Updating / Replacing Isolastic Mounts

dynodave said:

..iso's are being gently pushed forward and up to propel the bike down the road.

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How many HP to push a 800 lb lump with a drag coëfficient of 0.90 trough the air at 60 mph?

bill said:

the issue is it is definitely not a one to one ratio.

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The ratio will vary with speed and increasing air resistance.

along with throttle opening on acceleration. I can for sure say that weight in the saddle has an effect on vibration. if i was building a commando for 2 up i would place the shocks in a more upright position. this also goes back to the (lack of) engineering on grandpauls monoshock. with a near horizontal shock i would hate to guess at the forces put into the isolastics trying to push the powertrain out to the rear.

ludwig said:

The ratio will vary with speed and increasing air resistance.

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rvich said:

For the OP here regarding your isolastics. You are going to get a lot of advice here, some good, some bad, and that's just from me.

What I think you should do before being concerned about your isolastics is read up on layshaft bearings. Then make plans as to how you are going to verify whether its ever been replaced. If you can't verify that its been changed out since new then you can plan ahead for that project. While you are doing that there will be plenty of opportunity to examine and replace iso's.

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Thanks for the thoughts. I looked into that - can you confirm without doing the actual replacement? I.e. if the PO doesn't know should I just go for it? Didn't look to me you could tell without pulling out the layshaft?

Eric Bauer said:

Thanks for the thoughts. I looked into that - can you confirm without doing the actual replacement? I.e. if the PO doesn't know should I just go for it? Didn't look to me you could tell without pulling out the layshaft?

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You have to pull out the layshaft
The inner of the bearing should be on the layshaft

I'm not trying to freak you out about your layshaft bearing, however, it is fairly well accepted wisdom that you shouldn't ignore it either. My main point is that you have plenty of opportunity to find projects that will get you taking enough of the bike apart to check the isolastics without making it a priority.

I did my gearbox teardown last winter to go after the LS bearing . Did all seals, bushings, springs and other bearings while there. I followed the procedure from the Mick Hemmings dvd and OldBrittts step by step. I did it all with gb still in frame on bike.
My '74 850 had the dreaded Portuguese LS bearingfitted but it looked fine. Went with Hemmings recommendation to use a phenolic caged ball bearing rather than the more popular roller bearing.
This work gives peace of mind from an otherwise unknown situation.

bill said:

along with throttle opening on acceleration. I can for sure say that weight in the saddle has an effect on vibration. if i was building a commando for 2 up i would place the shocks in a more upright position. this also goes back to the (lack of) engineering on grandpauls monoshock. with a near horizontal shock i would hate to guess at the forces put into the isolastics trying to push the powertrain out to the rear.

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Dunno if relevant but a norton set up for max top speed runs went with an extended swingarm/loop/upright shocks setup.

Good plan to go after the LS first and think about the isos. As has been said there's a lot of opinions on the isos. I replaced my early 69 with what was supposed to be the softer ones from Old Britts (70 version). I was not happy. About 2 months later I took off the front iso to do some work and to my surprise the thing was compressed towards the bottom by about 1/4". I Installed the later style engine spring and that helped some with the sag, it got rid of the low rpm vibrations, but I still have buzz in the pegs and a bit on the bars. Originally there was none. I suspect there are iso donuts and then there are iso donuts. The swingarm mod is fairly easy also if it's not there, and necessary. Some find the new headsteadys great also, I didn't notice much, but I don't push the bike.

Lots to consider.

I am completely happy with my new CNW iso kit. And the Comstock headsteady. And the IKON shocks. And the Lansdowne dampers, and JS turcite bushings.. LMAO.

But the old '74 isos were knackered. There is no reason to have old rubber. Anywhere. Handling is as much a safety as a performance issue.

Sounds heavy duty alright might be suitable for a big interstate gas tank with loaded tanks bag, big guy/heavy rider, long distance rally goer, loaded his commando down with camping gear.

One group rally trip a friends bike broke down and we ended 2 up on my commando for the parts chase. I was much heavier back then, almost 200lbs, and my friend was probably over 225. I could immediately feel the iso's were compressing to the point the secondary buffers were fully compressed and loaded and it felt like a rigid BSA or Triumph.

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I don't see how the weight of rider and gear can compress the iso's.

I agree with Ludwig here.

The ratio of leverage the swing exerts downwards on the rear iso mounts is approx 11%, so the combined weight of 2 people at 425 lb, even if 70% of it was put on the back wheels would only be a pressure of 425 x 0.7 x 0.11 = 32 .7 lbs. Which is bugger all. That's a fact. Not a guess. But I'm only a mere automotive engineer so H T F would I know.

When my front ISO rubbers were rooted, I only felt the metal to metal contact on the overrun, so it was the torque of the engine forcing down on the front iso which caused that metal to metal feeling.

Dereck