Rod length and vibrations

Can anyone who has ridden both confirm that the 533s vibrate less than 447s? I've already gone to a rephase for the sound and vibe reduction and I've messed with my gearing extensively but I'm still not happy with the vibration. Maybe I should just get a smoother bike instead of throwing more money in the pit, but I'm a tinkering kind of guy.

With a stock crank lengthening the rod will decrease the secondary imbalance. With a rephase crank aside for some rocking couple the secondary forces are mostly canceled out so I wouldn't think it would make much difference. Have you tried setting the balance factor? For a 277 53% is probably optimal, for a 270 50%. I always wanted to try this but never did.

I actually sent the crank to Hugh, I don't know what (if anything) he does besides the cross plane conversion. I would say that it vibrates less than it did as a 360 but it's nothing like how smooth an Enfield Interceptor is. I understand that the Enfield has a balance shaft though.

Without balance shaft(s) the only twins that are inherently almost vibrationless are a 90 degree v twin and an opposed twin like a BMW. Having said that my 277 is not bad at all and quite a bit smoother than it was as a stock 360 with stock pistons. Looks like your bike is a 750, those heaver pistons will reduce the balance factor somewhat which is a good thing.

Here are some interesting observations I have made. I first rephased back in 2007 using my original '78 crank, Mikes performance rods and also bored it out to 700 cc with JE 10:1 pistons. It was pretty smooth like that especially at 5k RPM or so. Over the winter I took it apart and milled my pistons down to get close to 9:1 due to too many problems with detonation. Removing this weight from the pistons seemed to increase the vibration a bit... hard to tell for sure since I didn't ride it again until spring but I was pretty convinced it changed.

This past winter I bought a decent '76 crank and had Hugh do the rephase since my original crank needed a rod replacement. With everything else the same it is noticeably different with this crank. With my original one it had a rough spot between about 4100 and 4600 RPM, with the new crank this effect is reduced, it still smooths out past 5000 but the rough spot is much less pronounced. One other thing, with my '78 crank I have Mikes performance rods. My '76 crank has OEM rods.

So why the difference between the 2 cranks, could be differences in truing and possibly the balance factor?

Thanks for the useful info! You're probably one of the OG rephasers if you did it in 07.

I can't tolerate the vibes past 5k, so I know anecdotally my engine must have a major difference from yours. We both have 76 447 cranks that were rephased by Hugh. I'm certain he replaced both of my rods but I don't remember what with, that was ~10 years ago! I used to have a GT1000 Ducati that was very smooth at mid to high revs and it had a long ass rod ratio, I also had a Vstar 1100 that was extremely smooth and it has a pretty high rod ratio as well. Neither one of those engines are counterbalanced, so I know it's feasible to make a 75-90 degree twin relatively smooth.

The bike starts with one kick, the jetting is close to perfect, the cam timing is close to perfect, the ignition timing advance unit is working perfectly. I just don't enjoy riding it because it vibrates too much. I suspect the crank is the issue and I need to bite the pillow and get another one built by a shop that can balance it properly. Since I'm considering having another crank built this is the ideal time to go with a different rod length. I was hoping someone would hop in and say "I've ridden a euro and american back to back, the euro vibrates less".

The thread's less than a week old, so some old timers may still wander in with their wisdom. It might turn out that wisdom is "get a better bike and re home this one to an enthusiastic youngster" which is the way I'm leaning. I've spent a decade tinkering with this bike and it's been fun, but there comes a point where you just want to be able to ride.

Interestingly a lot of things affect vibration, my bike is noticeably smoother on a warm day with high humidity. I theorize a couple things are at play with that, the combustion process will be slower and smoother due to the humidity and the rubber mounts on everything get softer with increasing temps.

I would contact Hugh and see what he says about your vibration, assuming you have checked all the obvious things like bolts, etc. Is your bike a basically stock '76 frame, etc?

Yea, I've had the engine out and completely apart 3 times over the last decade. There wasn't any play in any direction last time I had it apart. The mounts are all tight. Higher octane fuel lessens the harshness of the vibes for sure, so you're on to something about the duration of combustion and harshness. Maybe that's what got me thinking about rod length, slower piston speed and more gradual compression/expansion should be less harsh.

What was it like before the rephase?

On my bike it made a significant improvement.

Pic of the bike?
handlebar mounts, grips, seat, exhaust mounts, other engine mounting points, all can make a huge difference in how much vibration gets through to the rider.
Kinda sad for me but I parked Madness for several years cuz I thought the crank had scissored, turned out it was just a broken muffler mount that made me think the motor was ready to frag.

It was bad before the rephase, very buzzy and harsh but the rods were out of it on the big end. Gary brings up an interesting point, it vibrates less with the stock type exhaust than with this old Hooker 2-1.. Excuse the dirt/grease/mismatched grips, I don't very much care about cosmetic stuff.

Very good points from Gary...

Where is the vibration most objectionable?

I added Vibronators to my handlebars to reduce it there.

One more thing, I have replaced my handlebar and foot peg dampers which probably help some too.

It's the handlebars that annoy me most, I'm going to put the stock 76 exhaust on this week and look into the vibronators too. I hooned around a bit this weekend and revved it out pretty high. It really makes great power up in the rev range but it buzzed so badly my muffler's endcap disappeared! It may just be the cheap 2-1 that's annoying me, since it's falling apart I'll remove it from the equation.

yamahammer said:

I don't very much care about cosmetic stuff.

Click to expand...

yamahammer said:

It's the handlebars that annoy me most,

Click to expand...

Well then, you my friend are a prime candidate for a set of Emgo foam grips! They may not be the sexiest looking grips, but they are cheap as chips and they do a very good job of insulating you from vibrations and they are extremely comfortable to hold.

https://www.ebay.com/itm/112555585955

I am running them currently on my ‘77D,

Nice bike, Mailman, it's super clean. I think I'm going to get a set of the vibronator inserts first, but I may go with foam if that doesn't cut it. I can see how the vibronators could work from a mechanical standpoint, they move inside the bar like a pendulum.

+1 on the foam grips.