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Voltaire 11-12-2012 10:55 AM

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....:cry

http://i129.photobucket.com/albums/p...dnzep/bmw3.jpg

Beater 11-12-2012 11:03 AM

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 :wink:

PS - Those are sure some nice looking scooters. Got any closer pics ... of all of them?

Voltaire 11-12-2012 11:46 AM

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....:rofl
http://i129.photobucket.com/albums/p...zep/3bmws2.jpghttp://i129.photobucket.com/albums/p...nzep/3BMWs.jpg

Airhead Wrangler 11-12-2012 11:51 AM

Quote:

Originally Posted by Beater (Post 20027154)
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 :wink:

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

Box'a'bits 11-12-2012 12:02 PM

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

bmweuro 11-12-2012 12:37 PM

They all flex at the swing arm pivots.

Ray R 11-12-2012 01:14 PM

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.

Voltaire 11-12-2012 01:53 PM

Quote:

Originally Posted by Ray R (Post 20028059)
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.:D

supershaft 11-12-2012 03:15 PM

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.

pommie john 11-12-2012 03:42 PM

I believe that tying the top of the engine to the top tube of the frame stiffens it all up quite well. The engine is only mounted with two bolts which seems to little to me.

How much frame flex you experience would be down to a number of things including how hard you ride it, how much power and torque it produces and how smooth you are as a rider. Oh and don't forget the track, I had mine shaking its head quite badly coming out of the last corner at Pukekohe, but then again, I think the steering head bearings were loose at that time :)

supershaft 11-12-2012 03:50 PM

Quote:

Originally Posted by pommie john (Post 20029123)
I believe that tying the top of the engine to the top tube of the frame stiffens it all up quite well. The engine is only mounted with two bolts which seems to little to me.

How much frame flex you experience would be down to a number of things including how hard you ride it, how much power and torque it produces and how smooth you are as a rider. Oh and don't forget the track, I had mine shaking its head quite badly coming out of the last corner at Pukekohe, but then again, I think the steering head bearings were loose at that time :)

+1!

I totally agree that bracing the rear part of the backbone through a engine mount is THE place to start stiffening a frame.

I also totally agree about flexing the frame. It isn't JUST speed. Smoothness and road surface makes all the difference. Smoothness? Your nuts if you don't think Wayne Rainey was a super smooth rider. Watch him ride that Yamaha his last couple of seasons. There is only so much any human can do along those lines IF you've got the speed.

Voltaire 11-12-2012 04:12 PM

Quote:

Originally Posted by pommie john (Post 20029123)
I believe that tying the top of the engine to the top tube of the frame stiffens it all up quite well. The engine is only mounted with two bolts which seems to little to me.

How much frame flex you experience would be down to a number of things including how hard you ride it, how much power and torque it produces and how smooth you are as a rider. Oh and don't forget the track, I had mine shaking its head quite badly coming out of the last corner at Pukekohe, but then again, I think the steering head bearings were loose at that time :)

Probably not pumping much power over a stock R100, the tracks are pretty good apart from Pukekohe which has the bike move around a bit. only been doing this for less than a year, lap times are falling so no plans to do any more mods on the engine at this stage. I want to see how much rider improvement can do. So far 5-7 seconds off lap times at different tracks in 6 months....thats just due to building confidence and getting used to bike. The rear brace idea looks good.

Rucksta 11-12-2012 04:28 PM

Quote:

Originally Posted by supershaft (Post 20028914)
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.

supershaft 11-12-2012 04:29 PM

Quote:

Originally Posted by Voltaire (Post 20029342)
Probably not pumping much power over a stock R100, the tracks are pretty good apart from Pukekohe which has the bike move around a bit. only been doing this for less than a year, lap times are falling so no plans to do any more mods on the engine at this stage. I want to see how much rider improvement can do. So far 5-7 seconds off lap times at different tracks in 6 months....thats just due to building confidence and getting used to bike. The rear brace idea looks good.

Your doing it the smart way Voltaire. How stiff does your setup need to be? It needs to be as flexable as you can get away with for whatever track and skill level you are at! My advise is to always err on the loose side of what you might think is perfect. Loose gets you 'feed back'. Too rigid in the back will get you going backwards down the track and too rigid up front will get you on your face. Or put another way: A little too much 'feed back" is always better than not enough.

Voltaire 11-12-2012 05:30 PM

Quote:

Originally Posted by supershaft (Post 20029467)
Your doing it the smart way Voltaire. How stiff does your setup need to be? It needs to be as flexable as you can get away with for whatever track and skill level you are at! My advise is to always err on the loose side of what you might think is perfect. Loose gets you 'feed back'. Too rigid in the back will get you going backwards down the track and too rigid up front will get you on your face. Or put another way: A little too much 'feed back" is always better than not enough.

Cheers SS, I'm quite happy at the moment 'just doin' it" as skills improve I can add power. I'm not sure how stiff I need or indeed what flexi feels like.... the bike feels really good. I have been told that on one downward hairpin I am very smooth, where other bikes seem to move around a lot. I think thats due to me transitioning before the corner ....not in it, and not using brakes past the turn in point.
I probably need to work on a bit of hang off as stating to scrape the raised motor on a few corners...then I react by closing the gas and the bike starts to go wide.....need to keep the gas on and lift the bike out. Good fun.


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