Broken Titanium rear spring - 2

[super slim]

please see attached Autocad LT drawing

From this photo the CURVED point contact on the spring can be seen, with the fracture starting in the middle

Filing the radius in the Aluminium only took 30 mins, with this in the first stage before fine filing, bluing the old spring bending it around the radius, then filing off the high points, sanding, then painting it.

The rear suspension is smoother now!!! But I think it is my imagination.

Regards

Super Slim

 

[WhiteSilvio]

So my question for John T. is
"Should I be stripping the rear suspension assembly from my Silvio and filing the radii at either end of the titanium leaf spring clamping assemblies?"

I have had a close inspection, well a good look anyway, of the spring and I can't see any obvious cracking, but I have a to say that the spring has developed an extremely shallow "S" shape to it. I can see and agree that there is a possible stress raising feature on the top of the "wheel frame/fork", and that the other end could also induce stress, but across the entire width of the titanium leaf.

What's the go?

Regards,
John R.

 

[trapdoor2]

Interesting. I'm left wondering what sort of peak loads we're looking at...and whether these TI springs are suitable for *ahem* heavy use. :?

By creating the radiussed contact area, you are extending the unsupported spring length (just a bit). To retain "normal" ride height a thicker spring ought to be in order.

Are there varying thicknesses available? Perhaps a true set of leaves might help distribute the load a bit over the length of the spring.

 

[super slim]

The new spring is 1.54mm thick compared to 1.3 mm, so 66% stronger, but it is 55 Kg for 15 mm (max) deflection, where as the urethane spring (old design with two orange rubbers)is 150 Kg for 15 mm, measured using a lever and digital scales (as I am only a light weight!)

The spring is there to keep the rear fork from dropping, + sideways top alignment if the bike goes over a large bump and the urethane spring looses contact with the fork top. There must have be enough friction between the deformed bottom of the urethane spring and the titanium spring clamp with its 4 holes to stop sideways movement.

If you pick up the bike from the seat, without the titanium spring, the rear wheel stays on the ground, as the Carbon fibre lower strut was only 5 kg for 15 mm deflection.

I rode 40 km with NO titanium spring, so the Urethane did its normal springing and the bottom Carbon Fibre stay kept everything in alignment. I did

Its an incredible design that still works very well, with half of the suspension broken!!!!

 

[super slim]

When the Titanium spring broke, I checked out web sites about titanium, and titanium valve springs last 5 drags compared to ONE for spring steel, so much higher fatigue life

 

[trapdoor2]

Well...TI has a different modulus of elasticity than steel (among other things). Fatigue life depends on many parameters so I can only assume that "TI springs last 5 times longer than steel" just means that whoever designed them weighted their design towards a bit more fatigue life. Hard to make blanket comparisons.

Now that you've posted this stuff, I'm realizing that this component's task is different than I initally thought. It isn't so much a loaded suspension component as it is a position/location/movement component. That is, one could substitute a pivot-shackle (no "spring") and realize the same result. Actually, I imagine one could indeed somehow delete the TI leaf and sub some sort of internal locator for the rear suspension without altering the ride characteristics. The current design is pretty simple though and well thought-out.

If I were a weight-weenie, I'd be wondering if the leaf mounts (nuts, bolts, etc) could be deleted and a lighter system implemented. Tension/compression stress on that leaf are probably quite light (keeping the top of the rear wheel fork in place). I guess if I were a complete weight-weenie, I'd be deleting the suspension altogether...

Not gonna happen here. I want the suspension for all the designed reasons. I had to put off my planned Silvio purchase as we're in the midst of buying a new home...and I haven't sold my Catrike yet. Ah well...

edit: I neglected braking forces (partially due to my ignorance regarding the connection made thru the elastomers) in my thought process. If the braking forces are translated thru the elastomer mount to the seat frame, the spring may 'see' only minor compressive forces. However, if the elastomer does not wholly locate the top of the rear fork, braking forces are reacted by the TI spring via compression. I can see where the combination of a torsion load and braking might severely stress the spring at the point load. If the spring is reversing itself (during loading and unloading cycles, say, over a rough road) and braking is applied, I can see where one might possibly exceed the yield strength of the spring over time...fatigue cracking, etc.

 

[Mark B]

My leaf spring looks to be in fine shape, with about 5000 miles on it.

Mark

 

[trapdoor2]

Mark B wrote: My leaf spring looks to be in fine shape, with about 5000 miles on it.

That's the fun part. We may never know what truly caused Slim's spring to fail. Might have been a material defect, might have been a hidden manufacturing defect (surface scratch, etc.). With such a low manufacturing quantity (how many Silvio's are on the road today?) it is very hard to make a determination.

Since you're 'mass enhanced' (though lighter than you were), I feel better that you've had no troubles. :mrgreen:

 

[Mark B]

trapdoor2 wrote:

Mark B wrote: My leaf spring looks to be in fine shape, with about 5000 miles on it.

That's the fun part. We may never know what truly caused Slim's spring to fail. Might have been a material defect, might have been a hidden manufacturing defect (surface scratch, etc.). With such a low manufacturing quantity (how many Silvio's are on the road today?) it is very hard to make a determination.

Since you're 'mass enhanced' (though lighter than you were), I feel better that you've had no troubles. :mrgreen:

My other observation is; the broken leaf in question looks pretty raspy... Even rusty? My spring is still the nice, dull color you expect from ti.

 

[Mark B]

The more I look at that picture, the more troubled I am. That thing is rusty and ti does not rust. Look at it closely and you can see pits in the metal. Something is not right here.

Mark

 

[super slim]

That is not rust, but dirt as it was raining heavily when I did the ride, and there was road works with clay and gravel we had to ride through

 

[defjack]

Just checked spring on my bike and it looks like new after 5000 miles. Jack

 

[Flasharry]

My rear spring broke today :cry: (older thinner version).
How or where can I get a replacement?
Many thanks.
Stuart.
Surrey, UK.

 

[John Tolhurst]

Flasharry wrote: My rear spring broke today :cry: (older thinner version).
How or where can I get a replacement?
Many thanks.
Stuart.
Surrey, UK.

Hi Flasharry,
Can you please accurately measure with verniers the thickness of the Ti plate and please email Maria http://www.cruzbike.com/founders.html with a photo.
Thanks.
John Tolhurst

 

[WhiteSilvio]

Flasharry wrote: My rear spring broke today :cry: (older thinner version).
How or where can I get a replacement?
Many thanks.
Stuart.
Surrey, UK.

Hi Stuart,
I'm curious to know a little more about the failure of this "spring".
I have "de-stressed" my Silvio spring by removing what I, and others, consider to be stress concentrator on the top of the rear wheel frame. And even at the mount on the frame side, although this is a line contact rather that a point contact (in simplistic terms).
SuperSlim's modification
Did your spring break at the point where SuperSlim's did?
Regard,
John R.

 

[Flasharry]

My spring broke in the same place as SuperSlim's, above the rear forks.

Originally my Silvio came with the 2 elastomers for the rear suspension and they were just too hard for the roads round here, so i bought 2 bump stops (before Cruzbike made them standard), one a bit taller than the other.

Having ridden for a while on the taller bump stop, the leaf spring set to a slight 'S' shape, then I changed to the shorter bump stop (which I have used ever since) and set the 'S' shape to a reversed 'S' shape.

So all in all the spring probably got over bent plus i probably should have radiused the edge of the fork and I didn't. Hence the problem.

Here is a pic of the broken spring, still on the bike, with a crude vernier showing it's thickness. (Seems to be 1mm). (Also sent to Maria. Thanks John.)

 

[WhiteSilvio]

Flasharry wrote: My spring broke in the same place as SuperSlim's, above the rear forks.

Hi Stuart,
Thanks for the info. I'm sure this will provide some field data for John T.

Just out of interest I measured the thickness of the spring and got 1.35 +/-0.05mm
I'm wondering of a spring failure is imminent? I have to say I have felt more comfortable since I radius-ed the rear frame, but if a modification or replacement is necessary I wouldn't be unhappy.
Regards,
John R.

 

[bubble]

On a ride today my rear titanium spring broke same place as the other's have broken, above the rear forks. I was not able to ride the Silvio after the break. I have around 4,000 miles on the bike.

John N

 

[JO_CONNOR]

My rear Ti leaf spring broke this past weekend in the same manner as depicted in these and similar threads. As I am still riding a S1.0 and this thread has been around for some time with no recent incidents reported, does anyone know if the problem has been fixed? If so, how can I find out the retrofit, or improved Ti leaf or whatever? Enquiring minds want to know, really.

Thanks.

 

[super slim]

I broke 2 off 1.5 mm thick rear titanium plate springs when experimenting with a 25 mm travel air bag, to replace the 12 mm travel Poly Urethane spring, and replaced it with a 0.47 mm 316 SS plate spring.
That was 2 years ago and still going strong. The edges of the 0.47 ss need to be polished smooth with 500 grade Emery to stop fractures.