Bike Repair AGAIN!

Last week on Skyline Drive I broke the left rear seat post bolt. I drilled it out and replaced it. On Sundays Boys and Girls club ride I took off and road around the parking lot to stretch my legs a little and POP, the other side broke. After a few choice cuss words I opened my bag and did the Zip tie trick again.

Have any of you RANS V-Rex or ROCKET Riders had this happen? This one was much harder to repair with the gears in the way but I figured it out and put a new bolt on it.

Tomorrow night I will get busy and clean this thing up.

 

My Rocket has over 23,000 miles on it and I have never replaced any seat supports or hardware. I don't know what quality of bolt RANS used, but as important as the seat posts are I would use at least a Grade 3 bolt. Might even be worth an e-mail to RANS about it.

 

Brian,

I have never had either of the sprint brace bolts break on my '99 V-Rex and I have no clue how many miles I have put on this bike over the past 12 years. Your's is a 2010 model isn't it? I agree with Dave, a note or call to Rans is in order.

 

I have TWO V Rexs and never had this happen.

OK, since I am never politically correct, do you weight a lot??? Rans says it can hold 250 pounds so if you weigh more than that, maybe that could be the problem.

My guess is Rans went cheap on the bolts causing them to break. Send them a note and let them know. They may send you new bolts.

I never thought about taking zip ties in my repair kit, but after reading your post, I think I will grab a couple and put them in there.

 

Pondering

1. Make sure the attachment screw is tight so there is more friction between the bracket and frame. I suspect the screw is loosening up just prior to breakage and the leverage on the seat bracket already has this 'head start' to failure. Install a lock washer on the screw(s) to help keep it snug.

2. There doesn't appear to be enough room for an additional screw, but the strength of this assembly can be increased by drilling a small hole (1/8"?) thru the frame directly against the lower part of the bracket and inserting a roll-pin or even another screw. This will serve as a 'wall' against which the bracket can push. Essentially. this nearly doubles the strength to resist shear.

3. Alternate to #2: Install a large dia flat washer (fender washer) on the skewer that would come in contact with the bottom of the bracket. Note: The edge of the washer could be ground in a 'cam shape' so it could be rotated until the desired contact location is achieved. It could even be shaped to 'cradle' the bottom of the bracket.

4. Drill & tap for a larger size screw(s) w/lock washer(s). This would be a last resort.

5. Redesign? A stiff compression spring in the bracket arm to absorb shock.

None of these ideas takes into account a failure in the seat support system elsewhere. A seat pad might even help to absorb pressure on this attachment system!

 

I don't think I am over the weight limit.. I am currently around 188 pds plus or minus a few.

I was thinking it was the seat angle but I am not sure this is an extreme angle is it?

 

Ed, I'm trying to visualize what you are saying in item 2......

 

Brian,

I have my seat laid farther back than yours so I don't think seat angle is the problem.

 

Shear stress in a 5mm BHCS root diameter with 188# load (*2/3 since that will be the force of your weight on each pedal stroke) is 5109#/sq. inch.

That's way below yield. Even if you throw safety factor on it.....

Are you sure those are factory bolts? or some cheap chinese crap from Lowe's? or a low grade SS bolt (stainless will actually shear worse than other carbon steels sometimes). By the way, why did you not get the old bolt section out of there and just put a new bolt in there instead of drilling a new hole????
If the bolt is not tightened down enough you are minimizing both the friction force and maximizing the bending force. At that point you are setting yourself up for trouble. Ed's suggestion of a lock washer will help correct these things.

Bill Gillette PE, Mr. Stress Analysis......

 

From the picture you have made a new tapped hole leaving less than the diameter of the hole at the outside edge. This will tear out.

I'll be glad to look at this for you. I charge only $150/hour for stress analysis and I'll give you a full FEA color analysis showing optimized hole location and mounting hardware specs. It will probably only take three hours.

Or you could just easy-out that old screw and put a new hi-grade carbon steel or SS304 M5 SHCS in there and tighten it down with a good spring lock washer for about $3.

Glad to help out whenever I can....


Bill Gillette PE, Mr. Stressed Out.

 

What I mean.......

Idea #2 would be a fix for a design flaw. Imagine the bracket in place and secured. Now if this is to 'factory specs', and still fails (assuming good screws are used, are tight, etc), perhaps a 'bumper' or extra 'wall' would help absorb the load. A roll pin or screw placed against the bottom of the assembled bracket might help to eliminate the shearing because now it would have to shear BOTH the screw and the pin.

When I said, "There doesn't appear to be enough room for an additional screw", I was referring to the flat on the bracket that already has the one screw provision. In other words, put 2 screws thru the bracket, but there isn't enough room (ON that bracket flange). Bill, hope that helps.

It's all very clear in my mind! If anyone needs further clarification, PLEASE ASK. If I had a clear side picture of the assembled parts, I'd be happy to down load it, mark it up with my ideas and re-post. It's issues like this (bracket screw breakage) that this group should be able to help resolve because everyone has the potential of exceeding a bikes designed limits. So.........., lets fix it!

 

After looking more closely at this and comparing it with a later picture, I realize that the seat bracket is not even shown in this picture. I assumed the bracket was that one to the left and just moved aside its mounting position, but that's a rack brace. I need a fresh pic of the assembled unit.

If I was guessing, I'd say the screw was way too small for the job to begin with. What size MM screw is it? What size are others using? Quite often early production sends out flawed assemblies, then when continual breakage is realized, it gets corrected on later production without updating earlier shipments. I'd bet! Compare S/N's to see if the other units that are OK were produced after yours.