DR400/SP Stroker Cranks

[SON]

I am about to get 4 cranks rebuilt 2 with standard length long rods
2 with shorter rods
Is there any demand for any more?
Price will be about AUD$425 including new rod kit

[ola_martin]

What stroke?

[SON]

What would you like?

[SON]

I am thinking of doing two different lengths
A little R&D
1 for MX
1 for Flattrack

[oldfart]

Interested in Mx version...  cams are problem they tend to wear out the carrier.

[evo550]

I am about to get 4 cranks rebuilt 2 with standard length long rods
2 with shorter rods
Is there any demand for any more?
Price will be about AUD$425 including new rod kit

What does changing rod length do on a fourstroke??

[SON]

Long rod is better for several reasons
However that will require a barrell base spacer, a longer cam chain and a bit of work to get the compression and valve clearances just right.
The squashed or shorter rod Might??? Alleviate the barrell spacer, allow the standard cam chain and is generally easier to set up, big downside steeper angles don't like revs but should be OK for VMX.
My TT 605 runs the Powroll short rod stroker but is for MX
I would not run a short rod stroker on Flattrack.

[ola_martin]

Sorry, been without a computer for some days.
I don't know much about these engines, just that I had one as a kid, and would love to build something cool for MX with a DR engine. I don't even have one anymore...
I don't have the knowledge to calculate the compromise between stroke and rod angle, but it would of course be best to have as much cc as possible.
If I where to buy a crank I would like the short rod type, as this engine is tall enough...

[SON]

Better question/answer is MX bottom end mild revs V Flattrack top end high revs
Answer your own question but shorter works for me

[ola_martin]

SON, what stroke do you reccomend as max with a shortened rod then? For MX.

[evo550]

Long rod is better for several reasons
However that will require a barrell base spacer, a longer cam chain and a bit of work to get the compression and valve clearances just right.
The squashed or shorter rod Might??? Alleviate the barrell spacer, allow the standard cam chain and is generally easier to set up, big downside steeper angles don't like revs but should be OK for VMX.
My TT 605 runs the Powroll short rod stroker but is for MX
I would not run a short rod stroker on Flattrack.

When you say shorter rod, do you mean standard length rod ?

[ola_martin]

When you stroke a crank you off course need to shorten the rod by half of the total increased stroke, to keep the piston stopping at the same height. This comes at the expense of a less ideal rod angle.

When you stroke the crank and keep the original rod you need a spacer of half of the increased stoke at the bottom of the cylinder.

[pancho]

 Somewhere in the "files" there is considerable discussion on con rod length and the associated variation in rod angulation (is that a word?)
I found it very interesting in light of my experience in the '60s were short rods were the "in thing".
 The summary in my head indicates that shorter rod equals more torque at some limited rev range, all the current experts seem to agree that longer rod is better overall.
  (still talking engine!)
pancho

[David Lahey]

Longer rod = heavier rod so there is a significant trade-off

[SON]

Long rod is fine for flattrack where you want the flywheel weight,
I have photos of the DR engine with the Yamaha rod, I just don't know how to post them,