I will make sure Jetmoto reads this post and see what we can do about this.

 

<div class="FTQUOTE"><begin quote>Originally posted by: outofline

Thanks, is there room for the head of the bolt?</end quote></div>

The heads of the bolts will rub the bearing housing very very slightly. That is why I ground them down some. You won't have to grind much, as a matter of fact I think only one or two rubbed. I just figured the hub was slightly warped.

 

EDIT: I just wanted to clarify a previous post of mine. I said "from those bolts coming loose" and they didn't, in fact, come loose. The threads are still in the hub, and they sheared off from impacting the sprocket. Just so everyone knows what's up.

I got some grade 8 3/4-16x1" bolts today and nuts to go on the backside of the hub.

I took the sprocket by the local ATV guru/shop today and the holes in the sprocket were sort of stretched open from the torque because of the bolts being too small for the holes. John@Raceway said today that these parts are in stock, and it won't be a problem getting them to me quick, which is just AWSOME and I might even be riding by this weekend (had plans with Dad to actually get him to ride his for the first time.)

Here is my question, because I'm getting conflicting reports from people on this:
What size hole should I drill for a 3/8" bolt to go through and be very snug. The guys at lowes said a 25/64 (slightly over 3/8) would be snug because it's not the outside of the threads that is measure. I'm not sure about you guys, but I always thought it was the OD of the threads, not the ID that showed the bolt size. =/

The ATV guy recommended using a 23/64 (or maybe 11/32) which is smaller than 3/8 to make the bolt very snug so there won't be anymore play (the stock sprocket twists and impacts the sprocket hub bolts every time you go from forward to reverse. He said you will almost have to twist it to get it to go through the sprocket, which is ideal.

It seems like this is a good way to accomplish this. I don't believe I will have to grind my bolt heads for them to clear the bearing/housing. =)

The best way to accomplish this easily (cheaply) for Jetmoto would be custom screws that have a non-threaded area between the threads and the head of the sprocket bolts which fits into the sprocket perfectly. That is, if it was the slapping of the sprocket on the bolts that was indeed the reason that they sheared off (as it seems to be.)

If Eugene/others from Jetmoto are going to stop by, I would be very interested to know the details on the rear sprocket! It appears to be a 4/50mm bolt pattern with a 428 pitch. Is this correct? I want to see about getting a custom sprocket made with 47-48 teeth.

We need not worry, John is on the ball! Raceway/Jetmoto definately deserve thier fantastic reputation!

EDIT: I gave up on getting the two axle nuts off and took the axle to a machine shop. They are going to remove the nuts, remove the bolt threads stuck in the hub, and drill and retap with 3/8x16 threads into the hub so all I have to do is drill out the new sprocket. I guess I need some advice on what size to drill with. The machinist recommended going 1/64 smaller than 3/8 (23/64) so that I don't mess up the threads on the 3/8" bolts. He said any smaller could be problematic in getting them to stay in the hub. Anyone have any experience with this?

-JNY

 

<div class="FTQUOTE"><begin quote>Originally posted by: pimpsmurf

EDIT: I gave up on getting the two axle nuts off and took the axle to a machine shop. They are going to remove the nuts, remove the bolt threads stuck in the hub, and drill and retap with 3/8x16 threads into the hub so all I have to do is drill out the new sprocket. I guess I need some advice on what size to drill with. The machinist recommended going 1/64 smaller than 3/8 (23/64) so that I don't mess up the threads on the 3/8" bolts. He said any smaller could be problematic in getting them to stay in the hub. Anyone have any experience with this?


-JNY</end quote></div>

I would use the smallest drill that allows the 3/8 bolt to fit. It will most likely be the 23/64 bit.

What do you think caused this pimpsmurf? It sounds to me like the bolts were just loose enough to allow movement, between the sprocket and the hub. That movement allowed the shearing to happen? Any ideas?

 

<div class="FTQUOTE"><begin quote>Originally posted by: mywifesquad

I would use the smallest drill that allows the 3/8 bolt to fit. It will most likely be the 23/64 bit.

What do you think caused this pimpsmurf? It sounds to me like the bolts were just loose enough to allow movement, between the sprocket and the hub. That movement allowed the shearing to happen? Any ideas?</end quote></div>

One bolt was still in when I got the axle off. It wasn't extremely tight, but the loctite was doing it's job. I don't know how the others were because I passed the buck. I took the whole axle to a machine shop to have them remove the axle nuts to get the sprocket hub off and to get them to remove the bolt-threads still in the hub, and drill/tap 3/8-16 threads for me. $65/hr, but I tried everything to get those big nuts off (hehe.)

I am willing to bet that what is left of the bolts in the hub is still loctite'd in place. The hole in the sprocket is too big for the bolts going through to the hub. When you put it in reverse, stop, and then go forward it snaps the sprocket clockwise, and then counterclockwise (from the left of the quad.) Everytime you go from forward to reverse, the sprocket spins just a little and impacts the bolts until eventually the bolts snap. Then they get caught between the sprocket and the bearing housing and eat the crap out of the bearing housing. I had washed the quad just prior to the incident, and I always clean the chain with a kerosine-soaked rag and relub it after washing (I powerwash the chain which gets most of the grease out.) I would have noticed the rear sprocket being loose come to think of it, because it would have rattled side to side as I pushed the quad while wiping the chain down.

I'm just saying that this can happen without notice, and it resulted in my rear wheels completely locking up on asphalt when a bolt head twisted and jammed into the axle bearing. I'm sure Jetmoto is going to be on top of this, as it is an unsafe design flaw, but I would say we should go ahead and change out to 3/8" Grade 8 bolts so as not to have a broken quad for a week while waiting on parts (not to mention, broken kids which are more expensive.)

-JNY

 

are these problems with the 4 bolt pattern hubs? I know some JetMoto hubs, like on my 110, use a 6 bolt pattern.

 

We are talking about the Jetmoto Sport 250, which has a 4 bolt rear sprocket.

-JNY

 

pimpsmurf, my 250 is still in pieces so I grabbed the sprocket on the hub and put it in a vice. I couldnt find a L handle allen wrench so i had to use a T handle allen wrench. To my suprise all 4 of the bolts were only hand tight. All 4 also had some sort of thread locker on them. but were loose enough to cause the back and forth shearing action you describe. That action happens everytime you are on and off the throttle as well as in and out of reverse. I think Jetmoto will either have to use stronger bolts and make sure they are tight. Or possibly go to a Flat Head Screw which will locate the sprocket as well as secure it to the hub. I measured my bolt holes compared to the bolts and the holes were .023 larger than the bolt. This allowed about 1/16" of play at the sprocket teeth.

 

Yeah, I didn't think about a flat head screw, but that would place all the presure on 1 part of the bolt (the angled area) instead of across the entire length of the bolt.

I think either larger bolts into the hub (ideal) which are tight into the sprocket, or a sprocket with bolt holes so the stock stuff will fit better. Red Loctite FTW! =)
Of course, the custom bolts I was talking about would be ok, but I think bigger bolt threads in the sprocket hub is the way to go.

-JNY

 

<div class="FTQUOTE"><begin quote>Originally posted by: pimpsmurf

Yeah, I didn't think about a flat head screw, but that would place all the presure on 1 part of the bolt (the angled area) instead of across the entire length of the bolt.

I think either larger bolts into the hub (ideal) which are tight into the sprocket, or a sprocket with bolt holes so the stock stuff will fit better. Red Loctite FTW! =)

Of course, the custom bolts I was talking about would be ok, but I think bigger bolt threads in the sprocket hub is the way to go.



-JNY</end quote></div>

Its just what most dirt bikes use. I have never had the sprocket come loose on my CR500 and it probubly makes a few more HP than the Jetmoto 250. [img]i/expressions/face-icon-small-wink.gif[/img]

Anything that would eliminate the play that allows and contributes to the shearing. And of course red loctite. [img]i/expressions/face-icon-small-smile.gif[/img]