Discussion FZR250 1986 to DeltaBox Frame? Possibility and Advantage?

I can confirm the engine mounts are the same, apart from the cylinder head as your aware. Most things could be made to fit I think, if your comfortable customising bits. Forks/ triple clamp is different but you can probably use a different bearing to get them to work

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The deltabox FZR250 frame isn't particularly light. In particular the rear subframe is heavier than needed.
At this performance level I see no advantage in going deltabox. The factory did it to make the 250 look like their bigger bikes and give owners the cachet of having a bike just like the big ones.
If anyone really wanted to build a very light and stiff chassis for the 250 four, I'd look at the KTM style steel trellis.

 

I think there's only one clear solution, and that is just get one of each. It's a bit scary seeing two nearly identical project bikes in your shed (currently my situation, 2KR and a 3LN3), but a glance at the redline number on the tachometer makes that go away

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The low modulus is compensated for by using deeper, wider section frame spars. The very simplistic way it's done on the 250's doesn't help weight - but undoubtedly helps cost. Nearly 3mm thick half pressings with a generous overlap welded in convenient spots makes for a quickly assembled frame. It's far less sophisticated than the bigger FZR's. And light-years away from a TZ.

 

personally i prefer the feel of the 2kr, different ergonomics.

but, i really had to scratch my head over the decision to make the frame on one side removable, whilst running coolant through the NON removable side of the frame.
the amount of rust that came out in my coolant was scary?

then, theres this nasty air pocket in the bit of tube above where the coolant enters. drill a tiny hole right up as high as you can, pop a screw and fibre washer in... then you can bleed the system properly (found this was also an issue on the zuki vx800 that does the same thing, coolant in frame? but it was that bad it would boil and spray all the coolant out the overflow until i figured it out...)

and, if you go 2kr... check the bottom shock pivot/engine mount. bit of square tube. mine was always a bit saggy at the rear, as after 30 odd years, the tube was starting to deform. every 2kr ive looked at since has signs of deformation in that tube...

thought nothing of it really, old bike... until putting it into a guard rail one night... well, lets say the tyre hit the frame hard enough to break all the lugs off the mudguard, and popped the CDI wires out? made a bit of a mess in that rear subframe... pillion seat and all that, took some bashing to get it all working again...

that tyre should not be able to get ANYWHERE NEAR the mudguard!

and here it got weird. the exhaust was completely crushed at the 4:1, it refused to start on the e-start, just spun over without a cough, but would clutch fine and ran fine. a certain kiwi will attest to how it put certain BMWs to shame in that condition, despite having the rear now sagging at least 50mm. tyre kept scrubbing in mudguard. lol, they see me clutch starting it and try getting in front... i was onto the second ciggie when they caught up

it sort of actually felt really GOOD with it all saggy!

i really must have hit that rail HARD! i swear i was only doing 30 or so...

and the bottom mount on its square tube was twisted like a pretzel.

stripped it down, replaced with a nice bit of round tube...

and then had another head scratch about the geometry of that bottom linkage, as no matter how one fiddled, it seemed that the linkages were all arranged so theyre already bottomed out before even starting to compress the shockie. cant move forward, engine in way. cant move back, makes rear sag. cant move down, makes rear sag. cant move up. engine in way. that pivot can only be where that pivot is located.

and the shockie itself is a bit unusual and whilst i thought of changing it, the oddball bottom pivot on it had me sticking with it.

yet it still seems to work fine? for a 30 year old shock...

and then i chopped it out again and used a bit of heavier wall tubing (OOPS!), and well... it did another 50,000KM after that with no issues...


2kr has 4mm valvestems, same seals as the 400 and 600... 3ln is 3.5mm.

other than the lugs theres no visual difference besides the number cast in, but there may be a difference due to the 3ln having a longer rod? that suggests the block may be 1mm higher, and that suggests the camshafts may be 1mm lower in the head to get the same camchain to line up properly degree wise?

iunno, that part is just speculation. maybe someone can verify or not? i could but im in no mood to tear the 2kr down just yet.


i can weld ali, but if i had to... i wouldnt be welding the 3ln frames?


oh. final thing. the 2kr has a silly metal bar above the carbs. the fuel tank mount.

for boosting, i feel the 2kr is more suited to a turbo for this reason (its the only reason i still have it really), whereas the 3ln has plenty of room to spare... i have an AMR300 waiting to get shoved into one, and the FZR engine has the advantage over the other 250s of having the generator stuck on like an afterthought, so one can tuck a pulley BEHIND the flywheel... making it rather neat and tidy and with some effort can be rather discrete... rather than all the others where its stuck out on an extension with belts hitting knees, holes in cases, and looking like absolute crap...

 

The longer rods are matched to shorter pistons. Block height is identical. Other than the valve stem OD the head geometry is also identical.

 

this is nice to know... so the deck heights a bit less?

could have made a mess if i threw the new pistons into the 2kr!

 

Deck height is the same so from crank > conrod > piston deck height - as cases & barrels come to the same height for both models - the compression height on the pistons is different, the 2KR piston is 2mm higher than the 3LN piston (centre of piston pin to deck height) - that's where the 2mm difference in conrod length equalises the difference between the two models and their pistons.

The 3LN1 service supplement details the changes from 2KR - 3LN - they make a lot of fuss about the conrods/pistons.

I think the 3LN is an outlier with 82mm conrods in the 250's, as research 'suggests' the others Honda, Kawasaki and Suzuki have 80mm conrods.

 

Not an uncommon development in an engine design which has been around for a while. Longer rods are a help in reducing angular friction if you're going to raise the revs in a later evolution of an engine.

In the days when the WSB was mainly 750 fours, there was a reg that you couldn't change the distance crank to head joint. Several race kits of the era - RC30 for one - had longer rods and shorter pistons.
Now I've observed that the late 1000's have what we'd have considered long rods as delivered.

 

As I understand it, longer rods and shorter pistons are better for high revving engines (along with the bore/stroke ratio being oversquare). I believe the reason is that it reduces piston velocity/inertia forces, so that the pistons don't decide to become a two-piece design and munch the engine.

 

i always considered "deck height" on the piston as crown to pin... awe hell, it was two decades ago that i spent hours running through wiseco catalogues and the akunar site hunting down different pistons for a ct110... maybe i just got the terminology wrong... there was crown height dome height, pin heights, blah blah blah blah...

whatever, 3ln and 2kr pistons and rods are different. good to know! anyone tried the next obvious thing? throw 3ln rods n pistons into the 2kr?

close...
when the piston is at half stroke, the crank is NOT at 90 A/BTDC. pistons dont perform sinusoidal motion, top half of the stroke is always slightly faster than bottom half. and the secondary imbalance is due to the piston inertia/acceleration producing this sort of "hump" at BDC...




the longer the rod for a given stroke, the closer the piston to midway at 90 A/BTDC, and the closer to the ideal "sinusoidal motion" it gets... less secondary imbalance. and also, as the rod is more "vertical", the sidethrust on teh piston/cylinder is reduced, and more of the combustion pressure goes into making torque.

dont get me started on engine design and ways to eliminate this effect! swashplates, scotchyokes, but my favourite has to be the "murray hypocycloidal"... AKA "cardan crank".





as you can possibly see, it raises certain "challenges" to make as a multi-cylinder! the piston rod tends to sort of get in the way...

they really do sound utterly different if you get to hear one... try YT?

i went mad with a few alternative designs on this. one day i might get my s**t sorted and actually make one... it doesnt have to have that ring gear

and then theres ways to totally go insane and produce variations on the atkinson cycle, cardioids, i have a design for an engine with two TDCs, two BDCs, and the full four strokes in one revolution... if only i had a rich uncle and nothing else to do but make mad engine designs...

more. see, im really good at doing this!



iunno if you ever played with pitbike engines, but i came across the weirdest outlier of all, a 10mm longer rod than any of the same type (same stroke)... and the cylinder was 10mm taller... and so the studs were 10mm longer, which is how i found out when i snapped one...

the so called "125"...um...1pm52? "clutch on crank" thats actually 119cc... yet the same cylinder height as the 140s (1pm54/55?) with the "clutch on secondary"... no chance of swapping over as the stud patterns are totally different... the studs are also a different thread between the 125's and the 140s...

i hunted through a pile of probably 1000 cases at a pitbike wreckers... hours... nope! utterly different! not one of the 125s had that extra 10mm.

lol. "shineray". ahhh, memories... having the clutch shred its splines overtaking a learner one day... complete junk but it did seem to rev out a wee bit higher than any other pitbike engine...

that was hands down the worst engine ive ever bothered playing with, though a vintage "danarm" chainsaw came pretty close. ever tried assembling crowded needle rollers through a transfer port?

 

I only made a differentiation between those two terms as I saw another question somewhere about the cylinder block height potentially being different, it wasn't to nit pick.

I also seem to recall someone getting some Aliexpress FZR250 pistons that were supposed to be for 2KR or 3LN (not interchangeable) and they ended up with the wrong ones.

Aliexpress sellers aren't always completely across their product listings...

3LN rods and pistons go straight into a 2KR engine, everything ends up the same.