Honda GXH50 - Best Gearbox/Ratio for Shift Kit

[Das]

Hi guys I'm doing a custom in frame shift kit using the Honda GXH50. The motor arrived yesterday and now that I know the dimension I am ready to start purchasing parts for the primary reduction and drivetrain. I did a bunch of searching but couldn't find a comparison between gearboxes and what the ideal primary ratio for use with a shift kit and GXH50 is. I wish to stay in the power-band for most common riding without having to whine the engine out to 7000 rpm all the time.

I see Staton uses his 16:1 gearbox and then uses a 12-16T pinion to drive a 48T chainring giving a very high final reduction. Then I see other kits using the Grubee 4G etc. which are only 4:1 or 5:1 primary reduction. It seems there is big gap between these 2 systems. Since I am building a custom belt drive setup I can build whichever ratio is ideal. Does that lie somewhere in between?

For my purposes I need good bottom end as I am a tall guy and I live in the hills and will have to climb 18deg grades, as well I plan to ride this mountain bike on the trails. Given that criteria what would you guys suggest for the primary ratio from the engine crank to pedal cranks.

Thanks for the help.

 

[cannonball2]

I have one set up with a 4G. The primary is 4:1 the second reduction is 1.7:1 and the final to the pedal crank is 4.4:1. So the reduction from the engine to the crank is 29.92:1. In the lowest gear it pulls like a freight train, with a top speed of only around 7mph. In the higest gear(6th) it will get to 35+mph. In low it would climb just about any thing you could throw at it. You could go to the Mega low gear, but I cant imagine needing it short of trail riding .

 

[Das]

Thanks for that info, I was unsure of additional reduction from the clutch sprocket to the jackshaft on those kits. So 30:1 vs 50+:1 on the Staton, I wonder then if people with the Staton typically never use the smallest chainring.

Are you using a single chainring with rear derailleur or a multi-speed hub since you list only 6 gears?

 

[cannonball2]

A derailluer/single ring.

 

[ocscully]

Das,

The information I provided below pertains only to the SBP 4-stroke Shift Kit and probably has no usable value to you and the Drive you are attempting to build. Some how I missed the Custom Belt Drive parts of your original message/Question. If you feel the information is distracting let me know and I'll deleate the body of the Post?

ocscully


I've been riding a 4-stroke shift kit now for almost a year and a half and pushing 16,000 miles. I started out using the Honda GXH50 with the Grubee III Gearbox (3.7:1 Reduction) and then this past Fall switched to the Grubee 4G for Honda (5:1 Reduction) One of the beauties of the SBP Kit is that it is very easy and relatively inexpensive to change/fine tune your drivetrain ratios. They have Chainrings in sizes from 48t Largest, down to 24t Smallest with 44t, 36t, 30t, to fill in the middle. The Output/Driver on the jackshaft can also be easily changed They offer sprockets in 9t, 10t, and 11t. These various parts can be mixed and matched to get you the right ratio or group of ratios for your terrain/riding conditions. When I was using the 3.7:1 gearbox I was using the 9t driver on the J-shaft and the 48t for the driven and the 36t for the final driver to a 7-speed mega range cassette. When I changed to the 4g Unit, I went to an 11t Driver on the J-shaft and switched to a 44t Driven on the Crankset. I've just recently switched the Chainring set up to a 36t Driven and a 30t Driver to the same rear Mega Range cassette. You really need to be aware of just how much room you have available in the front triangle when mounting this set up (SBP Shift Kit) in a straight tubed MTB frame set. In my experience you will need at least a 20 in MTB Frame measured from the center of the BB to the center of the Top tube where it meets the seat tube, to get every thing to fit and leave you room to make chain tension adjustments.

ocscully

 

[Das]

OCScully, no worries, thanks for sharing the info. Since I'm tall I ride 23" frames or bigger, the Trek 4500 I have up in the racks has a lot of room in the triangle.


CannonBall2,

Thanks, that is enticing to get rid of the front derailleur, though I think I may need 2 gears still. My original plan is to run the drive ring on the inside, rather than a fourth ring on the outside a la Staton. form what you've posted regarding your ratio and the ability to pull I figure I will have enough reduction already through my belt drive and gearbox that I can run a straight 22T freewheel for the drive ring. This should allow me to retain use of 2 outer chainrings with the derailleur.

Also does anybody know if you can run the GXH50 mounted at 20deg angle if you disable the oil shutoff and I weld an angled intake tube so that the carb sits level when attached. Or will their be issues with the internal oiling etc. if run in this manner that may cause premature failure?

 

[cannonball2]

Why are you retaining the 2 outer rings?

 

[Greybeard]

David Staton sent me some pics of a 7-1 gearbox to be used on the right side of the bike with a GXH50 turned around. The output shaft was to the inside of the gearbox. It would allow the use of the Honda or a clone without using a jackshaft. I have the pics in my e-mail of the gearbox, but don't really know how to post them. Will send them if someone wants, or wants to post them.

 

[Das]

Why are you retaining the 2 outer rings?

I may have not been clear. Basically I will be replacing the current 3 sprocket crank set with a freewheel on the inside that will connect to my gearbox output, and then retain 2 outer chainrings to drive the wheel hub/cassette so I can have a high and low range at the cranks.

 

[Joaquin Suave]

A couple of things...

It is not particularly difficult to build a gearing spread sheet in Excel.

Also, the Honda OEM Technical Check Sheet for the GXH50 & GXV50 states that the motor can be run "off-axis" up to 20 degrees.

 

[Das]

David Staton sent me some pics of a 7-1 gearbox to be used on the right side of the bike with a GXH50 turned around. The output shaft was to the inside of the gearbox. It would allow the use of the Honda or a clone without using a jackshaft. I have the pics in my e-mail of the gearbox, but don't really know how to post them. Will send them if someone wants, or wants to post them.

Interesting, is the clutch placed somewhere else or does this gear box bolt onto to his clutch spacer sytem (woulld be an anodized black machined block bolted between the box and engine)? I thought I may have saw something like that on his site, tried looking again but couldn't find it. Staton's site is a bit jumbled with all the different kits, options, and parts that it's sometimes hard to repeatably find things. His clutch system for the GXH50 looks really well built, but the engine is already really wide. Therefore I plan to use a plain GT2 or HTD pulley on the engine shaft that runs a centrifugal clutch on the input shaft of my inline gear reducer, thereby not increasing the width of the motor any. It looks like the EZ system uses a similar clutch method.

 

[Das]

A couple of things...

It is not particularly difficult to build a gearing spread sheet in Excel.

Also, the Honda OEM Technical Check Sheet for the GXH50 & GXV50 states that the motor can be run "off-axis" up to 20 degrees.

Thanks for that info, I'll fire up excel for final ratio's, I was trying to get in the ballpark from experienced riders as the majority of my reduction will be coming from a fixed ratio inline speed reducer. Looks like I'll be in the clear with that engine mounting angle.

 

[Joaquin Suave]

Looks like I'll be in the clear with that engine mounting angle.

WHOA! No you are not! Anything beyond that angle is out of spec! Thus, you are only ok if you are riding on flat ground. My motor is slanted forward at 10 degrees, so I am good to 30% climbing and 10 degrees dropping.

you might want to look at "slacking the cant" :0)'

 

[Das]

Canted 20% forward I'll be straight up and down on most of the hills around here and 20% on the flat. Don't need any throttle on the trails and downhill's around here, and I still plan on peddle assisting when I can.

Second question is if that spec is based off proper function of the gravity fed float carb, them not expecting people to defeat the low-oil shutoff, or whether it is something detrimental like the oil getting kicked up and will foam or not get to into the head etc. If it's one of the 1st 2 it could be fixed, or at least modified to allow a steeper angle while maintaining proper function.

Thanks for the head-up though.

 

[cannonball2]

Concerning the 2 extra chain rings I am running a 24t final driver with a 44t driven. My HS engine which is bone stock is maxed pretty much in 6th with this set up. I dont see how the Honda is going to be able to pull much more. Granted the 22t is somewhat lower. Seems to me the other 2 rings would be unusable except down hill maybe. Tell me what I missed.

 

[dtj6ppc]

I still plan on peddle assisting when I can.

QUOTE]

One thing nobody ever tell others is that you will not be able to peddle assist with the Honda motor unless reaching a motor to wheel ratio of 45 to 1, in order for the peddles to catch up with the motor, and even at that minimum ratio the motor with have to be throttle down to near idle speeds which makes for noticable oscillation, you feel every piston stroke through the frame.

Now if you are not interested in peddle assist, and only motor drive, then the Honda would be ideally ratio'd at 35 to 1 for optimal mountain climb and 35+ flatland cruise.

Staton developed his 50+ to 1 ratio based on the Subura low torque, low horsepower, higher rpm, 4 stroke motor, which is optimal for this motor and peddle assist.................Don

 

[Greybeard]

Interesting, is the clutch placed somewhere else or does this gear box bolt onto to his clutch spacer sytem (woulld be an anodized black machined block bolted between the box and engine)? I thought I may have saw something like that on his site, tried looking again but couldn't find it. Staton's site is a bit jumbled with all the different kits, options, and parts that it's sometimes hard to repeatably find things. His clutch system for the GXH50 looks really well built, but the engine is already really wide. Therefore I plan to use a plain GT2 or HTD pulley on the engine shaft that runs a centrifugal clutch on the input shaft of my inline gear reducer, thereby not increasing the width of the motor any. It looks like the EZ system uses a similar clutch method.

I'd not seen it on his site, and he sent me the pics in answer to an inquirey about clockwise rotation gearboxes. The clutch is on the engine output shaft. It looks a little like Grubee 4 t trans in overall shape, but has two gears. The gearbox would bolt straight up to the Subi or Honda 35 but has what appears to be a 1/2" to5/8" machined spacer for the GXH as the clutch isn't recessed into the crankcase as is the case of the two smaller moters.

 

[Das]

I still plan on peddle assisting when I can.

QUOTE]

One thing nobody ever tell others is that you will not be able to peddle assist with the Honda motor unless reaching a motor to wheel ratio of 45 to 1, in order for the peddles to catch up with the motor, and even at that minimum ratio the motor with have to be throttle down to near idle speeds which makes for noticable oscillation, you feel every piston stroke through the frame.

Now if you are not interested in peddle assist, and only motor drive, then the Honda would be ideally ratio'd at 35 to 1 for optimal mountain climb and 35+ flatland cruise.

Staton developed his 50+ to 1 ratio based on the Subura low torque, low horsepower, higher rpm, 4 stroke motor, which is optimal for this motor and peddle assist.................Don

Thanks Don,

That is the info I was after. I was figuring in the 40:1 would be optimal.

 

[Das]

I'd not seen it on his site, and he sent me the pics in answer to an inquirey about clockwise rotation gearboxes. The clutch is on the engine output shaft. It looks a little like Grubee 4 t trans in overall shape, but has two gears. The gearbox would bolt straight up to the Subi or Honda 35 but has what appears to be a 1/2" to5/8" machined spacer for the GXH as the clutch isn't recessed into the crankcase as is the case of the two smaller moters.

Ok, thanks, so it does use the clutch spacer on the GXH50.

 

[Greybeard]

My GF helped me with the pics. I pulled the muffler off my GXH yesterday and found it fit easily into my MTB when turned around with the stock airfilter facing forward. Thinking very seriously about doing this.