Topic: Structural integrity of Stock forks w/ out fender?

[Markcb750]

The only fault i can find is that front end is from a 75' 750F

[camelman]

Nikkisixx,

I like your drive to experiment.  You are definitely doing good work.  However, your experiment did not test the front end in a way that the fork brace was designed to help.  You tested to see how much a force on one leg would be transferred to the other leg in the vertical direction.  That isn't really the point of the brace.  The brace is to help resist the rotation of the front wheel with regards to the handlebars.  I don't have the space and tools to put together a test like yours...so... do you mind putting together a torsion test on that front end?

Thanks,
Camelman

[mlinder]

Hi Nikki.

First, let me say thanks for taking time out of your schedule to perform these tests.

Unfortunately, your results negate their validity.

Please allow me to explain:

Your first step was to measure a fender (brace) and it's properties.

While the results of the fenders rigidity were certainly nothing to write home about, they were measurable, and therefore, exist.

Which brings us to your later tests.

The fact that there was no difference in said tests requires that fenders have no qualities whatsoever, which you demonstrated as untrue in your first tests.

This means that either:

a.) Your testing method was flawed, or
b.) Your test sample was too small. In this case, it would be that you weren't influencing the subject enough to make a difference that your tools could read. Do you know how much force you were applying?

[scottly]

Well done, Nikksixx. Some had suggested that the fender brace increased rigidity in the planes you tested. Now we know.  

[mlinder]

The only fault i can find is that front end is from a 75' 750F

Please see above. A null difference with a real quantity added is impossible.

Also, a torsional test on the system was not performed, as far as I can see.

[scottly]

I rigged up a test today. With the forks turned to the left against the steering stop, I mounted an indicator to read the movement at the rear of the rim, and hooked a spring scale to the front. All readings are with 20 pounds pulling on the wheel.
No fender mount: .100"
Stock K7 mount with rubber bushings: .100"
K7 mount with bushings removed, sandwiched between washers: .080"
Aftermarket brace: .040"

[Markcb750]

The only fault i can find is that front end is from a 75' 750F

Please see above. A null difference with a real quantity added is impossible.

Also, a torsional test on the system was not performed, as far as I can see.

Only if the change is within the accuracy of the test equipment.

[mlinder]

Yep, saw yours, scottly.

I don't think Niki performed one, though.

The last test confuses me, though.
He compressed the forks less than 2 inches. This is far under the normal travel of the forks on a ride.

[mlinder]

The only fault i can find is that front end is from a 75' 750F

Please see above. A null difference with a real quantity added is impossible.

Also, a torsional test on the system was not performed, as far as I can see.

Only if the change is within the accuracy of the test equipment.

Or inaccuracy, or flawed test system.

[mlinder]

Off to bike-night, where I will not tell people it's ok to remove their fender while I help them.

[TwoTired]

Do you know how much force you were applying?

Yes.  For comparative analysis, one needs to know the amount of force needed for a certain deflection.  Not just unlimited force applied until an amount of deflection is obtained.

Without a strain gauge, I don't know how you can measure a hydraulic press's force applied.

I also agree that if you measure a part independently which clearly has some resistance to distortion, a "with and without test" suite must show some difference that it makes.  If the test is so insensitive that these differences can't be measured, then the test is too crude to demonstrate it's contribution.

Gotta say I'm impressed with your "go getter" and experimental attitude.  So, don't think my comments don't appreciate your effort.   Experiments are often tricky to set up and show the actual data you need exposed.  I've set up many that failed before getting one that bore fruit and meaningful results.  It was nice to be paid to do that sometimes frustrating and recursive endeavor.  To do it just for knowledge' sake is quite admirable.

Cheers,

[scottly]

The bottom line:
If you have a rigidly mounted, stock fender brace, keep it, with or without the stock fender attached.
If you have a rubber mounted brace, simply remove the rubber bushings and install some washers, and for less than a buck, improve the torsional rigidity (which is what counts) by 20%.

[Really?]

cb650PK I contacted the US rep for Tarozzi, and sent them the data they request for a new bike (my K6).  They cannot/will not adapt the brace to situations where the fender is more then 1 ½” above the lower fork top surface.

I am curious as to thr reasoning behind this.  Will it break?

[Markcb750]

cb650PK I contacted the US rep for Tarozzi, and sent them the data they request for a new bike (my K6).  They cannot/will not adapt the brace to situations where the fender is more then 1 ½” above the lower fork top surface.

I am curious as to thr reasoning behind this.  Will it break?

I asked if I cold make a spacer to each side to lift the crossbar above the fender, he assured me this would render the brace ineffective. 

I do not understand this butt who knows?

[scottly]

cb650PK I contacted the US rep for Tarozzi, and sent them the data they request for a new bike (my K6).  They cannot/will not adapt the brace to situations where the fender is more then 1 ½” above the lower fork top surface.

I am curious as to thr reasoning behind this.  Will it break?

I asked if I cold make a spacer to each side to lift the crossbar above the fender, he assured me this would render the brace ineffective. 

I do not understand this butt who knows?

I was thinking about the same thing: beefy spacer blocks between the clamps and the top plate. It would work, I think, with the type of brace I'm going to run. To fit a stock fender to the brace would be much more difficult, as the top plate would have to be machined to fit the curve of the underside of the fender.

[Goofaroo]

I'm sure he knows what he's talking about. It would be quite a puzzle to try to build something other than an arch design like the original that would add any extra strength. The fact that the fork tubes are shorter than the distance from the axle to the top of the tires make it hard to gain much functionality over the stock design. Even building one just like it but out of thicker material would only slightly increase the rigidity.

[Nikkisixx]

Nikkisixx,

I like your drive to experiment.  You are definitely doing good work.  However, your experiment did not test the front end in a way that the fork brace was designed to help.  You tested to see how much a force on one leg would be transferred to the other leg in the vertical direction.  That isn't really the point of the brace.  The brace is to help resist the rotation of the front wheel with regards to the handlebars.  I don't have the space and tools to put together a test like yours...so... do you mind putting together a torsion test on that front end?

Thanks,
Camelman

Couple of you wanted that info.  Didn't test it, here is why: if the front wheel is getting enough twist in that plane it will simply move the bars. If your handlebars are so tight that the axel twists in the saddles before the steering stem rotates you are a bad-ass.    The rider can't (and shouldn't) resist that movement.  

[scottly]

If anyone is interested, I can make some 1 1/2" spacer blocks and make the deflection measurements. (I won't take the time and effort unless someone really wants to know. While I'm curious, there are other things on my plate as well.  )

[MCRider]

"I was thinking about the same thing: beefy spacer blocks between the clamps and the top plate. It would work, I think, with the type of brace I'm going to run. To fit a stock fender to the brace would be much more difficult, as the top plate would have to be machined to fit the curve of the underside of the fender."



Well i don't have any science to prove anything but i assure you raising the top plate with spacer still yields a beefy piece of hardware. That's what i had done to mine. I bot a brace to use as the basis for the modifications. The machinist reduced the size of the holes of the clamps to clamp my puny fork tops, raised the center plate with 4 thick walled dowels. The fender is hung underneath. I like it and when it is all tightened up I dare anyone to twist it. Sure its not as strong as it would be without the spacers, but its darn strong.

[MCRider]

I'll get a closeup if anyone cares.

[Nikkisixx]

He compressed the forks less than 2 inches. This is far under the normal travel of the forks on a ride.

How do you know this?  Have you measured it?  I have:
http://forums.sohc4.net/index.php?topic=76383.msg851089#msg851089

The maximum I could get a stock setup to travel was 2.5 inches.  That is hitting a  speed bump head on.  After adding emulators, changing from ATF to 5wt fork oil, and upping the valving, I get 3.25.    

[scottly]

I'll get a closeup if anyone cares.

I would like to see a closeup. Thanks.

[MCRider]

He compressed the forks less than 2 inches. This is far under the normal travel of the forks on a ride.

How do you know this?  Have you measured it?  I have:
http://forums.sohc4.net/index.php?topic=76383.msg851089#msg851089

The maximum I could get a stock setup to travel was 2.5 inches.  That is hitting a  speed bump head on.  After adding emulators, changing from ATF to 5wt fork oil, and upping the valving, I get 3.25.    

Did you measure from full extension to your zip tie? I have but didn't write it down. Seems that way was more like 3.5 stock. But... I have no proof, sigh.

[MCRider]

I'll get a closeup if anyone cares.

I would like to see a closeup. Thanks.

Tomorrow.

[Retro Rocket]

I also like Nikki's efforts but in the real world they just don't stand up. The brace, simply because it is attached {without rubber washers} to both forks and has its own inherent strength, has to reduce movement, i don't understand why no one really understands this. While you are holding the bars straight on a ride and you hit an undulation, like a pothole you will get forces that will be very hard to simulate whilst stationary. Fibreglass fenders are commonly known to break where they join the forks, so we know that there is movement, adding a steel bracket of almost any type will help counter these forces, it has too because it adds resistance to those forces. I am a little amused that you guys are trying to prove otherwise..... I don't mind the testing at all though but just remember there are forces at work that you'll never test in a stationary situation like the gyroscopic effect of the front wheel, which changes with changes of speed...

Mick