4 stroke - 4G belt vs. Skyhawk Stage III

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darkcobra94

New Member
I'm going to be ordering a 4 stroke engine kit off of Bicycle Engines, Motorized Bicycles, Bicycle Motors, Motored Bikes: 2-Stroke & 4-Stroke Kits! and I want input on both 4 stroke engine kits. What one lasts longer? Less problems? Etc

4G T Belt Drive 4 Stroke Engine Kit
49cc 4G T Belt Drive Complete Gas Powered Engine Kit - $379.99

VS.

The Skyhawk Stage III Whopper Stopper
49cc Complete Stage III Gas Powered Bicycle Engine Kit - $369.99


Also I'd like input on websites that sell the engine kits, and how the service is. Thanks!
 
Hello darkcobra94,

Here is some information to help you decide. I'm sure others with chime in with more...

Gear ratio: 3.7:1 gives you more top speed than a 5:1. Keep in mind you are not stuck with the drive ratio alone. You can change rear sprocket sizes alter the gearing for your application.

Stage III gearbox: Uses 3 straight cut gears to alter the drive ratio. This gearbox has a ratio of approximately 3.7:1. This gearbox has been noted as being loud due to the straight cut gears (some are louder than others). The stage III gearbox is just like the stage II that it replaced, except that it uses 7 bolts to clamp the gearbox together and is designed to help keep fluid in when running the gearbox wet by using an improved seal at the output shaft. Forum members have tried several different lubricants in the gearbox. Some leak out too easy, some break down fast. Do a search for more info on gearbox lubes. It has a lever operated cable that allows you to engage and disengage the gearbox. Most people that primarily use the engine as their main source of propulsion lock the gearbox in 'engaged' to prevent engaging or disengaging when the engine is running. Engaging the engine at rpm other than '0' do this- the 'dog ears' on the sides of the gears can be torn apart.

4G gearbox: Uses a belt to alter the drive ratio. They come in both 4:1 and 5:1 ratios. The 80T 4:1 ratio box has a one-way bearing in them that have been problematic for some people. The 100T 5:1 gearbox has a solid pulley and a one-way 12T sprocket, which is much more durable. You can purchase a solid 80T pulley and belt to change the gearbox to the 4:1 ratio. Some forum members have experienced "teething issues" with this gearbox but come away happier with it than they were with their stage III (most cases I have read). With the 4G gearbox it is really important how you install the oil-lite bushing and that you keep it lubed properly. Some people have had the 20T pulley come apart from the clutch bell, usually when trying to get an oil-lite busing that has sat in it since it was made. This can easily be remedied with a small tack weld. Other than that it is quieter and more efficient than the stage III gearbox.

For reliability, I like the 4G: stock up on one replacement belt and get an extra oil-lite bushing (or get the one from mcmaster). Put a small tack weld on the clutch bell where the 20T pulley is pressed in. You will be good to go for many miles. The pulleys will never wear out. Only wearable items are the belt, clutch, clutch bell, oil-lite bushing, and bearings that hold the output shaft. I have never heard of the bearings in the output shaft failing (maybe if you had too much belt tension).

Some people using stage III kits (especially users on this site!) have gotten many trouble-free miles out of their gearbox, but being a straight-cut geared transmission, it inherently has more wear.

There is more but I gotta prepare for Turkey day... do some searches- there is a wealth of knowledge on this site.
 
Can the Stage III gearbox be installed on a HS 142-f engine that originally has a centrifugal clutch installed? Can I remove the clutch pack and install a gearbox, or is the shaft different? Is there a transmission that works like the 2-stroke type engines? I get the feeling that the centrifugal 4G transmission doesn't provide as much power due to slippage during uphill climbs. For instance, i read many people with the 49cc HS 4G have no problems going up inclines without losing power. I attempted to climb a moderate grade hill at speed, started losing speed slowly, and eventually came to a stop, while having full throttle the whole time. The engine still maintained roughly the same RPM, but feels as the centrifugal clutch cannot hold the force and is just spinning in place without actually driving the wheel. Gear ratio is 10T solid sprocket to 44T rear wheel sprocket. I only weight 150lb, so this 4G engine kit feels extremely weak compared to other users that weigh more and are still able to comfortably climb hills. Acceleration from stop also feels more sluggish with a centrifugal clutch. I get the general feeling that a lot of power is loss from clutch slippage. If possible, would changing to a stage 3 gearbox or any other direct drive gearbox allow more direct torque/power to the wheel?

If the stage III gearbox is always engaged, how does one start the engine? Or do all 4-stroke transmissions work by some type of centrifugal clutch from engine output shaft?
 
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Hello userix,
If the 142-f engine you are using has the 5/8 shaft, then you can use it with the stage III gearbox. Keep in mind there are a few different shaft sizes: http://motorbicycling.com/showthread.php?t=29312
Unfortunately, there is not a 4-stroke gearbox that use a manual clutch that I know of. If you think that your clutch is slipping during hill climbs, you can gear down or do some clutch adjustments. Softer springs/ weighting the pads will get your clutch to engage sooner and allow more force against the clutch bell. Also, make sure that you didn't contaminate the clutch pads by spilling oil, or over-oiling your oil-lite bushing. Is your engine new? You will gain a little bit of power after it is broken in. If you are at a high elevation, you could be losing power. Also, I noticed you were running a 10T solid sprocket...do you have a one-way bearing in your driven pulley? These are known to slip/go out, but usually when they do they go completely out. A stage III gearbox wouldn't get you anywhere as it uses the same type of centrifugal clutch and has taller gearing. The engager is just for disengaging the gearbox for easier pedaling.
 
BE-tech said:
Hello userix,
If the 142-f engine you are using has the 5/8 shaft, then you can use it with the stage III gearbox. Keep in mind there are a few different shaft sizes: http://motorbicycling.com/showthread.php?t=29312
Unfortunately, there is not a 4-stroke gearbox that use a manual clutch that I know of. If you think that your clutch is slipping during hill climbs, you can gear down or do some clutch adjustments. Softer springs/ weighting the pads will get your clutch to engage sooner and allow more force against the clutch bell. Also, make sure that you didn't contaminate the clutch pads by spilling oil, or over-oiling your oil-lite bushing. Is your engine new? You will gain a little bit of power after it is broken in. If you are at a high elevation, you could be losing power. Also, I noticed you were running a 10T solid sprocket...do you have a one-way bearing in your driven pulley? These are known to slip/go out, but usually when they do they go completely out. A stage III gearbox wouldn't get you anywhere as it uses the same type of centrifugal clutch and has taller gearing. The engager is just for disengaging the gearbox for easier pedaling.
Click to expand...

Thanks for the awesome info! So all 4-stroke engines, regardless of transmission/gearbox, work using a centrifugal clutch on the output shaft? I am aware of the crappy one-way bearing, which already broke after the first 5 min of riding. I already welded the bearing solid so it no longer freewheels. I am certain now that the bearing is in no way slipping. It definitely is the clutch slipping, as the engine RPMs remain constant during an uphill climb, while the speed gradually decreases until a complete stop. The engine is brand new, so the clutch pads on the engine areperfectly clean. I did not add any additional oil to the oil-lite bushing either, so no excess coming from there. What exactly do you mean by gearing down? Running a higher number of tooth sprocket will decrease top speed and increase power right? Where do I find softer springs, or how do I weigh the pads? Approximately how long is the break-in period for 4 stroke HS engine?
 
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Most of the typical 4-stroke bicycle engine kits (grubee) will have some type of centrifugal clutch on the crankshaft. You may just be going up too steep of a hill? Yes, gearing down would be running a higher tooth count rear sprocket or a smaller front sprocket. Typically, a 1 tooth change on the drive sprocket (small front) is equal to a 3-4 tooth change on the driven (rear) sprocket. This will give you more power for going up hills, but will reduce your top speed. If you are constantly going up and down hills, going to lower gearing will help a lot. It will also keep your RPM higher, which will increase the centrifugal force on the clutch, making it 'bite' harder. Keep in mind you are going to have to wind out the motor more for the same speed: at 25mph your motor will vibrate more and increase wear and tear. Also, these engines do not like sustained high rpm do to the simple design and flinger type oiling, so don't scream it at top RPM. I have seen a few thrown connecting rods and damaged big end connecting rod bearings (which destroyed both the con. rod and crank) do to over-revving. You might try the stage III clutch, it comes with softer springs and a rubber type of pad. Check out this thread: http://motorbicycling.com/showthread.php?t=35350
As far as weighting the pads, you would have to get creative. Drilling holes where applicable and filling with lead, etc. (better off just buying the stage 3 clutch imo). I know the pads on the stage III clutch are slightly heavier as well. Break-in usually consists of 5 heat cycles (whole other can of worms), but the motor will continue to make more power as you get more hours on it. The piston rings seat in increasing compression and parts free up reducing frictional losses. It would be interesting to see a dyno graph of a motor's power output over it's life. I would assume that power would increase up to a point and then start dropping off after awhile as piston ring and other wear started reducing compression. Newer motors should have more bottom-end power (compression) after break in, while old motors will rev more freely on the top-end. I'm sure the differences are miniscule however....little off topic but interesting:)
 
Forgot to add that if you go with too light of a spring then you will run into stalling issues as the clutch will engage at idle.
 
Thank BE-tech for the awesome info. Is there a difference in the shaft of the HS engines that have clutch already built-in ? Will I be able to remove the cluch and install the stage III clutch and gearbox without any modifications? I notice some sites state that the stage III kits only work on HS engines that do not have centrifugal clutchs built-in.
 
By built-in clutch, I will assume you mean sold with. All the clutches are removable. I believe the tapered shaft motors were sold with the clutch on the motor, while 5/8 shaft engines are sold without the clutch, but I am not positive. You will have to pull your clutch off and check out the shaft that it is on. If it is anything other than 5/8 then the stage III will not work. Keep in mind the 5/8 shaft is actually 4.976/8 or .622 inches, 15.7988 in mm.
 
BE-tech said:
By built-in clutch, I will assume you mean sold with. All the clutches are removable. I believe the tapered shaft motors were sold with the clutch on the motor, while 5/8 shaft engines are sold without the clutch, but I am not positive. You will have to pull your clutch off and check out the shaft that it is on. If it is anything other than 5/8 then the stage III will not work. Keep in mind the 5/8 shaft is actually 4.976/8 or .622 inches, 15.7988 in mm.
Click to expand...

Yeah, that's what I mean. So if it is tapered, then I am SOL? Thanks again!
 
The only other options if it is tapered would be:
-Find a tapered shaft 4G gearbox- Some suppliers carry the 4G gearbox for the tapered shaft. Check out grubee's website for pics of the 4g tapered shaft gearbox.
-If you want a stage III, get a 5/8 crankshaft and swap it for yours. I'm pretty sure that it is a direct swap but I'm not 100% positive that this is compatible, maybe others can give some info. From what I can tell, the motors are exactly the same minus the end of the crank.
 
Grubee is not making the 4G for taped/clutch HS anymore, :( I'm sure you could find one in the swap section. Or you could buy a chain drive like a mini bike, Dax has one and sells different drive cogs for different types of chains. Personally I would just buy a 5/8 engine and transmission instead of a kit and move away from the taped HS. If you want to continue towards newer transmission trends, the EZM is only made for the 5/8 engine as well.

As far as switching the shaft in the engine, I don't think its possible to do without also changing the casing on the clutch side as it has the built in housing for the clutch. But if you attempt it let me know!
 
Dang, should have bought a different kit. I thought at this point, all newer skyhawk 4g kits would use standard shaft, since the tapered one is outdated for use with newer transmission options.
 
At least the 5/8 engines are only $170. That's not too bad, if you were already saving for a new transmission.

Put up an add in the swap section for an older G4, there's bound to be someone with one they don't need.
 
Just to confirm with Tad Bit Tipsy, I reviewed some photos of the tapered shaft motor vs. the 5/8 and he is right. It appears the tapered shaft engine has a different casing on the clutch side.
 
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