Author Topic: Structural integrity of Stock forks w/ out fender?  (Read 46807 times)

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Offline Goofaroo

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #125 on: January 18, 2011, 01:59:32 PM »
Quote
I would love to see the responses in that section Mlinder, i feel compelled to answer here so some of these lesser informed guys don't make stupid changes to their bikes, some of the info in these threads is just plain dangerous....



I'm a bit puzzled about this myself and I understand your frustration but it seems like a losing battle. I hesitate to post this link because it is clearly on a modern race bike that no doubt has a superior frame and suspension to our old Hondas so it will be open for debate but the message is that without the factory brace or a suitable replacement, the bike will have a tremendous amount less resistence to doing this:


Sorry I don't know how to paste the video directly to the thread.


Offline Goofaroo

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #126 on: January 18, 2011, 02:03:07 PM »
Apparently I do know how to post the video to the thread.  :)

Offline Retro Rocket

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #127 on: January 18, 2011, 02:08:22 PM »
Goofaroo, the tank slapper i referred to in my post was about 5 time faster {the handle bar oscillations} and i had sore muscles for days, the thing that saved me was hitting the shoulder and riding it out in the dirt, i figured afterwards that the front tyre was getting no grip in the dirt and allowed me to gain control of the bike, had to check the underware to make sure control was maintained all round.... ;)

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Offline Really?

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #128 on: January 18, 2011, 02:18:39 PM »
I have had a tank slapper put me down.  SCARY CHIT!

I have learned a lot from this thread and appreciate it.  I for one will always have a fork brace, upgraded if possible.  I am not running the stock fork brace on my Venture, it is a superbrace.  Warbles in a high speed sweeper suk too.  Fixed that!

So, I have been further informed and glad I have been reading this thread!
I don't have a motorcycle, sold it ('85 Yamaha Venture Royale).  Haven't had a CB750 for over 40 years.

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Offline Goofaroo

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #129 on: January 18, 2011, 02:23:11 PM »
I've been there a few times myself but I've always managed to just flail around a bit and then recover-luckily. I've slowed down my street riding quite a bit as I've gotten older but I still find myself slipping back into old habits from time to time. Luckily my taste in bikes has matured with me so I'm not exposed to as much power as in my younger days.  

Offline Retro Rocket

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #130 on: January 18, 2011, 02:39:59 PM »
Yeah, i still get the urge to be a rat occasionally but self preservation has out grown my bravado, {thats a good thing i think} ;D
The single biggest improvement you can do on these bikes without bracing everything has to be a steering damper, no more tank slappers and improves stability immensely....Oh ..and always buy the best tyres you can afford...

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Offline MidnightLamp

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #131 on: January 18, 2011, 03:14:48 PM »
 :-\ This topic is rather strange.

I've ridden a few bikes here or there now, and I've got to say, most anything that was a production bike in this era and even after had spaghetti forks up front. The bigger the bikes got, the scarier they were to drive. IMHO the CB750 solidly falls under that class of "flex like you wouldn't believe" bikes. The CBX was actually a lot worse though  ::)

I think through all of this, it can be accepted (at least empirically) that A REAL FORK BRACE, offers a substantial improvement to the bike. As does a GL front end, due to the larger diameter tubes (which actually are not that much bigger internally, but mainly thicker wall). Other things such are boring for a bigger axle, steering dampers, and horseshoe braces really help on these old bikes.

As for the idea of the stock fender brace? As an Mech Eng, it's my opinion that the rubber mounted fender mount will do next to nothing nothing. I'll also say that the stock solid mounted fender brace would not provide a significant amount of stiffening. When you think honestly about the cornering forces these bikes endure and where the primary areas of twist are, it should be fairly apparent that the bending stiffness of the stock solid mounted brace is not significant (in all by one primary direction, the brace provides completely negligable stiffness). Also, there is no triangulation in any technical sense going on with the braces.

Now, as for the effects of the brace, this is where it gets interesting. It does have a notable effect. This was noted by many people (as can be seen in Hondaman's post) and from generally riding with or without a brace on most bikes with a similar front end. The fact of the matter is that when you have forks up front that are bendy like you wouldn't believe, a little means a lot. You have to consider how little stability is there in the older geometry and construction of the front end, and how fine a line it is between cornering hard and going down hard. Every little bit you have in terms of stiffness on these front ends is rather significant, and I wouldn't doubt that the brace makes a large difference in handling. Imagine yourself going into a corner and you throw yourself in slightly too hard, causing you to cut your available front tractive forces down to the limit. In this situation, even a slight oscillation of the fork would have a large effect, and may even cause you to loose what you have left in terms of tractive forces. All it takes is a fraction of a second of stability loss to cause a bike to try and buck you off. It's impossible (or close to it) to model how these old bikes respond when being pushed, and it was done partially from a design standpoint, but also partially through trial and error on many of these older designs. The amount of flex in each bike might even have been different enough that you can remove the brace on one bike and have an almost unnoticeable effect, but if you remove it off another it's night and day.  :o

For the honest truth though, The brace is really nothing special to make, and a stiffer than stock brace could easily be made using no more than basic hand tools. Even an older horseshoe style brace could be made at home and from parts from your local hardware/plumbing store. Arguing whether or not the stock brace is effective is probably moot, because FWIW, a proper brace is something that the bike could definitely benefit from anyways, and in the time it takes to argue, you could just make a decent brace  ;D

Hope that didn't offend anyone, and I just offer this as my $0.02 (canadian of course).

Cheers

Disclaimer The contents of this post are purely informally written, and should be taken with caution. No opinions expressed above should be used to remove or modify any suspensions components without proper knowledge and guidance. Read at your own risk!
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Offline mlinder

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #132 on: January 18, 2011, 03:20:11 PM »
"I'll also say that the stock solid mounted fender brace would not provide a significant amount of stiffening. When you think honestly about the cornering forces these bikes endure and where the primary areas of twist are, it should be fairly apparent that the bending stiffness of the stock solid mounted brace is not significant (in all by one primary direction, the brace provides completely negligable stiffness). Also, there is no triangulation in any technical sense going on with the braces.

Now, as for the effects of the brace, this is where it gets interesting. It does have a notable effect. This was noted by many people (as can be seen in Hondaman's post) and from generally riding with or without a brace on most bikes with a similar front end. The fact of the matter is that when you have forks up front that are bendy like you wouldn't believe, a little means a lot. You have to consider how little stability is there in the older geometry and construction of the front end, and how fine a line it is between cornering hard and going down hard. Every little bit you have in terms of stiffness on these front ends is rather significant, and I wouldn't doubt that the brace makes a large difference in handling. "


I can't tell if you were contradicting yourself or not.

/edit: I did make a mistake in triangulation, however. Fixing.
« Last Edit: January 18, 2011, 03:35:50 PM by mlinder »
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Offline Retro Rocket

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #133 on: January 18, 2011, 03:22:26 PM »
Good post midnight...
These old bikes can be made to handle well, but it does take some time, money and knowledge, but it is definitely possible.... ;)

You were talking about flex, ever ride a H2 750 Kawasaki hard.....Scary stuff.... :o

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Offline Really?

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #134 on: January 18, 2011, 03:32:34 PM »

You were talking about flex, ever ride a H2 750 Kawasaki hard.....Scary stuff.... :o

Mick

That is what I got the tank slapper on.  Just bought the bike and made it about 200 yards on the freeway, it started its thing at about 80mph.  I was about 15 years old!  Boy that bike was crazy!  I miss it too!
I don't have a motorcycle, sold it ('85 Yamaha Venture Royale).  Haven't had a CB750 for over 40 years.

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #135 on: January 18, 2011, 03:37:20 PM »

You were talking about flex, ever ride a H2 750 Kawasaki hard.....Scary stuff.... :o

Mick

That is what I got the tank slapper on.  Just bought the bike and made it about 200 yards on the freeway, it started its thing at about 80mph.  I was about 15 years old!  Boy that bike was crazy!  I miss it too!

Hi Tipper, the best way on those things to avoid that was to open the throttle, only one wheel to worry about then.... ;D

Mick
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Re: Structural integrity of Stock forks w/ out fender?
« Reply #136 on: January 18, 2011, 03:38:12 PM »
Please see figure B :/

Christ.

Yes, I saw figure B


Again, how is this not a rectangle?  The lower tree forms the top of the rectangle and the axle forms the bottom.  Or better yet 2 rectangles exist: axle to brace and brace to lower tree.  In order to turn the rectangles into triangles you would need a diagonal piece inside the rectangles.

The fork brace that ties both lower trees together solidly works becuase it reduces the length of the sides of the rectangle which in turn requires more force to distort.  This is the same principle used in gusseting.  Their is no triangulation going on here.  This type of brace only works when it does not pinch or push apart the lower legs and is a rigid structure that withstands deflection.

But this thread was not discussing Tarozzi style fork braces but the effect of the stock front fender mount as a brace.  The first thing going for it and the only thing is that it ties the lower legs together just like a brace would.  What doesn't the fender mount do?
1. It does not withstand deflection well (see nikkisix' tests)
2. It does not tie the lower legs together in a line parallel to the lower tree
3. It niether holds the lower legs from pinching or pushing apart
4. On some models it isn't even solidly mounted to the lower legs

These 4 things make the fender mount not a brace but a mount for the front fender.

Some issues with all telescopic forks:


The lower legs and tubes act as a lever and allow a fair amount of distorion when the tire encounters bumps.  A fork brace like the Tarozzi style can limit the amount of lever action by stiffening the lower part of the rectangle somewhat.  This is good?  Not always.  The less deflection that the forks can absorb the more deflection is put on the steering head and frame.  When you look at an effective steering brace and then look at a stock fender mount the differences should be obvious.

Scott

Offline mlinder

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #137 on: January 18, 2011, 03:55:19 PM »
Yeah, I fixed B.
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Offline Retro Rocket

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #138 on: January 18, 2011, 04:31:15 PM »
Quote
These 4 things make the fender mount not a brace but a mount for the front fender.

Once again, when these bikes were built the fender mount was indeed a brace, even if that was not the intention, removing it reveals this.
You are correct that the fork brace adds stress to the frame and thats why i brace the frame, but you will feel the improvements in handling way before the stress effects the frame to the degree you suggest, the frame stress mainly occurs under more spirited operation.
Nikki's test, while it looks good and does show that the brace isn't brilliant, {you only have to look at it to see that} is pretty irrelevant when compared to what happens on the street...The stock fender mount stiffens the front suspension and nothing anyone here can say will detract form that. I have 43mm forks {right side up} on my Honda and am still using a brace, always have , always will...
Sorry Srook, it sounds like we have you on the defensive because you aren't using a brace and are trying your best to justify it, my experience tells me otherwise...

Quote
The first thing going for it and the only thing is that it ties the lower legs together just like a brace would.  What doesn't the fender mount do?

Exactly the same thing only not as effective....

Mick
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Offline MidnightLamp

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #139 on: January 18, 2011, 04:34:25 PM »
The lower legs and tubes act as a lever and allow a fair amount of distorion when the tire encounters bumps.  A fork brace like the Tarozzi style can limit the amount of lever action by stiffening the lower part of the rectangle somewhat.  This is good?  Not always.  The less deflection that the forks can absorb the more deflection is put on the steering head and frame.  When you look at an effective steering brace and then look at a stock fender mount the differences should be obvious.

This is very true. Now on the contrary, having something there may not appreciably improve handling. I'd say the difference in handling when just putting the bike around would not be all that significant (if your definition of putting around does not involve scraping the sidecovers off), however taking the thing for a ride I'd suspect it'd the instabilities would become apparent. Even the effect of slightly adjusting the resonant response of the front end may have some unforeseeable positive influence. My point was that a solid mounted "brace" style mount is either a band-aid or a lucky fluke, because there is no way any competent designer would used such a brace as a structural element, but it somehow does seem to make a difference, and in a loop de loop way, the spaghetti forks seem to actually benefit from it. I think regardless of the testing or theory, if it makes a noticeable difference...it makes a noticeable difference.

You were talking about flex, ever ride a H2 750 Kawasaki hard.....Scary stuff.... :o

Thanks! And I haven't ridden an H2, but had an H1 for a while. I imagine making something heavier, more powerful, and with 70's suspension is a poor life choice.  :P

I can't tell if you were contradicting yourself or not.

Didn't intend to, meant to point out that even though the stiffness is hardly anything to write home about, little bits do can do a lot of good. Big things however will do exponentially more!  ;D
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Offline MCRider

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #140 on: January 18, 2011, 04:42:09 PM »
Its not an issue of competency with the designers. They are restricted by budget and appearance. The fender mount doubling as a fork brace does some good and will satisfy 95% of the potential buyers. The other 5% will do something better.
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Re: Structural integrity of Stock forks w/ out fender?
« Reply #141 on: January 18, 2011, 08:08:32 PM »
Honda engineers are not dumb..
 If they seriously considered this to be more brace than primarily fender mount, they would not have used 6mm bolts.. and the moun/brace would be thicker..

 I rest my case..
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Offline FrankenFrankenstuff

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #142 on: January 18, 2011, 08:21:22 PM »
 8) Mick and 754 have spoken. This thread may now be deleted. 8)
WAIT!...I am running a USD fork on mine, with a Vrod flimsy arse aluminum front fender....should I make a fork brace??
« Last Edit: January 18, 2011, 08:23:34 PM by Frankenstuff »

Offline scottly

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #143 on: January 18, 2011, 08:37:04 PM »
Honda engineers are not dumb..
 If they seriously considered this to be more brace than primarily fender mount, they would not have used 6mm bolts.. and the moun/brace would be thicker..

 I rest my case..
And they wouldn't have mounted it with rubber bushings  ;). The fender mount, while may be of some use resisting twisting of the fork tubes, is no where near as effective as a proper brace.
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Offline 754

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #144 on: January 18, 2011, 08:46:13 PM »
That mystery bRace is probably an ECLI.. at anyrate he made a complete fork incorporating that design.. looks very stiff.
 The drawback to fitting that to Honda forks, you have to machine them to fit that brace..
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Re: Structural integrity of Stock forks w/ out fender?
« Reply #145 on: January 18, 2011, 08:47:03 PM »
Honda engineers are not dumb..
 If they seriously considered this to be more brace than primarily fender mount, they would not have used 6mm bolts.. and the moun/brace would be thicker..

 I rest my case..
And they wouldn't have mounted it with rubber bushings  ;). The fender mount, while may be of some use resisting twisting of the fork tubes, is no where near as effective as a proper brace.

Thats correct, our point is that there is more flex without it and there is..... ;)

Mick
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Re: Structural integrity of Stock forks w/ out fender?
« Reply #146 on: January 18, 2011, 09:00:09 PM »
Dang you Mick, now I have to prove it to myself! >:( If I can find the time, I will conduct a twist test with/without a rubber mounted fender mount, like Frank suggested.
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Re: Structural integrity of Stock forks w/ out fender?
« Reply #147 on: January 18, 2011, 09:05:34 PM »
Its fairly easy mate, if you have a reasonably well set up bike that you are willing to ride hard, then try it with and without the front fender, just watch undulations on the road when braking into a corner, and don't hurt yourself mate.... ;) You will feel it flex more, especially if you have twin discs.... In saying that, i used to ride a little more aggressively than i do now and had a MAJOR scare without a front fender as i described earlier, Take the front fender off and put the front wheel between your legs and twist the bars, its even evident then....  ;)

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Offline Really?

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Re: Structural integrity of Stock forks w/ out fender?
« Reply #148 on: January 18, 2011, 09:25:00 PM »
So, will someone please make a fork brace for us folks that only have the stock option?  Will all this debate, the odds that the brace will just what we all need and can use.

@ Not Frank, if you do it, make sure it is not plastic!  ;D
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Re: Structural integrity of Stock forks w/ out fender?
« Reply #149 on: January 18, 2011, 09:42:59 PM »
Its fairly easy mate, if you have a reasonably well set up bike that you are willing to ride hard, then try it with and without the front fender, just watch undulations on the road when braking into a corner, and don't hurt yourself mate.... ;)
Mick
I was thinking more about a test using dial indicators and a spring scale, to measure deflection with force. I've been through one very scary tank-slapper before, and don't care to risk another, in the name of science! :D
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