Topic: Gear Ratio

[tolis]

This one is a bit tricky to explain, but i'll try.

I am swapping a K7 Engine for an older model. I don't know exactly which one as it had it's serial number erased and restamped with a generic one by the Dept of Transport here in Greece. The deal is i am trying to decide on which sprockets and chain to use as the K7 was the only K model up to 1977 to have 15T/41T sprockets and a 630 chain. The one i am replacing it with had a 18T front so i am assuming it also has the different gear ratio gerbox.

Older Model
Primary Reduction Primary: 1.708
Gear Ratio I 2.500   
Gear Ratio II 1.708   
Gear Ratio III 1.333   
Gear Ratio IV 1.097   
Gear Ratio V 1.939   
Final Reduction  2.667, drive sprocket 18 T, driven sprocket 48 T

1977 K7
Primary Reduction 1.708   
Gear Ratio I 2.500   
Gear Ratio II 1.708   
Gear Ratio III 1.333   
Gear Ratio IV 1.133   
Gear Ratio V 0.969   
Final Reduction  2.733, drive sprocket 15 T, driven sprocket 41 T

Assuming the new (older model) engine has the older ratio and instead i put a 15T/41T combination what will that mean? Am i losing on top speed? take off? Would it be a problem that the final drive 18T sprocket is about 2 mm thinner than the 15T one?

Hope i didn't confuse every one.

Thanks
Chris

Can anyone enlighten me on the subject?

[kghost]

The 15/41 will turn lower RPM at any given speed.

This will make the bike feel like its accelerating slower off the line.

For example: Stock !972 CB750 18/48 turns 5200 RPM at 70 MPH.

One of mine runs a 17/43 4200 RPM at 70 MPH.
Other runs a 18/40 4200 RPM at 70 MPH.

Neither leaves a dead stop as fast with the smaller sprockets but are more pleasant to ride on the highway.

As for running a 530 chain on a 630 sprocket or vice-a-versa, Dont do it.

Sprocket Specialists can make you a sprocket to fit the 530 chain. I just ordered a rear sprocket from them to fit a 77F rear wheel, Its a 530 spocket that will fit in place of the 630 (required because the bolt pattern is different for the 630). Cost was $72.00 including shipping.

[TwoTired]

Yeah, what kghost said. or...

Changing to smaller front sprocket and/or larger rear sprocket equals faster acceleration but lowered top end.

Larger front sprocket and/or smaller rear sprocket equals slower acceleration but higher theoretical top end, if you have the power.

Don't use a 530 sprocket with a 630 chain.  I think there is risk that it might throw off.  If so, it usually wads up and holes the crankcase.  Advantage; your replacement chain is oiled automaticaly.  Disadvantage, so is the entire back part of the bike including tires and must carry lots of spare oil.

[Clyde]

The K7 runs a 17 inch wheel compared to the earlier model which uses an 18 inch back wheel. You need to take this into account when calculating Speed vs RPM. The K7 wheel used a slightly large tyre, which will mean that there is not that much difference.
On my F1 I increased the front sprocket from 17 to 18 for smoother/less buzzy cruising.
I would get a 530 sprocket for the back wheel and probably run 18/45 (503 chain) to compensate.
If the bolt dimensions are different, it would be worth checking to see if an early sprocket carrier (from a K0-K6 or F1) would fit into the K7 back wheel. Then you could use the standard K0-K6/F1 sprocket range, which may be cheaper than a special sprocket.

[tolis]

Thanks to all again for your replies,

just to clarify i was not thinking of putting a 630 chain on a 530 sprocket or visa versa. I know they have different pitch and cause a whole big mess.

What i was saying is whether it would be a problem to put the complete kit, front/rear sprocket and chain of the K7, on an older model, lets say a K4. That i was thinking because i am using the K7 frame and rear wheel or should i just get a 530 chain a rear and front 530 pair of sprockets matched to the gear ratio of my K7 bike like some of you suggested?

Thanks again

P.S. Let's see if i got things right
So smaller front and rear sprockets give lower rpm at any given speed
Larger front and rear sprockets give higher rpm at any given speed
Larger front and/or smaller rear give slower acceleration and a higher top speed if engine can deliver
Smaller front and/or larger rear give faster acceleration but lower top speed

[dusterdude]

does anyone where i can find smaller than 48 tooth rear sprockets for a 750 k1,other than sprocket specialists.i didnt see one available on their website.thanks

[alfabeast]

I've got a K8 that I converted to 530, I'm running 17 front, 43 Rear and I have a 130/80/18 tyre.
It's a little bit long legged but great for cruising, plus I've stripped it down and probably lost 40Kg, so the taller gearing works pretty good.


http://www.home.earthlink.net/~alfabike/

[kghost]

Dennis Kirk has standard sprockets in different gearings.

[dusterdude]

thanks kg

[TwoTired]

P.S. Let's see if i got things right
So smaller front and rear sprockets give lower rpm at any given speed
Larger front and rear sprockets give higher rpm at any given speed
Larger front and/or smaller rear give slower acceleration and a higher top speed if engine can deliver
Smaller front and/or larger rear give faster acceleration but lower top speed

 Not quite, the first two statements aren't necessarily true.  Consider a 20 tooth front and a 40 tooth rear results in two revolutions of the front gets one revolution rear.
However, a 10 tooth front and 20 tooth rear, also results in two revolutions of the front gets one revolution rear.

[tolis]

Ok,
so what happens in that case?
Is it better to go with less teeth i.e. 10/20 or more i.e. 20/40?
And why is it different since both front sprockets 2 revs result in 1 rev for the rear sprocket?

Also how does the K7 secure the front sprocket since it doesn't have the drive sprocket fixing plate/washer that the older models have?

Thanks
Chris

[TwoTired]

A larger sprocket will put less stress/wear on a chain due to less bending of the chain on the larger diameter.
The down side is that it takes more chain to go around the larger diameter and that weighs more.  A lighter bike is a faster bike.  Do you want better longevity or more speed?

According to the Shop manual, the front sprocket for the K7 is held on with an 8mm bolt.   The picture shows a rather large washer, too.   I don't have a K7.

[tolis]

Now it all makes since!

Thanks a lot