Team GPR

**koba** · 17th December 2014, 13:32

Originally Posted by mr bucketracer

what is the higher cause that breaks rods , to much power , to meany rpm , down changes that make more rpm , pooly made rods , to heaver pistion for the rod .etc ( bad luck ) lol .

Bearing in mind I'm no engineer...

I'm guessing the piston is heavier?
That would increase the force on the rod, although increasing speed increases the force a lot faster. (At least thats what my school teacher told me)
I think doubling the mas doubled the force and increasing the speed was by a factor of 4 or something.

I don't think 'too much power' exists, it's probably that the rod couldn't handle the force generated by a psiton of that weight at the revs it was pulling.

I'm pretty sure I wasn't too harsh on the downchanges, although mechanical over-rev would have to stress it a lot more.

Grumph?

**koba** · 17th December 2014, 14:28

Ahh, this is interesting to work out.

force = mass x acceleration.

Acceleration can be worked out with some scary looking math:
http://www.codecogs.com/library/phys...f-a-piston.php

I don't have time to rip in to it right now but that's worth a look.

It'd be interesting to see the change in force from more weight and more revs.

There is bound to be an accepted guideline of how-much-is-too-much.

**koba** · 17th December 2014, 14:29

I guess the weight would have to be the piston, pin, clips rings, little end and the rod - or the top of the rod only perhaps?

**mr bucketracer** · 17th December 2014, 14:55

Originally Posted by koba

Bearing in mind I'm no engineer...

I'm guessing the piston is heavier?
That would increase the force on the rod, although increasing speed increases the force a lot faster. (At least thats what my school teacher told me)
I think doubling the mas doubled the force and increasing the speed was by a factor of 4 or something.

I don't think 'too much power' exists, it's probably that the rod couldn't handle the force generated by a psiton of that weight at the revs it was pulling.

I'm pretty sure I wasn't too harsh on the downchanges, although mechanical over-rev would have to stress it a lot more.

Grumph?

it snaped at the bottom of the rod so all makes sence what you have said , im getting another rod and lighting the small end ,bore out the small end just a little bit , even though the piston is light a bit here and there should help , the fxr rod is very light to this one and they dont seem to have problems , we put the rod under a hardness tester and reads 108 to a fzr600 rod been 105 . so not much in it

**husaberg** · 17th December 2014, 15:01

Originally Posted by mr bucketracer

it snaped at the bottom of the rod so all makes sence what you have said , im getting another rod and lighting the small end ,bore out the small end just a little bit , even though the piston is light a bit here and there should help , the fxr rod is very light to this one and they dont seem to have problems , we put the rod under a hardness tester and reads 108 to a fzr600 rod been 105 . so not much in it

Comparing it to FZR600 rods that have a reputation for being carrot like?

**mr bucketracer** · 17th December 2014, 15:12

Originally Posted by husaberg

Comparing it to FZR600 rods that have a reputation for being carrot like?

yes they are long so you think if they last with all that weight it should be a walk in the park with the ktm . think the early fzrs run the bigends a lot so i was told , the fzr 400 rod is so close to size its not funny

**F5 Dave** · 17th December 2014, 15:59

Earlier FZR600s had longer stroke but used to break rods in racing I seem to remember.

**Grumph** · 17th December 2014, 18:52

Originally Posted by mr bucketracer

yes they are long so you think if they last with all that weight it should be a walk in the park with the ktm . think the early fzrs run the bigends a lot so i was told , the fzr 400 rod is so close to size its not funny

i have the advantage of you in that the FZR600 we raced in pre 89 - very successfully till Kev bloody sold it - had race kit rods fitted..
Big end and shank all same same as std but the small end was slightly meatier and it was bronze bushed. They looked as though they were factory shot peened too.

Without seeing a pic of the KTM rod, if it's broken just above the big end, I'd pick poor design as the cause. Probably insufficient meat blending the eye to the shank. Cleaning up surface blemishes and shot peening will definitely help here.

If you do eventually do a rod from scratch the best way - as done in ChCh anyway - is waterjet cut from sheet then CNC the bores and the sides profile. Finally go down front and back to create the H section. For a bolt up bigend, you'd machine the joint faces and dowel it together and machine it as an assembly.

**koba** · 17th December 2014, 18:59

Originally Posted by mr bucketracer

it snaped at the bottom of the rod so all makes sence what you have said , im getting another rod and lighting the small end ,bore out the small end just a little bit , even though the piston is light a bit here and there should help , the fxr rod is very light to this one and they dont seem to have problems , we put the rod under a hardness tester and reads 108 to a fzr600 rod been 105 . so not much in it

Yeah, it would be a delicate balance there, the piston pin that led to Rich's big blowup at Tokoroa had a slight flaw in it and couldn't cope once they lightened it.
It'd be bloody good to be able to X-ray or MRI it like some people can!

**Grumph** · 17th December 2014, 19:01

Originally Posted by koba

Ahh, this is interesting to work out.

force = mass x acceleration.

Acceleration can be worked out with some scary looking math:
http://www.codecogs.com/library/phys...f-a-piston.php

I don't have time to rip in to it right now but that's worth a look.

It'd be interesting to see the change in force from more weight and more revs.

There is bound to be an accepted guideline of how-much-is-too-much.

Irving gives a worked out example of 54X54 with a piston weight of 6OZ - probably close to what is in the KTM.
Inertia load on the small end is around 2100 pounds at 12000 RPM.
If you work out the smallest cross section area of the rod and reference the yield point of a good steel, the rod doesn't have to be particularly large section to take the loads...but it should be shaped right, ie good blending radii. And a good finish so as not to have any stress raisers.

**Grumph** · 17th December 2014, 19:06

Originally Posted by koba

Yeah, it would be a delicate balance there, the piston pin that led to Rich's big blowup at Tokoroa had a slight flaw in it and couldn't cope once they lightened it.
It'd be bloody good to be able to X-ray or MRI it like some people can!

Go and see the reconditioners who do big diesels. Glen Durie here in ChCh bought Mace Engineering's old magnaflux machine when they wanted bigger (2 meter long cranks!!) He does a lot of competition crank and rods. Just done a GSXR1000 set for me...

**seymour14** · 17th December 2014, 19:22

I will make them, how hard can it be?

**husaberg** · 17th December 2014, 19:29

Originally Posted by seymour14

I will make them, how hard can it be?

water jet its step by step in the articles I posted esp when someone has access to a pretty flash cnc and a flash skill set.
Isn't about time we all seen the carnage rod pics though.......

**mr bucketracer** · 17th December 2014, 20:12

i will post some pics of the ktm with other rods buy it , i been told grain structures from round to plate can be quite diffient for making rods (whats the best)

**husaberg** · 17th December 2014, 20:49

Originally Posted by mr bucketracer

i will post some pics of the ktm with other rods buy it , i been told grain structures from round to plate can be quite diffient for making rods (whats the best)

have a read of the links the mvp is available in certified bar in suitable sizes..
bugger I am going to have to attach them aren't I?