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#Post#: 9858--------------------------------------------------
Jack shaft
By: NewbieNick Date: December 9, 2023, 12:20 am
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Question for technical types, what are the advantages of using a
Jack shaft. Can anyone recommend gear ratios.
Thanks as always to builders who reply
Cheers Nick
#Post#: 9859--------------------------------------------------
Re: Jack shaft
By: RhysN Date: December 9, 2023, 2:10 am
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Positives, jack shaft allows for easier ratio change, sometimes
makes packaging easier.
Negatives, adds cost (more bearings, extra chain and sprockets),
complication in that you have another level of alignment,
weight.
Some TAV systems , Max Torque, don't have a mounting plate like
Comet et al, so you are going to use a jack shaft anyway. Some
people prefer that.
You ask about ratios, if using a AV then ratio between sprockets
is, in my opinion, between 6:1 and 7:1. Theory would say you get
faster going towards 5:1, in practice it doesn't with an engine
tuned within the guidelines as the engine never gets to it's
"happy place at around 4500rpm as the gearing is too tall for it
to get there.
If not using a TAV then there is no simple answer, venue will
need the ratio altering to get the "best". Why do the racing
karters carry up to 10 sprockets and chains?
#Post#: 9860--------------------------------------------------
Re: Jack shaft
By: Jimr1999 Date: December 9, 2023, 2:19 am
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Advantages:
1 can help you place the engine where you want it (especially
useful if you run a diff centrally)
2 Choice of gear ratios is massive as you have 2 bites at the
cherry to gear down to the axle
3 You can run a smaller cog on the axle and this can save a
little length in the design
4 you could use it to be at the hinge centre of a rear swing arm
giving even tension on the chain through its swing
5 If you do number 4, the engine does not need to be unsprung
weight giving more responsive suspension
Disadvantages
1 More weight
2 more complexity
3 more mechanical loss in the drive (minimal)
4 harder to tension both chains when they stretch
5 cost
#Post#: 9877--------------------------------------------------
Re: Jack shaft
By: Mark Under Date: December 13, 2023, 3:34 pm
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The advantage is speed,
Being in the right gear is a massive advantage with such a small
engine, I have a choice of around 12 gear ratios that I can
easily change and do for most events.
In my experience gear ratios typically quoted are far too high
for our very short twisty circuits, meaning the engines are
labouring rather than revving.
#Post#: 9879--------------------------------------------------
Re: Jack shaft
By: NewbieNick Date: December 14, 2023, 12:02 am
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Hi Mark, are you changing ratios by swapping 1 sprocket? Do you
carry different chain lengths or use a tensioner .What ratios
are you typically using. Any chance of some photos.
I’m only looking at this as a possibility for my next Kart if I
can get it past her indoors.
Cheers Nick
#Post#: 9882--------------------------------------------------
Re: Jack shaft
By: Mark Under Date: December 14, 2023, 12:15 pm
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Nick
I mainly swap one sprocket on the lay shaft which is the easiest
to change, I have four different sprockets for that. This is
tensioned up by sliding the engine back. Each sprocket has its
own chain as they are too different in size to use a tensioner.
I also have 4 sprockets that drive the diff that I can change.
Tensioned by some packer plates under the bearings. Think you
can just see them in the attached photo.
You will have to work the ratios out, I think what works for one
kart probably won’t work for all. Plus the fun is in finding
out.
#Post#: 9883--------------------------------------------------
Re: Jack shaft
By: NewbieNick Date: December 14, 2023, 12:25 pm
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Thanks Mark. Hope to use something similar if I build another.
Cheers Nick
#Post#: 9886--------------------------------------------------
Re: Jack shaft
By: Granville Date: December 15, 2023, 4:35 am
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Hi Nick, I'm building a Jackshaft set up this winter for my
first Kart, found this link useful when looking at what size
plate wheels options I could use and what ratios I could end up
with.
HTML https://www.bmikarts.com/Go-Kart-Gear-Ratio-Calculator-Jackshaft-Setup.
#Post#: 9888--------------------------------------------------
Re: Jack shaft
By: NewbieNick Date: December 15, 2023, 9:48 am
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Thanks Granville really useful.
Cheers Nick
#Post#: 9891--------------------------------------------------
Re: Jack shaft
By: RhysN Date: December 16, 2023, 4:03 am
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Mark is correct in that gearing to "happy place" in the rev
range is what makes it work. Then again you may think it's
important to obey the club rule of 35mph, so just accelerate as
quickly as possible. That is, of course if you want to think of
cyclekarts as racing cars.
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