# Motor Mounting Concept Feedback



## Hollie Maea (Dec 9, 2009)

I'd put rubber isolation back in, honestly. Your chassis will try to twist, and with a rigid mount it will constantly eat at your welds. Demonstrator or not, you don't want your motor falling out.


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## George Rosebush (Aug 6, 2015)

Definitely don't want the motor falling out, but the question is will rubber mounts prevent that kind of failure when they've broken before under less torque? 

We're replacing the rubber mounts on the previous version with solid mounts, my guess is it will still break at the weld as it has before. That rubber, btw, broke in about two months of use, the one that had a crack at the weld had about 80000 miles. It's either the rubber or the welds that break, but FEA confirms it's not a strong design, this one however, any guesses?


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## Old.DSMer (May 18, 2012)

Isolation will be a tremendous help when compensating for instantaneous torque. I'd guess when your old rubber mounts failed, the brackets bottomed out and you ended up impact loading them. Without seeing the failed parts, I'm only guessing. However, I can confirm that rubber will not sustain the instant torque of electric motors. Even if it was half the size.

I'd recommend urethane mounts. Most performance shops offer them for engine and suspension upgrades.

Also check out evmetro's posts. I remember him posting information about a company that sold the inserts and collars which allow you to custom build your own.

Regarding your mounting, can you mock something up to represent the motor in there? I'm not quite seeing where things attach?

It's important to ensure your mounts are loaded in compression. Any shear on rubber or urethane will reduce their life dramatically.

Sounds like a wicked project with that much torque! Would love to see more when you can share.


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## George Rosebush (Aug 6, 2015)

Well the motor attaches to the big plate at the front to the circular pattern of holes, the gear reducer has mounting holes on it's sides (6 total) that attach to the plates with the angled arms going to the sides. The motor and gear reducer are bolted together but are offset (not concentric).

It's really not wicked at all because it's not any normal passenger vehicle, it's something much heavier and the current motor is just not enough.

It seems to make sense what you're saying about the rubber not being good for that kind of torque, they're just not meant for this purpose. I do agree isolation would be ideal, but how important would that be knowing that it will only be driven once?


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## Tomdb (Jan 28, 2013)

I would extend the "Motor plate" down to create more overlap for welding, possibly extend it so far as to be bolting the actual plate.


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## Duncan (Dec 8, 2008)

Hi George
You need to look at exactly how you are resisting;
Motor torque - bearing in mind that if the motor is bolted to the gear reduction you are resisting motor torque x gear ratio

Dynamic loads on the three axis
Pitch
Bounce
twist 

If your motor will produce 400lbft x 4:1 and your mounts are 2 ft apart
400 x 4 /2 = 800lbsforce at each mounting

Alllow for a two G "bounce" in each direction 
Motor + gearbox weighs - 300lb? - you may have to resist 600lbs in all three directions

There is no problem in getting a rubber mount to resist those sort of loads
The problems come if your mounts are too close together

You just need to understand the loads and load the rubber the correct way


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## George Rosebush (Aug 6, 2015)

Hollie Maea said:


> I'd put rubber isolation back in, honestly. Your chassis will try to twist, and with a rigid mount it will constantly eat at your welds. Demonstrator or not, you don't want your motor falling out.


Thinking about this again, if the stress is being distributed through the frame and it twists, wouldn't that mean there's a lot less stress on the subframe/welds? Wouldn't that twist either concentrate on some point on the frame or even get collected by the springiness of the suspension? Isn't the frame allowed to flex some, so long as it's not yielding?

You'll notice that the point where the big front plate contacts the big crossmember are very wide plates, doesn't that spread the stress over a large area? Would that be improved by instead attaching to rubber mounts that can absorb some stress but have a much smaller contact area?

If all you want to do is absorb stress, and not vibration, doesn't the flexibility of the frame do just that?


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## Hollie Maea (Dec 9, 2009)

Yes there is flexibility in the frame, and that's the problem. The frame rails can twist independently, and they need to. If you are on uneven terrain, if you are going around a corner, if you have torque from the motor...the two sides move a little relative to each other. If you mount your motor without isolation, now the frame is locked together. Instead of being able to twist a little, it pulls on your welds.

I could be wrong, but that's how I understand it.


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## electro wrks (Mar 5, 2012)

I think the forces HM is referring to are twisting and bending, road and vehicle weight forces transferring back through the motor mounts. These can be quite substantial. I recall a race car being set up, for the best traction and handling, to have no more than 1 degree of twist for a torsional force of 10,000 foot-pounds acting through the car's frame. This is 2+ X the next order of magnitude of the torque of even your large motor. If these are the kinds of forces that are present(they might even be much larger with your vehicle- you might give us a few more details), they would far exceed those of the motor and be the primary force on the motor mounts.

Rubber mounts would isolate the motor mounts and allow the vehicle frame to flex and minimize the transfer of these large forces to the mounts. Properly sized rubber mounts are just fine. Some huge ones are even used to base isolate entire buildings for earthquake protection! AIR it's best to set up rubber mounts with the primary static load in compression and the primary dynamic loads in shear to make the best use of the material.


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## Duncan (Dec 8, 2008)

Hi Guys

I find the idea that the chassis will twist that much simply frightening which is why I like "space frame" systems and consider ye olde twin rail chassis to be only useful for horse drawn carts

If I was making any type of performance vehicle I would be looking to ensure that it was stiff enough 
And the idea of mounting the engine/motor such that it's motor mounts have to cope with chassis twist.......


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## piotrsko (Dec 9, 2007)

You've never watched a top fuel blown dragster chassis at launch? I'm surprised they can go the 1/4 in a straight line.

How hard do you REALLY intend to push this? Stock is usually good to 12 sec / quarter, it's not like you're pushing 600 HP and 1000 lb ft torque.

Go back and read the early posts from the expert people. Btdt


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## Old.DSMer (May 18, 2012)

George, I don't know if you can tell us WHAT you are building. As Duncan said, you need to understand the loads. Is this a yard truck with a 20mph top speed? Rock crawler? Drag car? What are the chassis dimensions and vehicle weight?

There are hundreds of parameters that will affect chassis loads. Making specific recommendations at this point is impossible. Perhaps your application requires the most rigid frame possible? (usually preferred). Or maybe it requires some flex?


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## George Rosebush (Aug 6, 2015)

piotrsko said:


> You've never watched a top fuel blown dragster chassis at launch? I'm surprised they can go the 1/4 in a straight line.
> 
> How hard do you REALLY intend to push this? Stock is usually good to 12 sec / quarter, it's not like you're pushing 600 HP and 1000 lb ft torque.
> 
> Go back and read the early posts from the expert people. Btdt


I really don't understand what you're saying, "stock" what? This isn't a race car, and what are these early posts from what expert people?


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