Help with gearing yz400f

[andper]

Hi, im totally new to this and i need help.

I have been building a crosskart (https://www.retrade.eu/sv/aitem/302073/Crosskart_Hondamotor_125_kubik) that look something like this. But i need help with the gearing.

"Engine: yz400f -98

19" wheel

Transmission 5 - speed

1st 1.929:1

2nd 1.563:1

3rd 1.278:1

4th 1.091:1

5th 0.952:1

Sprockets 14/49"

And i going to use 8" wheel on my kart.

I have read other threads with 450cc karts using sprockets with less then 30t. Does that apply for me to, im thinking because of weight of the kart(450lbs).

andper

 

[Bwdeaton]

What size will the rear tires on the kart be?

 

[andper]

Wheels will be 8"!

 

[Bwdeaton]

The tires will only be 8" tall? or are you referring to the rims themselves?

 

[Tpdingo]

8" rims and 19" wheels that is the only combo of numbers that works.

 

[Bwdeaton]

8" rims and 19" wheels that is the only combo of numbers that works.

the stock wheel size on the yz400 was 19". To calculate gearing, the size of the rim isn't important. Only need to know the height of the tires themselves. The kart in the example picture appears to have 8 inch wheels and approximately 18-20" tires.

 

[andper]

The tires will only be 8" tall? or are you referring to the rims themselves?

Just the rims are 8". Must check the actuall of the tyre.

---------- Post added at 02:21 PM ---------- Previous post was at 02:09 PM ----------

Tyre are 21". And yz400 wheels are 110/90-19.

 

[Bwdeaton]

OK. If you are building a kart like the one in your picture, I can approximate the gear ratio.

To calculate based off a desired speed would be easier. At 8000 RPM in 5th gear, the motor should have sufficient torque to move a 450lb kart at 60mph, so I will use these numbers for an example.

First we need to find the RPM of the countershaft on the motor. The crank has a 21 tooth and the clutch has a 62 tooth gear, so the primary reduction is 2.952. To calculate the RPM of the countershaft in 5th gear at 8000 RPM.

8000 / 2.952 X 0.952 = 2,847 RPM

If you have 21 inch tires, the circumference is 21" X 3.14 = 66"

To go 60mph, the tire must rotate 960 RPM

60mph X 5,280 feet per mile X 12" in a foot / 60 minutes in an hour = 63,360 inches per minute

63,360/66 = 960 RPM

this gives 960 RPM at the axle and 2847 RPM at the engine sprocket. For easy building, the 14 tooth stock sprocket can be used. so the axle sprocket can be calculated with

2847/960 X 14 = 41.5, round to 41 or 42 teeth.

I believe these numbers are conservatively achievable with the torque the 400 motor produces.

The short answer would be to use a 41 tooth sprocket with 21" tall tires and have a top speed of just over 60mph while maintaining considerable torque in the lower gears.

 

[Joe-405]

So my 22” tires are a 66” rollout or diameter. So your will be smaller than mine or should be.

I did the actual math for every gear with a 66” tire (inches around the whole tire in the middle) a 14 tooth front and 41 tooth rear. You are pretty much geared exactly as the stock bike is geared with the other tire.

 

[65ShelbyClone]

You need a 2.74 final drive ratio to approximate the YZ400F's original gearing. Either 13/35, 14/38, or 15/41 sprockets will get you there.

The engine is capable of nearly 11,000rpm and can take a stock YZ400F to about 90mph/145kph. Make sure your safety systems are adequate.

 

[Bwdeaton]

You need a 2.74 final drive ratio to approximate the YZ400F's original gearing. Either 13/35, 14/38, or 15/41 sprockets will get you there.

The engine is capable of nearly 11,000rpm and can take a stock YZ400F to about 90mph/145kph. Make sure your safety systems are adequate.

With the additional weight of the kart, lower gearing than the stock yz ratio will likely be necessary to have better driveability. If keeping the stock 14t countershaft sprocket size, any 40-46 tooth axle sprocket could tested to see what you like best for driving characteristics, clutch release, and torque availability.

 

[65ShelbyClone]

It wasn't clear to me whether the 450lb number was race weight or just the kart alone. I assumed it was race weight, which would put it not far off from a YZ400F with a sizable rider.

If the kart + driver package is going to be 600lbs+, then yeah, lower gearing is definitely beneficial.

 

[itsid]

it's actually not possible to calculate the ideal ratio I'm afraid...

it's only possible to estimate a good one.

And IMHO that'd be between 4.5:1 and 5:1 for a dry weight of ~150kg
and a curb weight of ~250kg
(means 63-70 teeth on the axle keeping the 14T engine sprocket)

The thing is, it's a revver engine,
max torque is way above 5k rpm (I think @ 6500 or something)
and it's internally taylored gearing for a 110kg race bike doesn't actually compensate for heavier vehicles either

So in order to allow the engine to rev up quickly (which equates to fast acceleration)
you do not want to put much more load on it than the original setup would.

which means very conservative gearing!
essentially it boils down to
"new curb weight" * "original gear ratio" / "original curb weight"
keeping the original wheel diameter (that'd be 66 teeth)
as you can see I added some wiggle room, since your 21" wheels are smaller than the original ones, but then again the weight might be off a few kgs as well..

And with that, you work your way up,
letting the engine decide wether and how easy it could hadle a higher load
(higher chain gear ratio)

here comes the nice part: it's as easy as trying to take off in second gear.
No matter what, you must be able to do so,
then test again in third, most bikes are able to barely do so.
Once you hit that point with a new ratio, you have found about the best ratio.

The idea behind it is, that this way you still have a decent amount of torque in final gear
to actually hit top speed.

And while a 55 horse single cylinder engine should pack more than enough punch
to propell about any weight kart up to decent speeds,
it's sometimes the little things that count

And those little things are easier to feel than to calculate

'sid

PS I think you might end up with a 50-55 tooth axle sprocket in the end.
And yes I know that some people give a shirt and gear way higher (like low 40s maybe even high 30s) but for that you will pay with engine durability and lifespan of the gearbox (clutch will go first, followed by the pinion gear and shifting fork)
only to add more top speed (something you unlikely ever make use off)

 

[andper]

It wasn't clear to me whether the 450lb number was race weight or just the kart alone. I assumed it was race weight, which would put it not far off from a YZ400F with a sizable rider.

If the kart + driver package is going to be 600lbs+, then yeah, lower gearing is definitely beneficial.

Kart+driver(me) will probably 600lbs+

it's only possible to estimate a good one.


PS I think you might end up with a 50-55 tooth axle sprocket in the end.
And yes I know that some people give a shirt and gear way higher (like low 40s maybe even high 30s) but for that you will pay with engine durability and lifespan of the gearbox (clutch will go first, followed by the pinion gear and shifting fork)
only to add more top speed (something you unlikely ever make use off)

Actually I think its geared 14/51 right now, its a pain starting on tarmac,but maybe its me being to careful with the throttle.

 

[itsid]

As I said, it's still a race bike engine
below 5k rpm there's not much oomph in it..
it wants to rev, kick it! (or you'll ruin your clutch)

If you get above the 5k mark and it still doesn't want to start..
my guestimation was off (or your engine lost a serious amount of power)
and thus you will need to gear lower (back to 65-70 axle sprocket I'd say)

'sid

 

[Joe-405]

Leave the rear sprocket and go down 2-3 teeth in the front. Will be cheaper and easier.

 

[itsid]

I don't like that idea at all!

smaller front sprocket means increased wear (fewer teeth to spread the load across)
and you'll have a a shorter chain, which means every roller sees more abuse per full pass...
so as a result shorter lifespan of the chain and BOTH sprockets (the worn chain ruins the axle sprocket)

And while at first you save a buck, it comes ricocheting back and costs way more in the long run!

Do not forget we do NOT talk about any industrial 10horse but a 55horse motorcycle engine;
every tooth that grabs onto the chain carries about as much power as all of the engaging teeth on a prepped predator's clutch

So no.. I would not reduce the front sprocket in size!

'sid