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Author Topic: APS/Ω Gas turbine bike build  (Read 396118 times)
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Mobacken Racing
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« Reply #360 on: March 20, 2014, 02:47:19 PM »

Last night I spent three hours welding some of the gussets to the frame, I also removed the two thin bars between the lower frame tubes and replaced them with 1 1/2" CrMo tubes just like Saltwheels262 suggested. Looks much sturdier now, thanks for the hint! smiley









Cheers!
/Anders
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maj
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« Reply #361 on: March 20, 2014, 03:52:52 PM »

Anders, a mate had an interesting issue not long ago on a new build tube frame drag bike
the bike was not handling very well under full load, found it was twisting as the hp increased and putting the front wheel out of line.
more triangulation seems to have fixed it
it was a 4 bar perimeter frame and i would have thought looked fine 
made me more aware on a future build i am planning to actually check the stiffness between headstock and rear axle an see what the normal movement is likely to be .

After one of my bikes which handled bad and was found to have a 3.5 deg camber , and replaced with a frame that has less than 1 deg camber
i know for sure you want your front wheel positioned very nearly  perfect under load or the whole thing gets very fishtaily   

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Mobacken Racing
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« Reply #362 on: March 20, 2014, 04:32:59 PM »

Hi Maj,

I´ve been thinking about this quite a lot. A drag bike engine has tons of torque and leaves the line at full throttle doing its best to make a cork screw of the frame, my land speed bike has no more torque than a fairly standard hayabusa and leaves the starting line smoothly with the constant torque curve of a turboshaft engine.

I can understand that a drag bike needs a beefy frame to counteract all that twisting force, but in my case it feels like I should get away with a slightly less rigid frame.

How the frame will react at higher speeds with regard to fish tailing etc. I can only wait and see, I will build the frame like this and try it out and if it shows any tendency to become unstable I will have to do some thinking.

Cheers!
/Anders

Anders, a mate had an interesting issue not long ago on a new build tube frame drag bike
the bike was not handling very well under full load, found it was twisting as the hp increased and putting the front wheel out of line.
more triangulation seems to have fixed it
it was a 4 bar perimeter frame and i would have thought looked fine 
made me more aware on a future build i am planning to actually check the stiffness between headstock and rear axle an see what the normal movement is likely to be .

After one of my bikes which handled bad and was found to have a 3.5 deg camber , and replaced with a frame that has less than 1 deg camber
i know for sure you want your front wheel positioned very nearly  perfect under load or the whole thing gets very fishtaily   


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wobblywalrus
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« Reply #363 on: March 20, 2014, 10:49:06 PM »

This elaborates on what Greg is saying.  A real strong bracket is made to hold the rear of the frame onto the work table using the axle mounts.  This simulates the frame being connected to the rear wheel.  A big beefy steel rod is stuck through the steering stem with about 3 feet sticking out above and below the stem.

Then, you make some spider webs of sewing thread between different parts of the frame.

Last, you give the big rod a twist in different directions and you see how your frame flexes.  The threads go limp or tight to help you see what is happening.   
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Mobacken Racing
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« Reply #364 on: March 21, 2014, 12:05:17 AM »

This elaborates on what Greg is saying.  A real strong bracket is made to hold the rear of the frame onto the work table using the axle mounts.  This simulates the frame being connected to the rear wheel.  A big beefy steel rod is stuck through the steering stem with about 3 feet sticking out above and below the stem.

Then, you make some spider webs of sewing thread between different parts of the frame.

Last, you give the big rod a twist in different directions and you see how your frame flexes.  The threads go limp or tight to help you see what is happening.   

That is a great idea, I´ll make sure to try it after the frame is welded to get some idea how flexible it is.
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tauruck
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« Reply #365 on: March 21, 2014, 12:27:03 AM »

I've built a fair number of bike frames and I agree it's not as simple as it looks.

I see these TV shows where guys cut the backbone and change the rake and stuff.
I wonder how the camber changes on those?.

Defects show up on two wheels way before they would on four.
Bike frames are a SOB to get right.
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maj
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« Reply #366 on: March 21, 2014, 02:46:06 AM »

IMO salt bike alignment will be more critical than the drag bike ,
 i drag raced my cambered frame for years , never knew there was a fault,
it was a modified frame , raked by a very reputable Aussie frame shop , i never thought i needed to check the work
and the issues only showed up on the salt .

Your maximum torque will be happening right where you need your best alignment ,
i like Bo's idea  , even just forcing the bar will give you an idea of the frames elasticity or rigidity 
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wobblywalrus
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« Reply #367 on: March 21, 2014, 10:41:55 PM »

Go into Images on Google and click in "Tony Foale."  There are a lot of frames there.  Go into the "Articles" section on Tony's website.  These is a frame stiffening article there.  The two cages he made for the Kawa work pretty well on double down tube frames.  You will see how the lower frame cradles move in relationship to each other when you twist the steering stem.  His cages tie the two lower cradles together and this increases the support of the steering head.  A similar principal works in the back.

An advantage of a strong chassis is felt when racing on a rutted rack.  The front wheel will climb out of a rut with much less drama if the chassis is beefy.  The wheel tends to follow the rut with a flexi setup.  Eventually it climbs out and almost always there is some whipping and over correction.  This sets up a series of machine or rider induced wigggles and wobbbles.
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Mobacken Racing
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« Reply #368 on: March 22, 2014, 01:43:32 PM »

Wow, lots of useful info on that website!

The "problem" with my frame is that most of the holes in the frame where an X or diagonal tube would do wonders have to be left open to house the oil pump etc, so there is not very much that can be done to it except for cutting away the upper frame tube and fit a pair of them instead.

That fix is on top of my list in case the bike is unstable at speed, shouldn´t be overly difficult to do.

I´ll make sure to measure the wheel camber and correct any difference by making custom rear chain tension blocks to get the rear wheel angle the same as the front wheel.

Cheers!
/Anders
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maj
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« Reply #369 on: March 22, 2014, 03:38:41 PM »

Problem with a front end camber is not so much the angle but what it does to the wheel position on the ground and how that effects the whole bike , you would not notice so much initially as you would automatically correct , but when you get above a few % wheelspin it will become a handling issue
 

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Mobacken Racing
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« Reply #370 on: March 24, 2014, 06:25:35 PM »

With your concern for the frame stability in mind I went to the shed trying to figure out a way to stabilize the frame without having to rebuilt it completely, and I think I am on to something.



Adding one 1 1/2" tube on each side of the main upper frame should stiffen the frame quite a bit, a pair of tubes down each side connecting to the vertical tubes and I think I will have a frame that will behave like it should. At least way better than before.



Now when I look at the frame it looks ridiculously weak with only a single upper frame tube, you´ve probably thought so all the time but been to kind to say it to me.  smiley



I just need to verify that the engine and air box will fit before I weld them in place, don´t think it´ll be a problem but I better check anyway.



Cheers!
/Anders
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manta22
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« Reply #371 on: March 24, 2014, 08:12:45 PM »

Anders;

Tying the tubes together in a few places with short lateral tubes will help the stiffness but you were probably planning to do that anyway.

A 14 or 16ga steel sheet over the top of the tubes will be the best approach. Weld the edges to the outside tubes and put a number of rosette welds into the center tube.

Regards, Neil  Tucson, Az
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Regards, Neil  Tucson, AZ
Stainless1
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« Reply #372 on: March 24, 2014, 08:21:57 PM »

Anders;

A 14 or 16ga steel sheet over the top of the tubes will be the best approach. Weld the edges to the outside tubes and put a number of rosette welds into the center tube.

Regards, Neil  Tucson, Az

not only making it really stiff but also providing a nice blast shield in case catastrophe strikes...
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Stainless
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« Reply #373 on: March 24, 2014, 10:11:44 PM »


not only making it really stiff but also providing a nice blast shield in case catastrophe strikes...

Yep, what he said
G
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wobblywalrus
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« Reply #374 on: March 24, 2014, 10:59:06 PM »

You saw and fixed the biggest problem with those two upper tubes. 
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