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Old 11-12-2012, 09:55 AM   #1
Voltaire OP
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Frame Flex

I've been running my R90 track bike for nearly a whole season and so far it seems to go,stop and handle really well.

I have been expecting frame flex to turn up at every meet but so far its stayed away....

Running a 1983 R80 frame, racetech forks, fork brace, big tube shaft and Ikons.

Last effort I was manageing to lap the track in 1.27 minutes @ around 80 MPH average speed.

Is it only the /5's that flex as I weighed the two frames at the start of the build and the /5 one was from memory about 3 kgs

less...

Mine is the red one in the piccy....the least fast of the three....

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Old 11-12-2012, 10:03 AM   #2
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They all flex ... some more than others. I'll bet you've just gotten used to it. The /5's are known to be 'more flexy' and there are several builders out there that will not make a scrambler out of the frame due to it's propensity to crack.

I have thrashed a /6 and 'currently' a monoframe, which is essentially a /7 ... no cracks yet ... but get yourself on a modern rice burner, and then go back to the airhead. You'll feel it then

PS - Those are sure some nice looking scooters. Got any closer pics ... of all of them?
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Old 11-12-2012, 10:51 AM   #3
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Quote:
Originally Posted by Beater View Post
I have thrashed a /6 and 'currently' a monoframe, which is essentially a /7 ... no cracks yet ... but get yourself on a modern rice burner, and then go back to the airhead. You'll feel it then
I noticed the flex on my ST riding fast gravel roads watching my forks dance around underneath me, but didn't realize the extent until I rode a friends GSPD with HPN-style bracing up on the dalton highway in Alaska. 60-70 mph on potholed gravel and it felt solid as a rock. Night and day difference. Those speeds on the unbraced ST were scary and unsustainable, especially with the stock suspension.

Something about the /5, I believe they had tapered or ovalled (maybe both?) tubes for the downtubes coming off the steering head or something like that. I've read that Chris King, the guy that modifies airheads for off road use down in Texas, won't use /5 frames as he says they're too weak and flexy. He says that /6 and /7 frames are much stronger
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Old 11-12-2012, 10:46 AM   #4
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Most likely I am used to it...good point. when I say flex maybe I meant wallow.....
got another couple of pics.... Last handling bike I had was a 900SS ie..... I sadly never took it to the track ( too scared I damage a faring $$$$) I've had a ride on the track of Trustmes Triumph and thats a whole different machine....light and twitchy.
The other two bikes have been built by a local BMW guy who races the other red one, its got lots of go faster goodies on it. he's about 7 seconds faster per lap than me.....different league altogether....I'm a noob.
As you can see we like our small tanks, they look pretty good on a racer.
The Black one has a mono front, different brakes and I think both are over 1000cc......
Mine is stockish with a 336 cam, 36 mm Dells, ignitech ignition and original 1973 pistons....
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Old 11-12-2012, 11:02 AM   #5
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I think I remember portions of the /6 & /7 frames being double wall tubing. They went back to single wall with the R65 & later the monos. So the /7 frames should be stronger. The beefier swingarm will have helped.

I noticed a significant improvement with better forks.

Re the flexibility of the frame, watch how much the things twist around when the motor is on a very slow idle. Also the tracks you are riding don't have as many surprise surface variations that set off the weaves as road riding (albeit that you are at significantly higher speeds).

Would have thought Pukekohe would be a good test
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Old 11-12-2012, 11:37 AM   #6
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They all flex at the swing arm pivots.
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Old 11-12-2012, 12:14 PM   #7
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Which island are you on? Are you planning to run at the Burt Munro Challenge coming up next week?

I have a few mates who live on the south island and will be attending as spectators. Guzzi riders they are.
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Old 11-12-2012, 12:53 PM   #8
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Which island are you on? Are you planning to run at the Burt Munro Challenge coming up next week?

I have a few mates who live on the south island and will be attending as spectators. Guzzi riders they are.
I'm up the other end....about 1000 kms away. I'm racing this week end in the lower North Island, I'd like to go to the Burt but only as a specator.
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Old 11-12-2012, 02:15 PM   #9
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It will happen as soon as you get those lap times down some more. Personally, I am not having fun until my bike is wallowing, sliding, bucking, and tank slapping. Are you sliding your tires much yet? That will often get a wallow or a bit of a head shake when they hook up. Snapping them over into a corner too quickly will get a wallow everytime. I sometimes get a little head shake when the bike snaps upright again or changing directions in a chicane. Even at 80/90mph, my front tire often leaves the ground while flicking it over the other direction in a chicane. Very often in slower chicanes. That often gets me a little headshake. Then there is landing jumps a little crooked. There is a jump on one of my favorite rides that pitches me a bit sideways everytime at about 80mph. It always bucks and tank slaps a bit landing that. Man I am I glad my shock stud didn't brake there!

I broke my LS frame from flexing it back around '86. A cross member right below the swingarm pivot broke. The sourse of that flex is the backbone tube twisting. R65's and at GS's don't have double walled backbones. Monolevers do. The double walled frames STILL need beefing up there. Right where most people do nothing. Most people start with the steering head or side braces. The last place it needs beefing up IMO. The rear most section of the backbone twisting is the source of most of that.

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Old 11-12-2012, 03:28 PM   #10
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Originally Posted by supershaft View Post
my front tire often leaves the ground while flicking it over the other direction in a chicane. .
I got rid of this by aligning the back wheel with the front - no offset no skip.
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Old 11-12-2012, 07:42 PM   #11
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I got rid of this by aligning the back wheel with the front - no offset no skip.
What I am talking about has nothing to do with wheel alignment. All bikes do it. The corkscrew at Laguna Seca is a S turn that very often and obviousy gets riders into what I am talking about.
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Old 11-12-2012, 08:21 PM   #12
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What I am talking about has nothing to do with wheel alignment. All bikes do it. The corkscrew at Laguna Seca is a S turn that very often and obviousy gets riders into what I am talking about.
cuz it drops off like a cliff mid-turn.
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Old 11-12-2012, 09:04 PM   #13
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That's what makes it so obvious. All bikes will very often momentarily lift the front wheel in a turn like that on level ground. It just isn't nearly as noticable.
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Old 11-13-2012, 03:46 AM   #14
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I'm looking for a way to say you're right SS .
So far I can't find one.

I can't have the only bike in the world that doesn't hop thru esses.
Running a 21' front and a long travel suspension I'd expect the problem of unloading the front end and releasing the stored energy at a direction change would be worse.

Maybe I'm too old to push it that hard anymore
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Old 11-13-2012, 10:29 AM   #15
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What Shaft says is a real thing, however, you have to snap the bike from left to right violently, while being hard on the gas.
A large part of the phenomenon is in the geometry of the front end. To initiate the turn, you must turn the bars hard, to raise the bike to center. This loads the front suspension. At top the suspension unloads, the bike is hard on the gas, lifting the front, and the rake, and offset of the front swing slightly upward as well.
I used to roadrace superbikes, and seldom rode hard enough to experience this, and never on a road bike!

BTW if you have been unfortunate enough to have a bike go into tankslappers hard enough to leave black marks on the road (unfortunately I have several times, I've had some shitty handling bikes before) you will see the black marks are a foot apart, and non in the center.
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