honda step-tru racing

[TerraRoot]

It's that time of year again ;D
end of next month is the nifty fifty charity endurance race at mondello (Dublin), Last year my dads team won the 50cc class, this year im helping with the tuning to see if we can nail more of the 90's as well.
Last years winner, now engineless.

This years bike, it's a 90 which is in the process of an engine swap with the above bike

One of the local racer guys bike (derek wilson), it needs a rebore but we can't go mad have to stay within the factory bore and stroke.

Last spotted at the isle of man TT, circa the 70's. Maybe we should leave the aero mods till next season...


The rules state we have to keep it honda, so we are doing stuff like a 90 head on the 50 barrel, we are basically using the best honda made. Also i'm pushing for some mad tricks which i can't talk about just yet, but i'll post it after the race (terrible i know )

I'm not racing, figured last year would be just a flash in the pan, for the money it's great racing 2 hours cost the same as 60 seconds of hillclimbing, Have to get in next year just need three other able body people.

[davytelford]

not giving away any secrets then lol

[derek]

Best thing to do with he cub/pit bike engine is to port and polish it to within an inch of its life ;D then fit a nice fat polished manifold and good carb. (if you havent already done that ;D)

Good luck.

[cerberus73]

Big valve head from a c70... c90 barrel & piston... higher lift & duration cam from a SS50, bigger carb, and a 16 or 17t front sprocket, and as derek said port n polish the head & manifold and if you can find it a 4 speed box out of a ATC70 honda 3 wheeler gives you a extra cog and less revvy at top end so makes engine more reliable as its arse aint being ragged off. oh and make sure u use a 90 crank and connecting rod as the 90 has same bore as a 70... just longer stroke

[TerraRoot]

We are doing a 4 speed bottom end but we want to stay in the 50 class, we think we can get away with an aftermarket camshaft also some HD springs to combat valve float since mondello is huge and basically flatout for the the whole eight hours.
Right now it looks like we can get an outer rotor kit from a local guy who isn't racing this year, this will give infinite ignition possibility's and a redline of 14000rpm, perfect for our blend of 'ahem' pump gas. ;D

[davytelford]

so whats with that aero one looks unique

[TerraRoot]

The aero one is just something spotted back before i was born at the iom TT, we may have some tz fairings we could put on, but i think we could do better

[twoblackcats]

This looks a good laugh although all racing has a serious edge no matter how much you kid yourself it's just for fun

What you got planned for the fairings or is it all top secret need to know info till after the event? ;D

[TerraRoot]

IF we do some aero mods it'll be what ever fairing we have lying about the place, right now we are just concentrating on swapping engines about and making the standard (with 4 speed box) 50 run, once we find out how fast that is we can work from their. Their is only one top secret bit but i should be able to reveal that in a week or two just before the event, no one can catch us by that point

[TerraRoot]

Copy Pasta from the blog:



Most importantly the paint scheme has been selected, a joey dunlop traditional yellow with black pin strips. worth 5bhp at least.



50cc bore and stroke with c90 head installed, f**k all compression ratio and no squish band. but he won't fix it since he plans to run the 70 class next year and doesn't have a spare head.



selection of heads, bottom left is the fifty head, it's got 19in/13ex valves minuscule, all the other heads have 25in/22ex, the very early 90 head (top left) has a flat face with no squish band and would be perfect except the its the older cam in head design and the race for the cam is magnificently worn out.



Difference in inlet porting.



older style piston on the right would be brilliant if we could get a cam chain length to match the appropriately heightened barrel (piston is from the pushrod 50)



the shocks from my rxs have found a temporary new home.



Racing tyres,

pro: sticky, peaky profile might give us less drag on the straights, out psych the competition.

con: quite heavy, probably the wrong size, will tear the chassis to shreds in a hard corner



Last years trophy.

[davytelford]

good luck with it rooting for you

[TerraRoot]

Bit more done the other day



holes in everything and a seized center stand.



chop out the stand weld in the left over bit of the axle, hope the brake lever doesn’t break.



More holes, the rear torque arm has a guide/bung welded to it to allow a cable to work the rear brake, so we have a rear brake lever and a rear brake pedal, handy if you’ve no right leg.



Swing arm ready for painting.
this morning i got an mms from my dad:

[cerberus73]

with a C90 head on a 50 bottom end, the reason there is no compression really is cos the 90 is a long stroke, ie the conrod is physically longer, they use same size piston basically as a 50, just a longer stroke in the barrel to give a larger swept volume. so with a 90 barrel on a 50 bottom with 50 piston and conrod, the piston will only be coming so far up the barrel not to its optimim position, you will be running something like a 6:1 compression instead of a decent 9:1 or suchlike, that bike will never produce power like that, great if you wanna run it on paraffin or something! but for racing.... no ways, acceleration will be like a asthmatic fat kid. there is no squish band on these engines.... squish bands are for 2 strokes, the honda engines are 4 stroke, the area that looks like a squish nband is cos the diffrent engines over the yrs used diff shaped pistons, it was to give optimal shape to the combustion area. make sure you match piston to particular head, or it could be bent valves!, you can splice cam chains to give a longer length, so easy enough to make your own diy chain out of 2 others.

to use a 90 barrel you need to use the90 crank and conrod, either that or switch back to a 50 barrel, with a 70 big valve head, a much better racing set up. i would stick some heavy duty ebc clutch plates in there as well and a hotter cam, plenty available from monkey bike places... if you want tuning tips and guides use the monkey bike places, the engines the same and those guys ring the necks out of the limited power it can theoretically produce

[TerraRoot]

In the many many version of this particular honda engine, no 50 has the same bore as any 90, certain 70's yes have the same bore but with the 50's stroke. Our bore here is a 39mm with a 41.4 stroke.
A longer conrod will only help reduce the piston side thrust forces it does not affect the stroke.
Squish bands are for all internal combustion engines, valves get in the way on four strokes so they are more effective on two strokes but still needed anyway.
I hear you about the 6:1 compression but can't really talk my dad out of it, he wants the bigger valves, still it won't take very long to swap back to 19/13 valve head to see where that takes us. have some pics:

some weight turned off the secondary drive.

[davytelford]

she's looking good and very loud