Want to increase ground clearance of racing kart

[babzog]

Hey folks,

So, if I wanted to raise the ground clearance of a racing kart so my kids can muck about on the property and gravel laneway, I figure larger dia turf type tires will help gain an inch or two. Problem I have is getting the air intake/carb (honda gx140) to clear the rear wheel. Thought about raising the motor mount somewhat but that looks like it'll put the clutch right beside the driver's ear.

How would you go about handling this situation? If i raise the motor, is there an appropriate guard that can be had to protect the driver from the clutch and chain? Other solutions?

Not so concerned with maintaining speed, in fact, slowing it down is a good thing. So long as it goes forward under its own power and maybe puts a little wind in the faces, they'll be thrilled.

 

[babzog]

So, what would you suggest?

 

[r_chez_08]

Is the problem ground clearance from the frame or sprocket?
Have you got any pics of the kart?
If the frame, I would change the bearing position.
If the sprocket, you could get a jackshaft.
Here is a chain guard.

 

[landuse]

A jackshaft will be your best bet if it is for sprocket clearance. Getting larger tyres will just mean that you have to get a larger sprocket to keep your gearing the same, and you will still have a clearance problem

 

[anderkart]

These guys have a good point about you needing a jackshaft. Even with a 10 tooth clutch you'd need a large axle sprocket that wont have enough ground clearance for off-road use.

But If you installed this jackshaft kit: http://www.bmikarts.com/item/Motor-Mount-Plate-with-Jackshaft-and-Bearings-400361-6636 (between your motor mount and engine) It should help solve your gearing and air cleaner-clearance issues.

Then after we knew how tall your new back tire were, we could help you select a clutch and gears so you'd end up with a good gear ratio and several inches of clearance between your axle sprocket and the ground.

 

[babzog]

So, I guess I'm missing a bunch of information here, so please school me.

My thoughts were to get a kart for my kid so he can just toodle around the property, up and down the laneway. Not fast is good, at this time (he thinks otherwise, but he's only 5 and not in a karting league yet so I'll be the boss of speed at this juncture... lol). If it moves under its own power when he hits the gas and can stop before he parks it in a tree, he'll be thrilled.

The kart is an old racing kart, so the frame and sprocket are both close to the ground... figure there's about 1.5" frame clearance and maybe an inch more for the sprocket.

I see some folks are concerned about gearing and such and I'm not sure I follow the reasoning why.... if it's only to maintain speed, I gotta say, I don't care if it loses a few mph... I''ll be putting a throttle stop on it anyway so any loss in speed due to a slightly larger tire will be negligible (I would think) compared to the speed I'll be robbing from it. If there are other concerns about gearing with a slightly larger tire, then please educate me.

I picked up a (used) 12T clutch last night from a local shop. Fellow said to put on the largest sprocket I have (I think I have a 60+ T sprocket that came with) to further reduce the speed.

I had no idea what a jackshaft was but youtube came through. What sort of gearing would I need if I went this route? What are the outcomes of changing the gearing between the motor and JS and JS and axle? Not sure if I'd have the room to install one... will have to take a look next time I head to the shop.

I also picked up a new clutch guard last night but the spinning clutch will still be beside the driver's ear. Any decent full chain / clutch covers (that not only stop oil spray but are designed to keep hair and casual finger poking from being a hazard? Or am I into fabbing my own?

 

[landuse]

OK....I am not the expert here, but I can give you some pointers.

I am not too sure what your gearing is now (axle sprocket/clutch sprocket), but if you use a 12T clutch and 60T axle sprocket, then your gearing will be 5:1. This is a really high gear ratio. With karts, the general rule is to start from about 6:1 and up. At a ratio of 5:1, the change of your clutch slipping and burning up are really high, especially if you are on uneven terrain. Basically, a high ratio (like 5:1) will give you more top speed, but lower torque or 'pull off' power. A lower ratio (like 8:1), will give you less top speed, but will increase your torque or 'pull off' power.

When you increase the diameter of the tyres, and keep the same gearing, it is as if you are gearing higher. You will theoretically have a higher top speed, but you will probably never get there because of your clutch being burned up. To compensate for this, you will have to get a larger axle sprocket, which then cancels out the reason for the larger tyres in the first place.

The guy who told you to use the largest sprocket with your 12T clutch knew what he was talking about. He was telling you to gear low, which would reduce top speed and increase torque. The problem is that a larger sprocket just means less ground clearance.

The normal way to calculate your final ratio with a jackshaft is as follows:

(JS in/clutch) * (axle/JS out) = ratio. (You always want to divide the larger by the smaller.

You will see the first part of the equation depends on whether the clutch or the JS in has the larger tooth count. You divide the larger (whichever one that is) by the smaller. Using a jackshaft gives you a whole range of sprocket possibilities. What you have to do is decide on a final ratio that you would like, and then mix and match sprocket combinations until you get the combination that will give you the closest ratio to what you would like.

Have a look at this thread that I started a while ago when I was also clueless about jackshafts. It might give you some more insight.

http://www.diygokarts.com/vb/showthread.php?t=12786&highlight=jackshaft

About the clutch guard/covers....you might not find something that will just bolt on, so you might need to fabricate.

I can advise you to use the search function on the site for more info on jackshafts and how they work. There is loads of info available.

Post some pics of the kart so that we can see what we are working with

 

[babzog]

Okay, interesting read on the jackshaft there... thanks, I think I understand it a bit better now. So, a higher number (ie: 8:1 vs 5:1) indicates a lower gear ratio, therefore more torque and less speed and a lower number indicates less torque but more speed.

When you're saying karts start at about 6:1, are you specifically referring to offroad karts? I'd presume the racing karts would be fine with higher ratios (considering I got all sorts of small sprockets with the kart)?

I've attached a pic of the rear end of the kart.... not much room to mount a jackshaft setup. The motor shaft is already above the seat (though the seat needs to be properly mounted at the back.. it'll be about 2" higher when done). If I bump the motor up 1-2", the shaft will definitely be beside the driver's head. I'd then have to slide the axle sprocket along the axle ... not a lot of room before I'd be in the way of the seat or the frame.

A lot of work to gain a couple of inches.

 

[South Kart3377]

Okay, interesting read on the jackshaft there... thanks, I think I understand it a bit better now. So, a higher number (ie: 8:1 vs 5:1) indicates a lower gear ratio, therefore more torque and less speed and a lower number indicates less torque but more speed.

When you're saying karts start at about 6:1, are you specifically referring to offroad karts? I'd presume the racing karts would be fine with higher ratios (considering I got all sorts of small sprockets with the kart)?

I've attached a pic of the rear end of the kart.... not much room to mount a jackshaft setup. The motor shaft is already above the seat (though the seat needs to be properly mounted at the back.. it'll be about 2" higher when done). If I bump the motor up 1-2", the shaft will definitely be beside the driver's head. I'd then have to slide the axle sprocket along the axle ... not a lot of room before I'd be in the way of the seat or the frame.

A lot of work to gain a couple of inches.

View attachment 18502

ok, i wil try not to insult your info, but off-road carts are normaly 8:1 to 12:1, Racing carts are 5:1-7:1. Please, please use a js. if you don't want to be burning clutches please use a js.

 

[babzog]

How does the tire dia factor in to the equations?

 

[landuse]

How does the tire dia factor in to the equations?

Using larger tyres is the same as gearing higher. If you think your engine now has to rotate more to get one revolution out of your tyre. If you play around with some of the speed calculators (which always ask for tyre diameter)you will see that if you change the tyre diameter, you get totally different outputs. You will probably get higher top speeds, but your torque will be horrible.

I remember toystory once gave a good explanation regarding the influence of tyre size on gearing, but I cannot seem to find it now

 

[christeeezy]

Using larget tyres is the same as gearing higher. If you think your engine now has to rotate more to get one revolution out of your tyre. If you play around with some of the speed calculators (which always ask for tyre diameter)you will see that if you change the tyre diameter, you get totally different outputs. You will probably get higher top speeds, but your torque will be horrible.

I remember toystory once gave a good explanation regarding the influence of tyre size on gearing, but I cannot seem to find it now

I believe the engine crank rotates the same amount regardless of how big or small the tire is, to get one full revolution from the tire (given the sprockets setup stays the same)... am I right?

It's just that the bigger tire covers more land per revolution, which equals to higher speed, which equals lower torque.

I could be way off though

 

[gvfc2]

i actually believe you are correct. because the engine is not trying to move the tire it is moving the sprocket which in turn moves the tire. the bigger tire just covers more ground at a loss for torque but more top speed

 

[landuse]

i actually believe you are correct. because the engine is not trying to move the tire it is moving the sprocket which in turn moves the tire. the bigger tire just covers more ground at a loss for torque but more top speed

Yup, you are probably right. Like I said, toystory had a nice explanation somewhere.

The bottom line is that if you get larger tyres, your gearing will generally change for the worse.

 

[TeamCheap]

How does the tire dia factor in to the equations?

Its simple math (ok so its not so simple at first)

Starting from the engine and assuming the normal top engine rpm's of 3600 then the clutch sprocket diameter (determined by tooth count/chain size) then the diameter (tooth count/chain size) of the driven/axle sprocket then the diameter of the driven tire.

All those numbers calculated out will give you top speed under ideal conditions but overall weight and surface conditions are factors as well.

Anyway adding a jackshaft will allow you to change the gearing to suite your needs and can also be used to have a smaller sprocket on the axle to allow more ground clearance if needed by correcting for proper overall ratio at the jackshaft.

When I put a 200cc dualsport engine in a blaster frame I had to figure out the final ratio on the bike then calculate the wanted final ratio on the quad to be somewhat close to that of the bike so it would work right.
With the smaller diameter tires and then the added weight on the quad I wanted a bit more torque, I was nervous the first time out and wondered if I had it right but the first ride proved that doing the math and figuring it out got it right on.

OK now all that said I have just woken up and just finished my first coffee so I may read this later after work and laugh.

 

[DaiSan76]

http://www.diygokarts.com/vb/showthread.php?t=9591

This thread has everything you need on gear ratios.

 

[babzog]

But If you installed this jackshaft kit: http://www.bmikarts.com/item/Motor-Mount-Plate-with-Jackshaft-and-Bearings-400361-6636 (between your motor mount and engine) It should help solve your gearing and air cleaner-clearance issues.

Any chance of getting a better deal from these guys? The website has one price ($28) and ebay has another ($17 starting). So, obviously I'd like to get it for $17 or darn close to it (shipping is the same either way) but the girl I wound up emailing steadfastly refuses to budge. I've given up negotiating with her (don't want to be rude) but am just curious if anyone's gotten better deals and if there's someone I should speak to who can say Yes.

 

[TeamCheap]

Then just buy the one from EBAY the only difference I see is in the description stating there may be slight rust on the EBAY one.(i suspect the one listed at the BMI site has slight rust as well though)

I dont know how the shiping would change (it shouldnt at all) unless you were hoping to combine shipping costs or something IDK.

If you want it, buy it, or pass it up.

Your only talking about an $11 dollar difference which is what it costs me to eat wendy's for lunch.

 

[babzog]

I'm always on the lookout for a deal, especially when it means I can eat lunch with the savings.