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

[Goofaroo]

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.

Maybe I'm confused. From the picture it appears that your bike is a later model that has the longer fork tubes. I would guess that there are numerous aftermarket braces available to fit those forks and I'm sure that your setup with the factory brace and the aftermarket brace is solid as a rock. My understanding was that some forum members were trying to figure out a solution that would be more rigid on the old style forks with the shorter fork lowers and wheel clearance issues.

[Nikkisixx]

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.
[/quote]
My forks compress about 1/4 inch from full extension with the weight of the bike.  I added that to the overall measurement.  If you have a measurement I'm still curious what numbers other people are getting.  

[MCRider]

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

This source has front end travel at 5.6"! I think that's a little optimistic and may be without internal hardware.    

Nevertheless its definitely more than 2, or 3, or maybe 4.

http://www.sohc4.net/index.php/archives/85

[MCRider]

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.

Maybe I'm confused. From the picture it appears that your bike is a later model that has the longer fork tubes. I would guess that there are numerous aftermarket braces available to fit those forks and I'm sure that your setup with the factory brace and the aftermarket brace is solid as a rock. My understanding was that some forum members were trying to figure out a solution that would be more rigid on the old style forks with the shorter fork lowers and wheel clearance issues.

That's the beauty of the machinists art. The clamps are down low, the tops of the legs are below the top of the tire, you can see the aluminum dowels that are lifting the center plate, even with an 18" wheel (conversion). The machinist then put an extension on top of the clamps to hold the dust covers. From a distance it looks for all the world as you have noticed.

The factory brace has been removed. The fender is held to the bottom of the braces cross piece.

The close up tomorrow will show the detail.

If rather than dowels he had made a piece to go between the 2 bolts, one for each side, with its mating surface the whole width of the clamp and cross piece, it would be way strong. But we took the quick and easy way out. i can always have it done later.

[Nikkisixx]

WOW!     5.6 doesn't seem right.  Is there even that much room between the fender brace and the lower clamp?

[MCRider]

WOW!     5.6 doesn't seem right.  Is there even that much room between the fender brace and the lower clamp?

Agreed. I did a google on CB750 Fork travel and that was the first hit. But still its more like 3.5 to 4 in my mind.

[Retro Rocket]

http://www.motorcyclespecs.co.za/model/Honda/honda_cb750k%2074.htm

143mm or 5.629 inches of travel for a 750K.

Mick

[Nikkisixx]

Yeah Mick, but that must be the max possible travel? 

[scottly]

[quote author=Retro Rocket link=topic=82347.msg928355#msg928355 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
[/quote]
The purpose of my test was to determine the amount of twist in the forks, given a fixed load, with various bracing. It really doesn't matter if the force is coming from the gyro effect on not. By the way, when I had the tank-slapper, is was not at high speed per se, maybe 60 MPH tops, probably less, nor during hard cornering. It happened when the front wheel touched down after a wheelie. Maybe I landed with the front wheel slightly turned? Whatever caused the start of the very rapid oscillations (as in your experience), the feeling was that the front end had turned into a spring connection between the handle bars and the wheel. This happened with a K1 fork WITH NO fender brace. It was the main reason I fit the K7 fork in the first place, and while I never had a tank-slapper, it still felt rubbery, no doubt due to the rubber mounted K7 brace.  

[Retro Rocket]

There are gyroscopic forces involved with the wheelie as well which adds load, the wheel slows when the front end is lifted, as it touches down it instantly comes back to speed, twisting at the same time and instantly increasing the effect....Also there is less resistance to the gyro effect whilst the wheel is airborne because the ground or road isn't making contact with the wheel which also helps keep the wheel straight....I have seen hundreds of bikes shake while coming down from a wheelie, even braced, actually i have seen it at races many times on high performance machines braced and set up way better than our bikes, i think the wheelie head shake is a different thing to what i have experienced.

Mick

[scottly]

Well, do you agree that increasing the torsional resistance of the front end is a good thing, how ever it is measured?

[Retro Rocket]

Well, do you agree that increasing the torsional resistance of the front end is a good thing, how ever it is measured?

Of course.....
I hope no one thinks this is an argument, it is an interesting discussion, i just think some answers are a bit simplistic....


Mick

[mlinder]

The measurement between the lines on my forks caused by leaky seals is over 4 inches. (I do believe the 5+ inches is correct for total travel available. The 'long travel' forks of the vey nice riding bmw /5's was ove 6", if I remember correctly. They did have a smooth ride.)

Anyway, my earlier arguments against Nikkis tests still stand.

Null difference with non-null variable = impossible unless erroneous testing or tested variables too small for detection under testing conditions.

And, one of the most important tests untested (torsional), except where they were tested under different circumstances in a different environment by different personel which showed improved characterisitcs.

Sorry Nikki.

Where are you in NC? Maybe I can go visit a friend of mine there, drop by, and we can conduct proper tests with your equipment?

My friend is in Ashville. Are you nearby?

[mlinder]

BMW f650 has around 7" of travel

[mlinder]

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

Flawed argument. Have you gone through a chicane at triple didgit speeds? Or even at just a mildly brisk pace? The bars will fight you. You must apply real pressure to get the bike to flip 110+ degrees in a hurry.

[mlinder]

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.    

Perhaps you were riding uphill at a 35 degree angle, sitting behind the rear shock mounts, weigh 110 lbs, were going 7 mph and the speed bump was a gradual 10 meter long, 2 inch high convexity?

[Retro Rocket]

You want to test the front fork travel then use the front brakes, i have had to use preloaders on a few bikes to stop them bottoming out.

Mick

[MCRider]

I'll get a closeup if anyone cares.

I would like to see a closeup. Thanks.

Basic front view. Where my finger is pointing is the clamp level of the brace and the top of the fork leg. Extending that across you can see it wouldn't even clear the tire and that's an 18" reduced from stock 19".

This much was added to the top of the clamp portion (more than was necessary) and has a groove in it to hold the dust cover.

This much would need to be added to the dowel's height if you wanted to clear a 19" wheel:


I was just randomly buying braces off of eBay, usually $20 to $40. This one may have been for a CB400 IIRC. None of them had clamps small enough to grip the tiny diameter of the origianl CB750 fork leg. The machinist inserted a tube into the clamp, made to the proper size, welded it in place, then cut it apart so it would be part of the clamp. That was probably the hardest part. Raising the center bridge was easy.

This allowed me to hang the front part of the CB350F fender (18") from the crosspiece eliminating excess hardware.

[stueveone]

Just installed my new fork brace. If nothing else, it sure looks cool. . .

stueve

[somesuch]

Any idea how accurate that statement in red is?

Accurate, it should actually be stiffer in my opinion. You have one disc for a start so centrifugal force is trying to twist the front end without braking. If your front end gets a wobble it can and will spit you off because there is no brace to hold everything tight. I NEVER ride without some sort of front brace and more often than not i use a better brace.I have had some really bad "tank slappers" with the standard brace, would not want to try and control an un-braced one. You will get a lot of guys say that you don;t need one but they aren't paying for your medical........

Mick

100% agree, feels like wet noodles without one, I usually add some sort of aftermarket brace. The billet ones from Tkat.com are great.

--Nick

[somesuch]

if its a brace its a flimsy one,i can bend them with one hand,the axle is clamped fairly solid,the trees are fairly solid,so a piece of tin half way down is going to make or break you?

the axle diameter is very small and it is a two-piece....all the modern stuff is much larger diameter.

Just stand in front of the bike, put the front wheel between your knees and try to move the bars...you'll see how easy it is to deflect it...now try with a brace....much better....better yet with a nice billet one.....you can actually measure the force and the deflection with some ingenuity...Google the article on the CBX that was raced in a super dinosaur class in AFM some years ago. The guys did a great job of quantifying the flex.


--Nick

[somesuch]

Got glossed over in the heat of the moment I'm afraid!
Does anybody know if an early style, k1 (non-rubber grommet) fender brace will work on later applications, ie my K4?
And I haven't been able to make out if there is an aftermarket fork brace available for early K bikes?

The bolt pattern changed on the fender bracket somewhere between K1 and K3 so its not a slam dunk.

Also, there is no aftemarket fork brace for the earliest of CB750s. (That I know of, and I've looked) The diameter of the top of the fork leg was smaller for a while up to K2 or so. The top of the leg was lower than the top of the wheel/tire assy. So no brace.

I had one made for mine by starting with a 5 piece brace from ebay (2-2pc clamps and a bridge between the 2 clamps.)  The machinist shimmed down the clamps to fit my leg tops, and raised the center bridge to clear the tire. Not cheap.

Then the diameter of the top of the leg increased to accomodate a larger seal. Still the tops were below the tire top, so the brace had to have an arch in it to clear the tire. Braces for these are available from somewhere overseas for about $200+ and a very long wait.

Then, the legs got longer, above the top of the tire, and a brace for them is still available for about $100 from Tarrozi and such.

The guy at tkat.com made them, you just used the dimension sheet from his website to give him dimension, and he would make to that spec.....not too expensive either. Great quality.

--Nick

[stueveone]

Ok, they definitely aren't just cool looking. . .
The difference in handling with a fork brace and without fork brace is DRAMATIC!!! My front end no longer wanders when I come to a parallel seam in the road or uneven pavement. The responsiveness and tighter feeling of flicking the handlebars is greatly improved. So much more road feel and quickness. Feels like the bike doesn't have a rear wheel but turns like a unicycle!
Really great upgrade. I can't wait to pair this up with a new set of shocks and rebuilt swing arm!

[MCRider]

Got glossed over in the heat of the moment I'm afraid!
Does anybody know if an early style, k1 (non-rubber grommet) fender brace will work on later applications, ie my K4?
And I haven't been able to make out if there is an aftermarket fork brace available for early K bikes?

The bolt pattern changed on the fender bracket somewhere between K1 and K3 so its not a slam dunk.

Also, there is no aftemarket fork brace for the earliest of CB750s. (That I know of, and I've looked) The diameter of the top of the fork leg was smaller for a while up to K2 or so. The top of the leg was lower than the top of the wheel/tire assy. So no brace.

I had one made for mine by starting with a 5 piece brace from ebay (2-2pc clamps and a bridge between the 2 clamps.)  The machinist shimmed down the clamps to fit my leg tops, and raised the center bridge to clear the tire. Not cheap.

Then the diameter of the top of the leg increased to accomodate a larger seal. Still the tops were below the tire top, so the brace had to have an arch in it to clear the tire. Braces for these are available from somewhere overseas for about $200+ and a very long wait.

Then, the legs got longer, above the top of the tire, and a brace for them is still available for about $100 from Tarrozi and such.

The guy at tkat.com made them, you just used the dimension sheet from his website to give him dimension, and he would make to that spec.....not too expensive either. Great quality.

--Nick

Nice find. That's essentially the design my machinist came up with using 5 pieces. The bridge being spaced up to clear the fender. I wonder what he charges. Too bad i already have mine. His looks real beefy.

[Retro Rocket]

Ok, they definitely aren't just cool looking. . .
The difference in handling with a fork brace and without fork brace is DRAMATIC!!! My front end no longer wanders when I come to a parallel seam in the road or uneven pavement. The responsiveness and tighter feeling of flicking the handlebars is greatly improved. So much more road feel and quickness. Feels like the bike doesn't have a rear wheel but turns like a unicycle!
Really great upgrade. I can't wait to pair this up with a new set of shocks and rebuilt swing arm!

Good stuff mate, they definitely work and all those that say they don't either think they are fast and really aren't or just don't know what they are talking about. Fork brace and steering dampers make massive differences to these old bikes.... You would be surprised to find out how many guys don't actually know that you counter steer going into a corner, you don't turn in you turn the bars the opposite way to the corner.....

Mick