Dual Suspension Frames with rear steel dropouts!? exists?

[BiGH]

hey all. I'm finding the bumps a bit much at high speed, just wondering if anyone can suggest a dual suspension frame with steel rear dropouts?

also something that would support a fairly good rack too (i'm not sure a single beam rack would support my batteries sufficiently (13kg).

 

[albie]

i'm liking the schwinn s60... was telling you about it the other night.

Although pretty hard to find her in australia... thats probably the frame i'll end up with.

 

[ngocthach1130]

i have the schwinn s60 too. The rear dropout is not steel though. It's an aluminum frame. The shock is crappy. The bike doesn't fit my height at all. I'm around 5 feet 8. With the seat set so my leg is fully extend when i pedal, i can barely reach the floor when comming to a stop. Lowering the seat to reach cause my leg to not fully extend so it hurt the knee. I have tried adjusting the shock so the bike ride lower with preloaded shock. But on bump it cause the rear to drop since it was preloaded. Upon hitting the ground again the spring smash against the stopper. Still a very great bike.

 

[Mark_A_W]

Why don't you just make BEEFY steel reinforcements and use a normal aluminium frame?

I wouldn't trust a steel frame anyway, not with high power. The dropouts are open, you really should have a closed section around the axle IMO (yes, I'm a friggin professional mechanical design engineer).

Go a downhill frame and you get 150mm wide dropouts - lotsa room for rear cassette and a disc brake

 

[getadirtbike]

Big H,

If you're quick, I can make you a set of torque arms that will work fine in an aluminium frame. We've been testing these things for over 9 months and the droputs have worked just fine.

If you're after a dual suspension frame, my friend is selling this one for $1000 AUD to finance a new set of dual suspension frames that we're building.

http://www.e-mtb.com.au/images/AZ12Ah.jpg

He's only selling the frame, swingarm and rear shock, but the torque arm situation is all sorted out so you'd just need to bolt some forks in and add a set of cranks, wheels and brakes and it's ready to go. He might sell the whole thing as a package. The frame goes on Ebay in the next 2 weeks.

I would hate to think how you'd fit a rack to it though.

Mark is right. Whatever you do, you need a closed section around the axle. We've had twin 6mm thick open steel sections spread apart from the torque of the motor. We've learnt it the hard way.

 

[deardancer3]

Big H,

snip

Mark is right. Whatever you do, you need a closed section around the axle. We've had twin 6mm thick open steel sections spread apart from the torque of the motor. We've learnt it the hard way.

I am a little dense on the mechanical stuf; can you give a pictoral example of good closed and open rear dropouts?

thnx

dick

 

[getadirtbike]

Here's the closed section torque arm which I make. They are very strong and I haven't had one even look like it's going to fail yet. I can mill the slot on any angle, and The actual arm is made of thinner gauge material to lighten it. Once it's all done the arm can be cut down and drilled, or bent or modified in whatever way is needed to make it fit the bike.

50 bucks a pair if anyone wants them.

 

[curious]

How much is shipping to US ?
Is the arm welded to the base (it looks like that on a picture)? If yes the weld may be prone to the failure as it is a very high stress area. Also do you temper the base after milling it ?

 

[getadirtbike]

Yes it's welded. We're talking about a very small force working against that weld compared to what the weld could hold. If your motor could break it I'd be overly impressed and ask that you send me a motor straight away. The milled portion is 6mm thick and made of EN36A steel which is more than strong enough to do the job without tempering. Shipping to the US?? Around $20 AUD.

 

[curious]

Do you have any suggestion as to how to implement the frame clamp that can hold the arm both ways (I am planning to use regen braking) ?

 

[getadirtbike]

Well without regen is easy because the axle will only ever try and spin one way. I haven't had much experience with regen... are the forces high enough for it to be of any concern?

Most bikes have a provision for a rack to be mounted. You could use that hole. Drill a hole in the torque arm, and bolt it directly to the frame. That way it wont move in any direction. It does mean that you have to drill your own hole in the torque arm to suit the bike. I'm sure most people can do that though.

 

[curious]

I just checked my donor bike. It does indeed have eyelets for the rack but in my view they they are fairly week - essentially a small nut welded to the frame in a narrow spot. Granted mechanical engineering is not my strong side so this is just a feeling. What is worse they are located about only 34mm away from the axle center so the load will be substantial. Also it requires the torque arm to be flat (and short).

 

[getadirtbike]

The torque arm is indeed flat and as short as you want to cut it down.... 34mm isn't bad. If you do the calculations .... the actual formula is... Torque = Force x Distance.

Hypothetically if your axle nuts were loose and there were no slot in the dropouts and you had a 10mm axle (5mm radius), ie..... If the rim was held fixed and the axle free to spin... At 34mm, the force is 14.7% of the force that it would be at the edge of the flat on the axle. If you were using 2 torque arms, the force would then become 7.35% of the force at the edge of the flat on the axle.

If you do the calculations, it works out to about a 18OKg shear load or 1800N shear force per bolt that would be opposing the torque of the motor at 34mm from the centre of the axle.

Your rack would be held by a mild steel M6 bolt.

If you consider that the ultimate tensile strength of mild steel is 400Mpa and use the formula of ....... Shear Force = Shear Stress x Cross Sectional Area, the shear stress due to the actual shear force on the M6 bolt would work out to about 63 MPA.... Considering that the bolt will fail at 400Mpa, you're well below the safety limit of the bolt.

The results of these calculations will vary slightly because I did the calculations quickly and assumed that the diameter of the M6 bolt is 6.0mm when an M6 bolt actually has an effective diameter of around 5.8mm due to the thread. Having said that, the results will vary just a little.

Once it's tightened up and your axle nut is tightened up there isn't actually that much force on the arm itself. The clamping force of the axle nut will distribute a lot of the forces between the actual dropout and the milled part of the torque arm. If your nut was entirely loose then all of the force would be put through the torque arm and the calculations above would be pretty accurate. But you'd be a fool to ride with loose axle nuts for a variety of reasons.

I've converted at least 15 bikes and used much the same method on all of them. Some of the bikes have run at 84V and 40A and have been severely abused..... there has never been a problem.

I'm not going to justify the construction of the torque arms any more. The wont fail.

 

[fechter]

That does look like an excellent design.

The only thing I could ask for on top of that would be some way to make the slot angle easily adjustable, like a spline of some sort, but that would add expense. I wonder how many angles are commonly used on bikes? There does seem to be quite a bit of variation there.

Some dropouts, especially front ones, have a recess around the axle nut that the torque arm would need to fit into. As long as the outside diameter of the slotted part fits, it would work perfectly since the arm is thinner than the slotted part.

On a high powered setup, the forces on the edge of the flats could reach extremely high levels if the axle nuts are loose. The strength of the axle material could be a limiting factor. Your extra thick slotted section will definitely help in this situation.

Another idea I had is if you put a small raised edge along the outside diameter of the face that goes against the dropout, the edge would bite into the dropout and help prevent it from spreading.

 

[getadirtbike]

Yeah, you're right on all fronts.

There are a lot of different angles. That's why making it in 2 parts like I do makes it easier. All I need is for someone to send me a good square on photo of their dropout slots, and I can weld the arm at the angle they need. As for the ones with a recess, the round part is 6mm thick ( I can make it thicker) and the flat arm itself is 3 or 4 mm thick depending on what I'm making it for. I can offest the flat arm a bit if needed so that it can fit in to a recess. Splining is very costly. I could weld 10 pairs of them for the amount of time and money that it would take to spline just one.

The milled slot is a pretty snug fit so the forces are distributed along the the flat pretty well. I was thinking of that when I was making the first few because if it were a little loose there would be a lot of force placed on the very edge of the flat as you've mentioned. As I said above... I can make them any thickness if need be.

I think I understand what you're saying about the raised edge on the OD of the face, but it's another process, which means more time, which means more money. It can be done though. I always check the condition of them when I remove them and so far none of them have shown me any sign of failing.

 

[getadirtbike]

Here's a side view of it

 

[curious]

getadirtbuke -
Please don't get me wrong - I actually like your design. I was just trying to understand how I can make it work on my bike.

 

[getadirtbike]

That's cool mate, I make them simple like that so that people can cut them or drill them to suit their own bike. The important part is getting the angle of the slot right with respect to any mounting features that you might want to use.

Because they only twist one way (without regen), a lot of people simply bend them under one of the frame tubesso that the arm sits up against the frame. When the power is applied the arm pushes against the frame tube and everyone's happy 8)

 

[curious]

Because they only twist one way (without regen), a lot of people simply bend them under one of the frame tubesso that the arm sits up against the frame.

How wide is the arm ? I may be able to bend it such that the arm is locked between chain stay and seat stay.

 

[Nimbuzz]

I use a hard tail, aluminum dropouts with a torque arm and Maxxis Hookworms 26 X 2.5 at 50 psi (can go up to 65). These tires give all the suspension I need on moderately lumpy roads going 30+ mph. With a hardtail I can use a sturdy supported Topeak rack for 20lbs of batteries, a laptop and a few sundries.

 

[getadirtbike]

I can make it 13mm and up. If I use wider material then you can cut, grind and drill it to suit yourself. If you bent it between the chain stay and the seat stay wouldn't it still have some room to move? You don' t want any movement whatsoever. I would make use of the bolts on the rack. It also looks a lot neater.

 

[Nimbuzz]

This one is bent a bit under the stay so it catches there then held in place with a P clamp. It is plasma cut out of SS with a similar design to the one under discussion except it is one piece.

 

[getadirtbike]

Yes, it's a similar thing, but with my ones I have a 6mm thick face on the flats of the axle. This saves the axle from wear. This also means that to plasma or laser cut one out of one piece you'd need to use the same 6mm thick material for the whole arm. It starts getting heavy, and harder to work with when you need to modify it for whatever purpose. I have done it that way before and it worked very well. But it also means that you have to send the plasma cutters a new 2d drawing each time you want to make a torque arm for a different bike. With the method that I use, I can mill out the slots from the round material, and then weld the arm on to whatever angle I need to. This means I can work from a photo and get it done accurately and relatively quickly.

 

[Nimbuzz]

I see, very well thought out -- and I see that the axle will wear less with your 6mm -- then again my axle barely gets out of the nut as it is.

 

[getadirtbike]

You must have an unusually short axle or a very thick dropout. There is usually room for a 6mm torque arm as well as the nut and then most of the time the axle still protrudes through the nut.