Steel frame stretch a real concern?

Joko
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8/25/2016 7:02pm
After listening to the 8/23/16 Pulpmx show when Steve M talked about how their steel YZ250's frame would stretch 1/4" after its first time out, and JT saying it was be a noticeable change in handling, and the new KTM steel frames are changed 5x a season at the pro level makes me wonder if this is a real problem when considering owning a KTM for an extended length of time? I also recall reading RC would go through KX250 frames on a monthly basis. Is the stretch dimension realized from the centerline of the steering head to the centerline of the swingarm pivot? Can it be repaired or reinforced to prevent stretch? Most of us probably couldn't stretch a frame in a season, but maybe after 3+ seasons of use? What is the noticeable change in handling on a stretched frame ... Does it gets too raked out and doesn't turn as sharp?
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gsxr6
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8/25/2016 7:10pm Edited Date/Time 8/25/2016 11:46pm
For us mere mortals, a seasoned steel frame becomes preferable to brand new. A softer " feel" is realized. Example: I owned a kx250 2000 model. Bike was stiff as all hell. Made my wrists hurt, had a big push in the front end. Ten years I rode that bike off-road. Around year four or five, I swear I could feel it flexing when it was too rigid in the past. The front end push disappeared as well, and it wasn't because my riding style was any different. The bike flat worked better in years five through ten.
HardTaco
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8/25/2016 7:12pm
You're welcome for that question... Lol. I'm moto'n a 02 yz pretty hard, and so far, so good. Bike was beat before I rebuilt it, so no telling on hours. I guess time will tell.


Josh
tempura
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8/25/2016 7:16pm
I have a 16 year old KX250. It's been beat pretty hard over the years. Frame is still good. Pretty much everything else on the bike has been replaced lol..
Jmicmoto13
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8/25/2016 7:18pm
Very interesting topic. Doesn't get spoken about much but I also heard the same of the steel frame. The flex is its shining star compared to the stiff aluminum chassis. Except that the frame could eventually get out of a desired spec when the aluminum wouldn't.

The Shop

Crush
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8/25/2016 7:23pm
Would you rather some flex and stretch or a crack and sheer?
gsxr6
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8/25/2016 7:35pm
Crush wrote:
Would you rather some flex and stretch or a crack and sheer?
Exactly. My 02 cr 250 is getting up there in years. The wear from the shrouds makes me wonder how long before I make it all the way through.....as in the shrouds have cut a groove in the twin spars where it rubs. I always crack the suckers near there too. Squeezing with the legs.
8/25/2016 7:35pm
Crush wrote:
Would you rather some flex and stretch or a crack and sheer?
Give me sheer and crack any day of the week, bruh.

newmann
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8/25/2016 7:56pm
Guy Cooper says that you guys overthink shit.

1
newmann
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8/25/2016 8:00pm
Kevin Windham says you guys overthink shit.


hvaughn88
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8/25/2016 8:15pm
Pingree says you need to think about some shitTongue


Crush
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8/25/2016 8:37pm
newmann wrote:
Kevin Windham says you guys overthink shit. [img]https://rx.iscdn.net/2015/08/107529_team_bull2.jpg[/img]
Kevin Windham says you guys overthink shit.


I'd say they were both definitely UNDER-thinking shit on these occasions!
Jack mehoff
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8/25/2016 8:39pm
newmann wrote:
Guy Cooper says that you guys overthink shit. [img]https://s-media-cache-ak0.pinimg.com/736x/6e/77/97/6e7797712d6722c06cd8f248be00204d.jpg[/img]
Guy Cooper says that you guys overthink shit.

I know that soil unadilla, where is windham s ? And I sure those shocks where toast after that moto
8/25/2016 8:59pm Edited Date/Time 8/25/2016 9:17pm
newmann wrote:
Guy Cooper says that you guys overthink shit. [img]https://s-media-cache-ak0.pinimg.com/736x/6e/77/97/6e7797712d6722c06cd8f248be00204d.jpg[/img]
Guy Cooper says that you guys overthink shit.

I know that soil unadilla, where is windham s ? And I sure those shocks where toast after that moto
Because the swingarm leverage ratio on (non forward-mount) twin shocks is so low, they do almost no work at all compared to modern shocks.
mark1960
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8/25/2016 9:55pm
newmann wrote:
Guy Cooper says that you guys overthink shit. [img]https://s-media-cache-ak0.pinimg.com/736x/6e/77/97/6e7797712d6722c06cd8f248be00204d.jpg[/img]
Guy Cooper says that you guys overthink shit.

I know that soil unadilla, where is windham s ? And I sure those shocks where toast after that moto
Because the swingarm leverage ratio on (non forward-mount) twin shocks is so low, they do almost no work at all compared to modern shocks.
So thats why my 1979 yz yamaha frame creased on the backbone.
Cause i had factory rider speed.
Moto75
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8/25/2016 11:07pm
hvaughn88 wrote:
Pingree says you need to think about some shit:P [img]https://p.vitalmx.com/photos/forums/2016/08/25/148535/s1200_image.jpg[/img]
Pingree says you need to think about some shitTongue


Yep, those aluminum triple clamps Tongue
Crush
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8/25/2016 11:45pm
I know that soil unadilla, where is windham s ? And I sure those shocks where toast after that moto
That photo of KW is from his house, the party in the pasture two years back I think.

Nutjob!
gsxr6
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8/25/2016 11:48pm
I thought I read kdub bent the forks or footpegs pretty good on that jump.
8/26/2016 12:55am
Everything bends when you jump a vintage mxer.
8/26/2016 12:57am Edited Date/Time 8/26/2016 12:58am
I know that soil unadilla, where is windham s ? And I sure those shocks where toast after that moto
Because the swingarm leverage ratio on (non forward-mount) twin shocks is so low, they do almost no work at all compared to modern shocks.
mark1960 wrote:
So thats why my 1979 yz yamaha frame creased on the backbone.
Cause i had factory rider speed.
Yamaha monoshock, forward mount, and catilever shock vintage mxers have pretty high leverage ratios compared to the bikes pictured, though still low compared to modern bikes.
philG
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8/26/2016 3:14am
The reason guys didn't jump them like that back in the day, was because they knew what would happen ..

Metal flexes, and keeping a track of it is it pretty hard , I know when I have scanned bikes in the past , getting good numbers is hard work. Measuring a bare frame is easy , you just need a point of reference that isn't going anywhere, and on a dirt bike , that is hard to do.



The way you measure it can make a huge difference to the perceived results as well

davis224
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8/26/2016 3:20am
I would almost think that factory teams would replace steel frames often just for the sake of settings, if there is a "break in period" on steel frames where they know exactly how it reacts and based their settings off of it, that they would replace them often to keep their specs in line. Just a guess.
BobPA
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8/26/2016 4:52am
Overthinking.
Cadpro18
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8/26/2016 5:21am
On our mid 1990's KX250's we just moved the axle all the way forward and measured the wheelbase... they definitely stretched.
philG
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8/26/2016 5:38am
Cadpro18 wrote:
On our mid 1990's KX250's we just moved the axle all the way forward and measured the wheelbase... they definitely stretched.
But that isn't a true measurement of the frame, that's two points at the extremes of the bike .

1/4 a degree of movement in the frame is nothing , but measure that at the end of the forks and its huge almost 5mm over 1.2metres.

that equates to 0.3mm movement in the headstock.

Road race boys here in stock classes get frames pulled to the steepest allowable angle ( think the tolerance is half a degree) to help them turn.
Cadpro18
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8/26/2016 6:18am
philG wrote:
But that isn't a true measurement of the frame, that's two points at the extremes of the bike . 1/4 a degree of movement in the...
But that isn't a true measurement of the frame, that's two points at the extremes of the bike .

1/4 a degree of movement in the frame is nothing , but measure that at the end of the forks and its huge almost 5mm over 1.2metres.

that equates to 0.3mm movement in the headstock.

Road race boys here in stock classes get frames pulled to the steepest allowable angle ( think the tolerance is half a degree) to help them turn.
Precisely the reason we measured wheelbase. Since we didn't have the ability to measure frames, we measured wheelbase and extrapolated as you just did. One of the guys in our group would see 1/2" lengthening of the wheelbase. As I recall it had the net effect on trail of a 3mm change in triple clamp offset. Feel free to double check my math... I'm going from memory on this.
philG
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8/26/2016 6:38am
philG wrote:
But that isn't a true measurement of the frame, that's two points at the extremes of the bike . 1/4 a degree of movement in the...
But that isn't a true measurement of the frame, that's two points at the extremes of the bike .

1/4 a degree of movement in the frame is nothing , but measure that at the end of the forks and its huge almost 5mm over 1.2metres.

that equates to 0.3mm movement in the headstock.

Road race boys here in stock classes get frames pulled to the steepest allowable angle ( think the tolerance is half a degree) to help them turn.
Cadpro18 wrote:
Precisely the reason we measured wheelbase. Since we didn't have the ability to measure frames, we measured wheelbase and extrapolated as you just did. One of...
Precisely the reason we measured wheelbase. Since we didn't have the ability to measure frames, we measured wheelbase and extrapolated as you just did. One of the guys in our group would see 1/2" lengthening of the wheelbase. As I recall it had the net effect on trail of a 3mm change in triple clamp offset. Feel free to double check my math... I'm going from memory on this.
No, i agree its as valid a check as you can get , and your maths is fine.

I just know people who do the same and think the frame has gone that much LOL.

We sold adjustable clamps here in the UK to pull in the head angle to get race geometry out of road frames, we built a bike for a guy , he built his van to take the bike, then bought a second bike and fitted his own clamps, couldnt understand why it was shit.. i asked him to bring them both over , the fact that the second bike didnt fit in his van wasnt enough to tip him off that the inserts were in backwards.

I measure for a living , and its easy to get weird numbers if you dont get everything right.
Bearuno
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8/26/2016 6:44am
They routinely replace Aluminium frames, too.

Anything that's being hammered, fatigues.

It's wise to retire things, before they fail.

And everything , can, and will fail when taken past it's limits. Nothing lasts forever.
seth505
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8/26/2016 6:46am
L-O-L

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