DT2MX transmission

[JC]

Although these percentages is some guide don't you also have to calculate the primary drive ratio. Doesn't the primary drive ratio impact on the overall and the % drop between gears.

This ain't my expertise (very dodgy actually ). And isn't there a difference in the primary drive ratio difference between the 250's and the 360/400's (clutch basket etc.).

Graeme, primary (& secondary) ratios certainly affect overall ratios, but don't affect % drop from gear to gear. Its simple maths.


Vandy, no criticism intended. I was just crunching the numbers so people could see what % drops tell you. Yr descriptions of how they feel when riding is exactly what I would have expected from the % drops. It works for your intended purpose. I might have used the 29T 5th gear from an MX box in the 360, to close up that top gear drop if it was mostly used for racing, but each to his own.

[Davey Crocket]

Trust me, they rip....std gearing all they want to do is wheelspin and fishtail.....vandy's riden it....he cant work out why I'm so slow.

[firko]

Boys...This is great stuff. It's an area that I fall a bit down on so I'm finding it all very interesting. I knew JC and Vandy would come up with valuable information....keep it coming guys . In all of your opinions, what would be the best trans ratios for a piston port 360 RT1 Yamaha using what's available from the Yamaha range? I'm building another engine for my Cheney using the best I can muster. Lozza's doing the ports and supplying a 'Lozzatronic' CDI and Frank Stanborough is balancing the crank. If I'm going to the trouble to get it going fast, I'd like the best ratio spread available.
I've got a DT2MX trans in the engine currently fitted to the bike and have a YZ250A trans in my DT1 from Hell engine. The DT1 was/is perfect but going by your opinions, the DT2MX ratios might not be all that great for the 360. Because of the torque the 360 produces, would the stock ratios be better than the DT2MX? I'm starting to think they may be, even before I ride the bike.

[Freakshow]

MY PP 360 honked along quite well, 1st was a bit of a screamer and then it was great till to hit top then it stopped.

It tended to load up the carby a bit, so maybe with a bit more power it would clean out.  if i was do do anything to the gears i would stuff around with 1st and 5th.   I cant start in 2nd and 1st needed a change metres of the line, so if you can back it down to just under 2nd i reckon that would be a good thing, then you can still pull sprocket changes. 

[Davey Crocket]

Use the DTI from hell gearbox Firko.

[firko]

Use the DTI from hell gearbox Firko.

I would John but I don't want to rat that engine. It's a bit sentimental to me plus it's still a goer (in need of a bigend).
But........if you could sell me a YZA trans from your extensive stash my problem would be solved .

[Davey Crocket]

Unfotunately I only have 1 spare g/box....old mate in the west has a few.

[JC]

Mark,
 
There's no question that if you want a FAST engine, the closer the ratios (eg YZ-A/B) the better. The simple reason is that when you change up it doesn't drop as low in rpm on a close ratio box so you are in a meatier part of the powerband.  (Thats one major reason why YZA/Bs & VR Montesas are so fast.) But of course it requires a bit more tap-dancing on the shifter. (Hence my point about YZA/B ratios being a bit too close IMO, esp for a 360. Fine for Gary Jones back in the day, but for regular VMXers..?? Is it needed?)

A  DT2M or MX-A gearset really needs a closer top gear for a fast engine, eg from YZ, as that std gap is really too big for a fast bike. Depending on the track, you may get away w gearing the bike to use 2nd to 5th, which means % drops of  24%, 21% & 14% (forgetting the 1-2 drop of 27%) w a YZ top gear. Not as fast as a full YZ gearset, but less tap-dancing

Using std DT/RT ratios, you'd also need a wider powerband & closer top gear, say from the DT2M, & likewise probably gear it to use 2nd to 5th, which means % drops of 27%, 23%  & 21% (forgetting 1-2 drop of 29%) w the DT2M top gear (Only requires one gear changed IIRC). Still less fast than the above, but even less tap-dancing - a bit of a 'lazy-mans' race bike. With the right final drive ratio, you should be able to overcome Freaky's problem mentioned above (ie 1st too low; 2nd too hi) on his PP360 w this set-up.

Another important consideration is overall gear-ratio spread (bottom ratio divided by top ratio). You typically need about 2.2 - 2.3 in a 250 racebike; perhaps a little less for big-bore. (eg 4sp Maicos were 2.0 IIRC)

[firko]

Another factor that occured to me is the ignition curve and flywheel weight and their relationship to gear ratios. We know that the difference between the heavy external flywheel DT400 cdi unit and the internal flywheel lightweight YZ or PVL style ignition will produce different power characteristics and therefore different gear spacing needs. I realise I'm putting an over complicated take on it but I know from my DT1 that the ignition differences make dramatic changes to the engines power to the point that a corner taken in second with one ignition can be taken in third with the other. Am I reading too much into the importance of this in relationship to the transmission ratio? Am I morphing into Ji Gantor?

[vandy010]

In all of your opinions, what would be the best trans ratios for a piston port 360 RT1 Yamaha using what's available from the Yamaha range? I'm building another engine for my Cheney using the best I can muster.
 If I'm going to the trouble to get it going fast, I'd like the best ratio spread available.

Firko,
for an MX race motor, any of the close ratio MX/YZ or DT2MX boxes will be fine.
in the 360 donk, i'd be aiming for the tallest ratio 2nd gear i could lay my hands on as starts are gennerally done in 2nd and a tall 2nd will allow the motor to pull it for just that bit longer and if your silly enough to go back to 2nd out on the track it won't rev its tits off quite so much.
Daveys YZ250A is flat amazing.
it felt like a bigger motor to me but my RT1 makes smoother torque.
when young Davey's done with it, he reckons he's gonna wrap it in shiny plasticy stuff and leave it under my tree
what a mate

[vandy010]

Another factor that occured to me is the ignition curve and flywheel weight and their relationship to gear ratios. We know that the difference between the heavy external flywheel DT400 cdi unit and the internal flywheel lightweight YZ or PVL style ignition will produce different power characteristics and therefore different gear spacing needs. I realise I'm putting an over complicated take on it but I know from my DT1 that the ignition differences make dramatic changes to the engines power to the point that a corner taken in second with one ignition can be taken in third with the other. Am I reading too much into the importance of this in relationship to the transmission ratio? Am I morphing into Ji Gantor?

STD DT400 monoshock flywheel wieghs 1800grams
                RT/DT1,2,3   "          "        1650grams
my RT1 is using a YZ490  "         "         1450grams.
the 1650grams flywheel gives a good torque/rev mix for my liking
as to how it all ties in with gear ratio's?
we need beer and a good pub after a good days ride...

[mx250]

Change the flywheel weight changes the feel of the power delivery not the amount of power - general makes power delivery easier to control, smoother.

Taller gearing also changes the feel of the power delivery reducing the multiplier effect of the gear ratio, reduces torque applied, and also means you can hold the gear longer, over a greater speed range, which can also be a handy 'tuning tool' for specific situation (but would only normally be applied at the highest levels of motor sport).

Normally a heavy flywheel goes well with a w/r gear box. Even the most viscous 2t can be made like a 4t with enough flywheel weight . 

I would use either for one or the other rather than both. But that's me .

[firko]

I left a key word out of the sentence in my post "differences make dramatic changes to the engines power characteristics". I like the DT400 flywheel on my DT1 as it's fairly radically ported and the heavy flywheel tended to soften the 'on/off' type power delivery that hindered it when the MX250 internal flywheel ignition was being used. The extra inherrent torque of the 360 puts a different slant on it though. I'll initially be using 'Lozzatronic' ignition....not sure of the flywheel weight......... Lozza?

[Freakshow]

I had a PVl on mine so you would be thinking similar rotor to a lozza knock up, so refer PP360 info above again .

[martian170]

I have a complete NOS gearbox but not sure what its from RT1 i think includes shafts and shift folks all in original packaging 5 speed close ratio dont know what its worth make me an offer
regards
   Martin