Let's Talk About Adding Weight to an HPI Inner Rotor

So in the middle of a long ride, my inner rotor HPI Pinto sputters out. The crank seal had been spit out. It happens. But upon closer inspection when taking off the ignition (without splitting the case), the crank now has developed some play all around. Either the crank is f'ed, or just the bearing. Engine had about 500-700 miles on the rebuild. Bottom line is WTF.

Helpful Euro Puch tuners have suggested that an inner rotor on one side and a standard tight spring two/three shoe clutch's weight differential on both sides of the crank will eventually cause the crank bearing to fail -- due to the amount of stress. That is why HPI / Puchshop sells a 300g weight to balance the two sides out. The bearing could have failed for some other reason, not because of the HPI. Obviously, I must inspect it.

There is a good discussion about adding weight to a mini rotor on a ZA.

https://www.mopedarmy.com/forums/read.php?7,3572099,page=1

Before this motor is rebuilt for the third time, I was wondering if anyone had some insight into whether adding weight to an HPI is an ok idea (even if it does hinder spin up time). I really do not want to ditch the HPI set up, but I need my Puch not to develop major issues, as it is my daily / rally bike. Thanks! (edited)

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

If the question is can you successfully add weight I am sure it is doable. You seem to have shaken hands with the duality of physics. Any other questions?

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

HPI does indeed sell weights for their setups as you can go too light. Whether that has consequences related to stress I wouldn't say, but idle can be compromised. It will not hurt anything but spin up time if you do add.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

it's worth a shot, if you believe that the weight imbalance is at fault for the bearing failure. flywheel weights are commonplace in the dirt bike world, to tame down high-strung MX bikes for trail riding, etc. it's an OK idea, let us know how it works out

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Thanks, guys. I was not sure that if this was a real, or imagined, problem with the HPI, but I am planning on rebuilding the motor and adding the weight. If nothing else, I hear that the weight permits better idling (HPIs never seem to quite consistently purr at idle).

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

GET REALIZED! BE HERE NOW! /

yeah, obviously a light fly wheel is going to discourage a nice even/low idle...go read about woods bikes and stuff....

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Mars just had a similar failure. Thats about 3 as of recently? Bad batch of hpis?

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

#CrazyWayne™ rocks. #CrazyWayne /

Most likely your E-50 bearings are floating in the case haves when at temp.

Sand/deck the haves till your plastic gauge reads 0.0015'' to 0.0025'' 10mm from where the bearings sit with new bearings in the carters at room temp.

With the cases bolted.

Use the right sealer like yahambond

And assemble/load the cases warm 150/180f, toque and place in fridge to cool 1hr and re toque . (edited)

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

#CrazyWayne™ rocks. #CrazyWayne /

*.* HONDA Wrote:

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> Mars just had a similar failure. Thats about 3 as

> of recently? Bad batch of hpis?

Mars fried his piston and scored the cylinder with metal grindings, he did not clean out the pipe.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

That's one of your better renditions Wayne

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

#CrazyWayne™ rocks. #CrazyWayne /

Flagrant - Wrote:

-------------------------------------------------------

> That's one of your better renditions Wayne

The E-50 bearing float fix or mars walking the dog in the closet?

Thanks

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

#CrazyWayne™ rocks. #CrazyWayne /

Crazy Wayne™ rocks your Mom's sister's niece's daughter's grandma Wrote:

-------------------------------------------------------

> Most likely your E-50 bearings are floating in the

> case haves when at temp.

> Sand/deck the haves till your plastic gauge reads

> 0.0015'' to 0.0025'' 10mm from where the bearings

> sit with new bearings in the carters at room

> temp.

> With the cases bolted.

> Use the right sealer like yahambond

> And assemble/load the cases warm 150/180f, toque

> and place in fridge to cool 1hr and re toque .

just remembered, wipe cases and bearings with brakeklen and cote with loctite 609 bearing retaining compound.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

My e50 as built by Paz with a gila kit has a hpi mini I put on correctly. I now how some play on the hpi mini rotor. I'm thinking it's the main bearing on that side. Hpi has a 280 gm add on. I contacted hpi about the part. I have two others on hobbits and wonder why this problem should be puch alone. So I asked for three. I looked on the puch shop and there is a add on weight but it was added several years ago and it's my belief that the hpi mini is a newer issue.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

dual snap rings.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Daniel please elaborate. I see on the wiki one side has a different bearing that I guess is a snap ring. Can ya post the specific bearings if they differ from the wiki or what treats sells.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

#CrazyWayne™ rocks. #CrazyWayne /

Marc your going full retard you just run two of the snap ring bearings.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

case needs machined to fit the second 6203 NR bearing.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

CW said I can run two snap ring bearings, so I'm going to. Might even run 4. He knows everything.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

#CrazyWayne™ rocks. #CrazyWayne /

But are your snap rings held by snap rings.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Hi , i run points. Because.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Can somebody with a scale or if they otherwise know the exact weight of a stock puch oem flywheel please either post it here or pm /email me. I will email that weight to Willi at hpi and encourage him to make a weight which when added to the hpi mini will yeild the exact weight. Yea the performance will suffer on some aspects especially off the line but then folks can dispense with it or use the 300 gm weight already available.

I have a stock hobbit flywheel and will do the same for that. I assume there isn't something unique to a e50 design vs other small two strokes that would

Make this premature bearing failure isolated to puch.

That will then only leave the actual curve issues of the hpi on the table.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

RingringBANG ! ( austin) — Sep 12, 2011 01:19AM

was doing a little homework tonight and weighed my flywheels laying around...

Just in case anyone was needs this in the future:

Stock: 875g

Treats 12v CDI: 892g

HPI CDI: 430g

MVT CDI: 251g

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Big Al's Moped Shop /

I weighed some rotors when doing some research for the mini rotor. I think a 575g weight would be neat for mini. With DAS' numbers maybe 450-500 for the inner.

The HPI mini rotor weighs 335g

treats cdi za rotor 904g

stock e50 rotor 875g

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Michael Mike Naz Wrote:

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> case needs machined to fit the second 6203 NR

> bearing.

this.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Ok can we get these numbers exact. If I am going to press this with Willi this needs to be exact! Das says 430 and harsky says 335. Both agree on stock at 875. So it's somewhere between 445-540 gm. Let's get it exact or Willi will never never never pay any attention to a moped er

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Big Al's Moped Shop /

Marc - I didnt mean to confuse. The numbers I gave are accurate. I think you didnt notice that the difference was between the mini and the inner. I am also certain that there will be little variation on stock type rotors. So 335 for mini and 430 for inner and about 900 for stock.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Ahh I was wondering. So is the consensus that this happens with all hpi or is the additional 95 gm loss the deal breaker. If it's just the minis and not the inner hpi then is it reasonable to assume that it is solely a weight issue ?? the mini's smaller radius may magnify the disparity in the angular momentum differential between the two weighs on opposite sides of the crankshaft.

I have a feeling that whether it's both the weight and the radius or just the weight will need empiric testing. I'm going to start with the 300 gm weight and simply put it on the slightly deteriorated bearing. The play at this point is slight. And run sparingly. When I am done with some other mopedisms I am gonna change the bearings and use the 300 gm weight and see how long the bearings Last. Then figure out how to get it to five hundred gms unless the overwhelming opinion is it's not just a weight thing alone. My friend Bobby who does all kind of race car engines is my next stop for wisdom. He helped me on the gila rebuild after I toasted it.

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

I have a theory about things going wrong with crankshafts with lightweight ignitions. It has to do with Torsional Vibration.

By swapping the stock flywheel for the much lighter mini-rotor ignition, there is greatly reduced inertial dampening on the ignition side of the crank. You also have the force impulses from the piston/combustion increasing in both frequency and amplitude due to your more powerful and higher RPM setups.

Every time you get a pulse from the cylinder, you are winding/twisting up the crankshaft in between the clutch side and the crank cheeks like a torsional spring. Then it unwinds. Back and forth. At 10,000+ rpm. When you lighten the ignition side, you take away dampening and create a more asymmetric reaction on each end of the crankshaft.

I deal with this on propeller shaft installations on boats. For every boat we design we have an outside company evaluate all the components in the drive train (Propeller, Shaft, Engine, PTO Items, Etc.) They look at the engine impulses driving everything as a system of torsional springs. Looking for harmonics especially. There is a lot of Finite Element Analysis and I only read the reports, but we just had a boat that we had to add a mass disk to, for this very reason. Without it they were predicting that we would have destroyed the drive shaft in no time.

My prescription, add rotational mass back to the ignition. It's not weight, it has to do with rotational inertia. You may not have to add all of it back.

http://www.livephysics.com/physical-constants/mechanics-pc/moment-inertia-uniform-objects/

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

Matt that sounds totally correct and if ya look at the hpi 300 gm weight it looks like they were on the same thought direction. So do ya agree with my empiric trial?

Re: Let's Talk About Adding Weight to an HPI Inner Rotor

I rode my kitted Puch everyday with an inner rotor. Fast/slow, rallies, stop lights, potholes, and the bearing tanked after maybe 500 miles. Maybe some folks' unweighted HPIs have held up better over the long run, but given that people are having similar problems on Puchs and Hobbits suggests that it is not simply "user error" in assembling the motors, or just crappy bearings.

Yeah, the Euros have been trying to tell us for years about the HPI weight imbalance thing on one-speed mopeds. If you look at HPI's website, the inner rotor is sold as a "universal one-speed ignition," and not designed specifically for Puch. The weight thing is made precisely to avoid unbalancing cranks and destroying bearings. :).

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