Upside down forks-I donít get it! - Stromtrooper Forum : Suzuki V-Strom Motorcycle Forums
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post #1 of 15 Old 10-09-2018, 12:07 PM Thread Starter
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Upside down forks-I donít get it!

I know itís not V-Strom directly, but I really need someone to help explain the benefits of upside down forks.
My Strom has a fork brace which helps to give extra rigidity 1/2 way up the forks. I can see the logic in this because otherwise thereís a long travel end-to-end with only the internals holding everything in line!
But on upside down setup this isnít possible! At the wheel hub there needs to be quite a sizable chunk of metal to keep the forks in line.
I used to assume the upside down advantage was just in unsprung weight, but this extra lump of metal at the hub must offset a large portion of unsprung weight advantage.
So can someone explain..... what are the benefits??


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post #2 of 15 Old 10-09-2018, 12:27 PM
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post #3 of 15 Old 10-09-2018, 02:18 PM
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In laymans terms what you are overlooking is the fact that the "upside down" forks don't need a fork brace. It is pretty much built in. Look at the tubing USD forks are made of. It is very large diameter. The clamps that hold them have a much easier job holding large diameter tubing. While this tubing is thin, it is very strong due to larger diameter. This larger diameter is considerably less apt to twist under load. The same loads the regular forks move easily against.

Why not simply increase the tubing diameter on regular forks? Then you get into stiction problems with seals.
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post #4 of 15 Old 10-09-2018, 03:03 PM
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FBD

See the attached diagram. On the left are normal fork and on the right are inverted forks. To the right of each graphic is a (crude) free body diagram (FBD). Schematically, on the normal forks a large mass is at the end of a long thin shaft so when braking or turning forces from the wheel are transmitted to the thin shaft it wobbles and moves about (uncontrolled) which is bad for handling. If you invert the shock then you move a lot of the weight from the bottom to the top of the shock/shaft and (effectively) make the thin portion of the shaft shorter with less weight on the end. In the USD shock the FBD shows how the smaller weight at the end of a short shaft is easier to control and thus improves handling.

The shock brace that many DL riders use is a way to stiffen the thin shaft in the FBD on the left and thus reducing uncontrolled wheel movement.
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post #5 of 15 Old 10-09-2018, 08:13 PM Thread Starter
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Thanks for the feedback guys. Interesting topic!


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post #6 of 15 Old 10-10-2018, 06:59 AM
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Ben Spies ran fork brace on his USD GXSR in AMA and Marzocchi had design with hex axle to tie-up lowers and reenforce them so it isn't that it can't be done just no need mere mortals.

Thing is that resistance to bend proportional to power of 4, so small 10% increase in diameter yields 46% increase in stiffness. Even increasing fork diameter from 41 to 43mm increases stiffness by 21%.

Also most of the flex on USD forks comes from seals/bushings brace wouldn't do much about that.

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Last edited by cyclopathic; 10-10-2018 at 09:07 AM.
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post #7 of 15 Old 10-10-2018, 10:54 AM
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Jay, Sasquatch of Adventure Power Sports, explained or at least talked about the different forks at a Cache Rally a few years ago.
Memory is faulty but the gist was there wasn't much benefit of Upside Down over conventional forks. Maybe more of a fashion statement as he saw it.
I do know that the Smaller diameter portion of the fork is going to do all the flexing. Flex it high or flex it low, it's still gonna wiggle like a worm if viewed in slow motion.
A stout larger diameter fork tube will be of more benefit that one design over the other. Then you start talking about weight of material and weight is the enemy, at least on race bike.
That should be less of a concern for a street/touring machine. You could have a bit heavier bike and YOU lose the 10-15 pounds of fat to keep the weight down.
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post #8 of 15 Old 10-10-2018, 11:24 AM
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It's pretty simple. USD forks have the thickest, strongest part of the forks where the bending moments are most concentrated, at the triple clamps, and generally the upper half of the forks. While the sliders may not be the heaviest parts (despite their large size they are fairly thin-walled and made of light alloy, as opposed to the heavier steel of the stanchion tubes), the design also allows the stanchions (fork tubes) to be shorter, and thus lighter, so there is little trade-off in terms of unsprung weight. It IS possible to brace an USD fork - the brace is made in essentially the same way as the front fender - but less necessary since the fork is inherently stiffer. There is usually also a very large diameter front axle to provide rigidity at the bottom of the fork.

I'd have to ride the same bike back to back with conventional and USD forks to be able to say if they're really any benefit on the street. However, my current bike (a Yamaha Fazer 8 with USD) does have by far the most responsive steering of any bike I've owned. Great front-end feel, you can really tell what the tire is doing, and no feelings of flexing.
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post #9 of 15 Old 10-10-2018, 12:28 PM
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On the street, at legal, (or at least sensible), speeds, it probably won't be noticeable. Higher speeds, or pushing hard in the dirt- lower speed but more stress- the inverted fork will be stiffer and thus more stable.
FWIW, I rode behind a guy with an older BMW, Earles type front suspension, and he was a difficult rider to keep up with.

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post #10 of 15 Old 10-10-2018, 01:55 PM
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This article hits the high points of why USD performs better. One point is often understated: "Increased Stanchion to Slider Overlap"
This aspect has the most impact when either braking or cornering. That longer "overlap" spreads the positioning "grip" on the fork tube. Imagine trying to control a long-handled garden tool while dragging it across uneven ground with your hands together vs your hands spread apart.

My point is that while tube rigidity matters, keeping that tube from wiggling around inside the upper housing is critical. That keeps the caster, rake and contact patch more faithful to the geometry design and not constantly changing. It is that reduction of wiggle which makes magazine testers use words like "planted", "stable" and "confidence".

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