GIII - They look alright, particularly the stops built into them, but if you were going to use them , you'd want to double check a few things.
The two very important things to remember with any sort of rubber/spring mount system are :
Resonance and engine speed.I know, I'm starting to harp on about it but I can't stress this enough. As Guy_Fawkes has pointed out in the "how to kill yourself with a listeroid" thread, there are large amounts of force and energy lurking in your engine. 10 minutes with a pocket calculator will tell you whether it's reasonably safe to proceed with the mounts you plan to use.
Unless you have a very firm mount, your engine will pass through a resonance point as it spins up and down. If you can't get your mount to resonate above the maximum RPM of your engine (and I'd suggest it'd be a long way past your rated max RPM), you need to make sure it resonates at the lowest frequency possible. Two things help with that - 'bendy' mounts and lots of mass. You can substitute one for the other if need be, or use both.
I can spin my engine over by hand faster than the resonant point in the system that I'm going to use - it's point is calculated at about 70-80RPM. It's a low enough speed that the imbalance forces in your engine are not particularly large (due to v-squared). It's safe because the engine once started, will fire once or twice and it will be well above that speed then and off up to 650RPM.
Looking at the PDF brochure, those Lo-Rez mounts resonate at about 133 CPM (counts per minute - equivalent to engine RPM) , which is pretty good for a spring mount. Adding weight on top of that mount lowers the resonant frequency of the setup. The more weight, the lower the frequency (and the lower the up-and-down movement at normal speeds as well) Remember that there will be a smaller resonant point at twice the RPM noted due to firing pulses being at only half engine speed.
Any more than that - eg 200CPM , and I'd start to get concerned. Once you reach the area of 200-650RPM, the forces applied go up pretty fast. At 650RPM, you're getting in the order of 150 kilograms of force pushing up and down on your engine and frame due to the piston mass alone. 10 times a second. This is why you need a solid frame.
A dangerous situation is having mounts that would resonate at 600-650RPM. You've got a large amount of force that will - if you have no bump stops or restrictions - exercise your mount and frame to destruction if it resonates at that speed. But there is a worse one - a system that resonates at 800RPM. You get an overspeed for some reason, things are already loaded just because of the rotational forces, and now you hit a resonant point.... that machine will be chasing you around the shop, if it doesn't fly apart when it drops off the mounts. For safety's sake, keep any resonance below 200RPM. This is my concern with the try-it-and-see approach. Works fine at 650RPM..... at 1100RPM one day with the rack gets wedged at full fuel, it tears itself to bits.
If you can't figure out the natural resonance of the mount yourself,
you need to ask someone. Ask the manufacturer. If they can tell you the deflection under load, you can work the resonance out. Ask me, if they don't have a clue - I may be able to help.
But having said all that GIII
- going off the criteria I've specified, those mounts look reasonably suitable, especially their bump-stop arrangement built in. Wonder how much they are?
Guy.....actual weights from a Vidhata 6/1
Thanks for those weights xyzer, I'll plug them into a spreadsheet I've got at work tonight. It spits out a graph of deflection Vs engine RPM which is very enlightening, so I'll see if I can post a shot of it tonight.
I'll double-check my spreadsheet over the next few days with the vibration engineer I've got on the case, and let you guys play with it then.
If you feed it a couple of things - the mass of your setup, the spring rate of your mounts, a damping factor, it will show you a nice set of graphs from 10- 800rpm which have :
- The amount of imbalance force applied to the frame and mounts.
- The amount of movement (up and down) in mm because of that force. You can spot the resonant point pretty quick, it's the great big spike in your graph. My system will end up with about 1.5 to 2mm of jiggling from the looks of things. A bit more with torque pulses under load.
- The amount of force transmitted to the ground, on the other side of the mount. Due to a curious quirk in the physics of it all, you can actually have a lot more force transmitted to the ground at certain frequencies near resonance than if it was just a solid mount.
This kind of info really helps in the design in a lot of ways. Seeing when your engine is about to leap from the frame is one good reason of course.
But there are others reasons to do the maths.
For me, I'd want a frame that can not only carry the weight of the engine, but the weight of the engine + ten times the imbalance forces applied. So if I build a frame, set it up on a solid mount and apply (150 x 10) + 750 kilograms on it and it bends more than 'a bit' ... I'd be making something stronger. "A bit' , is something I'd like to define down the track a little more.
I know, it looks like overkill. But as someone famously said, you can never have too much overkill. I did question the frame design as suggested by mobile_bob originally, but looking at the forces applied, it's better to be on the safe side.
(And man! I need to lay off the coffee or something! Pretty wordy post.
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