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Old 02-12-2019, 03:09 PM   #1
Gixxer343
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Fork extensions?

Why?
Just bought a bike that was a race/track bike that has fork extenders on the tops. Whatís purpose? I know the game of lowering and raising the forks in the triples and the affects of each but this seems counterintuitive to a race bike. If I need extensions itís because Iím lowering the forks in the trees which would raise the chassis and result in a slower turn in.
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Old 02-12-2019, 03:16 PM   #2
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They're generally combined with more rear ride height.

there's an ideal rake/trail and an ideal swingarm angle. with many bikes, getting both means raising both ends significantly.
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Old 02-12-2019, 06:05 PM   #3
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What Daniel said-It is done to increase stability.
You see it on Track GSX-R's
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Old 02-12-2019, 06:26 PM   #4
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They're generally combined with more rear ride height.

there's an ideal rake/trail and an ideal swingarm angle. with many bikes, getting both means raising both ends significantly.
This.

Geometry is a variable. It changes to suit the needs of the track, rider, pace, and conditions.
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Old 02-12-2019, 06:29 PM   #5
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Quote:
Originally Posted by Gixxer343 View Post
Why?
Just bought a bike that was a race/track bike that has fork extenders on the tops. Whatís purpose? I know the game of lowering and raising the forks in the triples and the affects of each but this seems counterintuitive to a race bike. If I need extensions itís because Iím lowering the forks in the trees which would raise the chassis and result in a slower turn in.
shortening the forks does not make the bike turn in faster. it makes the bars easier to turn, but thats not the same thing.

race bikes need to go as fast as possible. decreasing stability of the front end can be a good way to go slower around a track.
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Old 02-13-2019, 08:12 AM   #6
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Watch videos from Dave Moss and you'll see...
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Old 02-13-2019, 08:43 AM   #7
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Related question:

if ground clearance isn't an issue, how big of a difference will you see between raising the bike front and rear vs adjusting swingarm pivot height to get the same swingarm angle (and rake/trail) with a lower ride height?
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Old 02-13-2019, 11:22 AM   #8
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Quote:
Originally Posted by Junkie View Post
Related question:

if ground clearance isn't an issue, how big of a difference will you see between raising the bike front and rear vs adjusting swingarm pivot height to get the same swingarm angle (and rake/trail) with a lower ride height?
the taller bike will for sure pitch more. I think its also less stable in roll, but cant be sure. so it will wheelie/stoppie easier and may fall into corners faster. it will use a little less lean angle, which further helps it get to the required lean angle. the shorter bike will pitch less, and therefore wheelie and stoppie less. it will also use a little more lean angle.

however, I have no idea how far ud have to go to feel the differences. I can occasionally feel 2mm of ride height on either end. but those changes affect other variables too like trail. changing CoG in isolation would likely be much harder to feel. maybe id need 4mm to feel it, or 8.
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Last edited by stangmx13; 02-13-2019 at 11:29 AM..
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Old 02-13-2019, 11:55 AM   #9
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Old 02-13-2019, 02:32 PM   #10
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Attached is part of the HRC suspension setup manual given to HRC race teams (pg 4-5). It explains what effects each adjustment of ride height gives you. There is more. However I just included these pages for this discussion and a small upload. If anyone wants the entire PDF from HRC (Honda Racing), hit me with a private message, and I will send it to you. It just covers the basics of motorcycle suspension setup.

I know it's not super clear, but I was trying to keep the file small
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Old 02-13-2019, 02:53 PM   #11
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Originally Posted by stangmx13 View Post
the taller bike will for sure pitch more. I think its also less stable in roll, but cant be sure. so it will wheelie/stoppie easier and may fall into corners faster. it will use a little less lean angle, which further helps it get to the required lean angle. the shorter bike will pitch less, and therefore wheelie and stoppie less. it will also use a little more lean angle.

Not sure if true. Bike is an inverted pendulum (among other things). Longer pendulum means longer period. So I think change in lean angle is slower. But perhaps more stability in return. Everything else you said makes sense though.
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Old 02-13-2019, 05:11 PM   #12
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So looking at the top of the shock it looks like they may have shimmed it a bit. I’m going to have to wait for a track day to see if it’s something I want to keep this way or drop back to stock height. I think I’ll be surprised if I don’t like it. Who doesnt like a bike to be responsive and drop into corners well?
Oh... that’s right, Harley riders
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Old 02-14-2019, 08:11 AM   #13
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Quote:
Originally Posted by Gixxer343 View Post
So looking at the top of the shock it looks like they may have shimmed it a bit. I’m going to have to wait for a track day to see if it’s something I want to keep this way or drop back to stock height. I think I’ll be surprised if I don’t like it. Who doesnt like a bike to be responsive and drop into corners well?
Oh... that’s right, Harley riders
If it's a GSXR, it's very common to shim the shock 2-6 mm, and use fork extenders. It makes transitions from turn to turn much easier ( think esses at Sears Point, T3-T6 at Thill. In fact for the 2006 and up 600s and 750s it is usually the first thing done.

There's a rule of thumb, if the bike gets nervous and starts rotating around the headstock on hard braking, you have the rear too high. If it won't finish out a corner, too low.
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Old 02-14-2019, 10:32 AM   #14
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^^ good rule of thumb.

too much rear ride-height will tend to spin a lot as well.
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