber mounted to frame, it has bearings to help positively stop that twist. You with the leaves though have added forces to let more twist in the 'suspension' if you could call it that, I certainly don't. More like mounted solid as a rock with all forces going into those puny little rear bolts. BIG mistake my friend.............................
Leaf springs are NEVER considered suspension only, they cross the line to be part of the structure (frame) guaranteeing driving wheel(s) stay straight, look how they are used in cars and trailers. They ceased being 'suspension only' parts when you began to ask them to stop wobble which is what this thread seems to be about! Suspension alone does NOT stop directional integrity yet clearly you have asked the leaves to hold that integrity.
Again, no insult intended, if you were my best friend I would be telling you even more.
DON'T, your basic idea is dangerously flawed. I just got a free Nighthawk 750 because someone simply extended swingarm and the frame runners at top to mount standard OEM shocks further back by 5 inches. The frame extensions were 3/8" steel plate but extended like that they flexed and then tops of shocks moved side to side to make for an uncontrollable wobble over 30 mph.
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amc49
CB750 Guru
Laugh all you want, nobody said we had to be smart about it did they? You are most certainly no mechanical engineer....................wanting to weld on spring steel makes that patently clear. You have no understanding there because if you did you would know the bike is junk. Cafe racer??? First time you lean into a corner at 60 mph wobble will absolutely be the least of your worries when those rear bolts come loose. You have no understanding of how leafsprings mount whatsoever. You cannot mount the ends one sided; you either get that or you don't. Meaning you need to start completely over, no rewelding on the planet is going to address that. The springs must be moved to be directly over the pivot point like every other set in the universe is. You're in defiance of every other mechanical engineer on the planet with the way it is.
AND, if you were a mechanical engineer, then why are you asking others if your setup will wobble???? A true engineer would already KNOW. I know and I'm not one.
I'm done here, you can spout your crap to others until you are blue in the face.
AND, if you were a mechanical engineer, then why are you asking others if your setup will wobble???? A true engineer would already KNOW. I know and I'm not one.
I'm done here, you can spout your crap to others until you are blue in the face.
