2004 KX125

gs317
Profile picture for user gs317
Posts
111
Joined
3/10/2020
Location
Reno, NV US
Fantasy
584th
gs317 2/23/2021 12:52pm
2/23/2021 12:52pm
I may post this to the tech forum, but ill try here first, the kxf forks are sprung too stiff for me on the 125, I am 155 w/o gear. FC recommended going down to .92 spring rate, RT calculator suggested .84. Anyone thats done the swap care to weigh in with what rates you switched to? Thanks
Phillip_Lamb
Profile picture for user Phillip_Lamb
Posts
1924
Joined
12/14/2010
Location
ORANGEVALE, CA US
Fantasy
2447th
Phillip_Lamb 2/23/2021 10:30pm
2/23/2021 10:30pm
gs317 wrote:
I may post this to the tech forum, but ill try here first, the kxf forks are sprung too stiff for me on the 125, I...
I may post this to the tech forum, but ill try here first, the kxf forks are sprung too stiff for me on the 125, I am 155 w/o gear. FC recommended going down to .92 spring rate, RT calculator suggested .84. Anyone thats done the swap care to weigh in with what rates you switched to? Thanks
id call one or two more shops, the RT calc suggested stock springs for me but im clearly bigger than the target weight

The Shop

See All »
gs317
Profile picture for user gs317
Posts
111
Joined
3/10/2020
Location
Reno, NV US
Fantasy
584th
gs317 2/24/2021 7:18am
2/24/2021 7:18am
gs317 wrote:
I may post this to the tech forum, but ill try here first, the kxf forks are sprung too stiff for me on the 125, I...
I may post this to the tech forum, but ill try here first, the kxf forks are sprung too stiff for me on the 125, I am 155 w/o gear. FC recommended going down to .92 spring rate, RT calculator suggested .84. Anyone thats done the swap care to weigh in with what rates you switched to? Thanks
Phillip_Lamb wrote:
id call one or two more shops, the RT calc suggested stock springs for me but im clearly bigger than the target weight
Got an email from FC later in the day that suggested .88 and when I stopped at the local shop they recommended the same, so I ordered a .88, maybe after the respring it'll be able to get to run an inside line finally.
gs317
Profile picture for user gs317
Posts
111
Joined
3/10/2020
Location
Reno, NV US
Fantasy
584th
gs317 3/11/2021 7:25am
3/11/2021 7:25am
Well after about 8 hard motos, it looks like the carbon mounts I made are throwing in the towel. I'll post pics later when I pull them off, but they appear to be breaking down. I still call this a win for my first time working with CF like this and I know what the design flaws I could fix for a second set would be. I can go into a breakdown of the points of failure later when I get them removed if anyone is interested in that sort of thing.
1
Kidkawie
Profile picture for user Kidkawie
Posts
1129
Joined
8/31/2012
Location
ქუთაისი GE
Kidkawie 3/11/2021 1:17pm
3/11/2021 1:17pm
No way Id run that cf headstay. That joint has to be rigid. Too much flex in cf for that unless you make them 5mm thick, then just use aluminum.
1
Phillip_Lamb
Profile picture for user Phillip_Lamb
Posts
1924
Joined
12/14/2010
Location
ORANGEVALE, CA US
Fantasy
2447th
Phillip_Lamb 3/11/2021 9:56pm
3/11/2021 9:56pm
gs317 wrote:
Well after about 8 hard motos, it looks like the carbon mounts I made are throwing in the towel. I'll post pics later when I pull...
Well after about 8 hard motos, it looks like the carbon mounts I made are throwing in the towel. I'll post pics later when I pull them off, but they appear to be breaking down. I still call this a win for my first time working with CF like this and I know what the design flaws I could fix for a second set would be. I can go into a breakdown of the points of failure later when I get them removed if anyone is interested in that sort of thing.
need to test a set back to back against the stock pieces.
1
gs317
Profile picture for user gs317
Posts
111
Joined
3/10/2020
Location
Reno, NV US
Fantasy
584th
gs317 3/12/2021 7:25am
3/12/2021 7:25am
Phillip_Lamb wrote:
need to test a set back to back against the stock pieces.
I would have to remake the mounts from carbon first, which I might do, but for now I am going to throw the stock mounts on and keep the CF models as a souvenir of a fun project.
gs317
Profile picture for user gs317
Posts
111
Joined
3/10/2020
Location
Reno, NV US
Fantasy
584th
gs317 3/12/2021 7:30am
3/12/2021 7:30am
Kidkawie wrote:
No way Id run that cf headstay. That joint has to be rigid. Too much flex in cf for that unless you make them 5mm thick...
No way Id run that cf headstay. That joint has to be rigid. Too much flex in cf for that unless you make them 5mm thick, then just use aluminum.
Carbon should actually make a stiffer mount than aluminum by a significant amount given that the Youngs modulus of 2 way CF such as what I was using is ~90.5-190 GPa vs aluminum at ~ 70 GPa. This can vary with the grade of aluminum used but either way, the issues experienced were due to build quality issues, not using proper molds, and the ability of the fatigue resistance of the material under the loading conditions. I wouldnt hesitate to use CF again provided I can improve my MFG processes to make a more consistent product.
Phillip_Lamb
Profile picture for user Phillip_Lamb
Posts
1924
Joined
12/14/2010
Location
ORANGEVALE, CA US
Fantasy
2447th
Phillip_Lamb 3/12/2021 11:06pm
3/12/2021 11:06pm
Kidkawie wrote:
No way Id run that cf headstay. That joint has to be rigid. Too much flex in cf for that unless you make them 5mm thick...
No way Id run that cf headstay. That joint has to be rigid. Too much flex in cf for that unless you make them 5mm thick, then just use aluminum.
gs317 wrote:
Carbon should actually make a stiffer mount than aluminum by a significant amount given that the Youngs modulus of 2 way CF such as what I...
Carbon should actually make a stiffer mount than aluminum by a significant amount given that the Youngs modulus of 2 way CF such as what I was using is ~90.5-190 GPa vs aluminum at ~ 70 GPa. This can vary with the grade of aluminum used but either way, the issues experienced were due to build quality issues, not using proper molds, and the ability of the fatigue resistance of the material under the loading conditions. I wouldnt hesitate to use CF again provided I can improve my MFG processes to make a more consistent product.
math's hurt my brain
1
garasaki
Profile picture for user garasaki
Posts
200
Joined
1/20/2021
Location
Mount Vernon, IA US
garasaki 3/17/2021 7:19am
3/17/2021 7:19am
Hey, just curious, what is special about this time frame of KX125's?
gs317
Profile picture for user gs317
Posts
111
Joined
3/10/2020
Location
Reno, NV US
Fantasy
584th
gs317 3/17/2021 11:05am
3/17/2021 11:05am
garasaki wrote:
Hey, just curious, what is special about this time frame of KX125's?
The 2004 Kx125 was the last 125 to win an AMA championship, which was won by James Stewart. It was also the bike he raced for all but 1 race of his lights career and was on when he created the "Bubba Scrub" at I want to say Budds Creek in 2003. So the bike has a good bit of racing history to it. PC also ran them and had a pretty iconic look to them in 2003. As far as technologically, they were a solid motor at the time, matching the YZ125 for second best motor behind the KTM. 2003 was a half generational change with new body work and chassis from the 2002 model, but they made further changes in 2004 that carried through until 2005/2006 when the bike was discontinued. So it is also the last generation of the bike, so adds another layer to it. Its still a really fun bike to this day, and I race mine in 30+/125C/250C, but I am also running the least HP even with the mod motor so I am always at a bit of disadvantage.
1
Kidkawie
Profile picture for user Kidkawie
Posts
1129
Joined
8/31/2012
Location
ქუთაისი GE
Kidkawie 3/17/2021 9:53pm
3/17/2021 9:53pm
Kidkawie wrote:
No way Id run that cf headstay. That joint has to be rigid. Too much flex in cf for that unless you make them 5mm thick...
No way Id run that cf headstay. That joint has to be rigid. Too much flex in cf for that unless you make them 5mm thick, then just use aluminum.
gs317 wrote:
Carbon should actually make a stiffer mount than aluminum by a significant amount given that the Youngs modulus of 2 way CF such as what I...
Carbon should actually make a stiffer mount than aluminum by a significant amount given that the Youngs modulus of 2 way CF such as what I was using is ~90.5-190 GPa vs aluminum at ~ 70 GPa. This can vary with the grade of aluminum used but either way, the issues experienced were due to build quality issues, not using proper molds, and the ability of the fatigue resistance of the material under the loading conditions. I wouldnt hesitate to use CF again provided I can improve my MFG processes to make a more consistent product.
Its your bike man. My point is that you were planning to sell them which could lead to someones frame and/or engine tabs cracking due to a loose connection. Loose mounts or some other underlying condition is what lead to the original headstay failing.


gs317
Profile picture for user gs317
Posts
111
Joined
3/10/2020
Location
Reno, NV US
Fantasy
584th
gs317 7/22/2021 3:40pm
7/22/2021 3:40pm
Well ended up selling the project bike to a kid that was moving from 80s up to the big bikes. Hope to see him do well at the races.

As for the motor mounts, I found the point of failure was the bends in the material from the constant flexing. Ultimately I did not make them uniform enough in thickness, and likely would need to add additional layers. If I revisit the mounts I will spend more time on proper prep and molding to improve the end result. It was a fun part to try to make and it worked well despite the short lifespan.
1

Pit Row

See All »

Post a reply to: 2004 KX125