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Engine mount to chassis question
FERRARIST - 4/8/14 at 08:11 PM

New on locosts, i started few days ago single seater mid-engine project and different questions pop-ups.....
Question is - If i mount the engine directly on the chassis not using any PU bushes, can this cause any structural damage in welds and joints.
Details - i'm using 40mm square tubes, 2 and 2,5mm walls, and 35mm tubes for front and rear cell, 40mm square tube to connect both.......Alfa 166 3.0V6 engine transverse mount, 200kg with gearbox......
PU is not a problem for me cause we make bushes from PU, but it will save me a lot of time......I don't care about vibrations while driving as long as it can't cause any damages.....
Thanks....

Badger_McLetcher - 4/8/14 at 09:15 PM

I wouldn't advise it - you may find that the completely undamped vibrations may cause welds to fracture. Basically you may end up with undamped harmonic resonance at certain RPM where it hits the natural frequency of the chassis or other components, and that can lead to... interesting effects.
You may end up with nothing, you may end up with severe issues that you cannot foresee. I'd stick with at least some damping

FERRARIST - 5/8/14 at 05:45 AM

Note taken, PU bushes will be used.....

drt - 5/8/14 at 11:02 AM

I disagree.
Any bushings need to be very large and very soft to be of any help.
This because of the natural frequency of a low mass chassis and the frequencys you may expect from the engine.
PU will be to harsh and the standard car ones way to soft due to comfort

If interested I can post the maths

Slimy38 - 5/8/14 at 11:14 AM

Bike engines have been solidly mounted with no ill effects... although I would expect the resonant frequency of a high revving four pot to be a bit different to a torquey V6.

mcerd1 - 5/8/14 at 11:58 AM

why not use the stock engine mounts ?
its one less thing you'll need to make....

coozer - 5/8/14 at 12:00 PM

I used landrover engine mounts, no problems.

Then I used a turnbuckle type bar with rose joints from the top of the engine to the top of the chassis. Just to stop the engine moving around too much on the track.

The results was tingling numb fingers from the high pitch vibs coming through the steering wheel.

FERRARIST - 5/8/14 at 02:41 PM

Original mounts pillar are outside chassis dimension, i'll use same fitting points, but different pillars....
May use 3 of the bushes pictured below - one at rear, and 2 in front of the engine, can make 4th mount in top of it as well - 85Sh hardness of the PU guess it's hard enough to hold engine stiff and suppress vibs......

Sam_68 - 5/8/14 at 06:31 PM

quote:
Originally posted by drt
If interested I can post the maths


I'd be interested.

motorcycle_mayhem - 5/8/14 at 06:38 PM

Debated this with myself for many a month, on all the vehicles so far.

All moped engines, so I can't speak for the car derived things. Plenty of opinions on how to do this from plenty of people.
Essentially, solid mounts are apparently inviting resonance and harmonics. I have some evidence that hard mounting a modern Blade engine will create a resonance that will annoy the knock sensor, or it seemed to, solved by using Westfield rubber suspension bushes at the engine mounting points. Cause/effect, unsure.

The older engines (GSXR) without a knock sensor have been fine, as have the chassis they're attached to, hard mounted. No real vibes, no real resonance.

I saw a soft rubber mounted R1 in an SR1, where the diff was suspended along with the engine in a unitary cradle. Boy, did that thing rock. I'm suprised the shafts didn't come out of the diff!

I've seen some Harley tractor engines rock violently on idle, where Harley have clearly mounted the thing in the bike on rubbers.

mcerd1 - 6/8/14 at 07:16 AM

quote:
Originally posted by FERRARIST
Original mounts pillar are outside chassis dimension, i'll use same fitting points, but different pillars....
May use 3 of the bushes pictured below - one at rear, and 2 in front of the engine, can make 4th mount in top of it as well - 85Sh hardness of the PU guess it's hard enough to hold engine stiff and suppress vibs......


if you mount the engine at 4 points you risk over constraining it - this will likely increase the load on at least one of the mounts and can cause failure

that's why all factory ones are mounted at 3 points, but you'll also notice that the factory ones are not all at the same orientation - in transverse layouts this usually means that the engine is hanging on the left and right mounts, these mounts deliberately allow lots of 'forwards/backwards' twist (if you get what I mean) this twist is then held by the rear mount which often doesn't do much else - all this is to avoid over constraining it


also however soft those PU bushes are I'm sure your mounts will end up more than 10 times or more stiffer than the stock ones - so you've got to expect the same kind of increase in vibrations to !



as for bike engines - you've got to remember that they are designed to be hard mounted - car engines are not....

[Edited on 6/8/2014 by mcerd1]

40inches - 6/8/14 at 08:22 AM

quote:
Originally posted by FERRARIST
Original mounts pillar are outside chassis dimension, i'll use same fitting points, but different pillars....
May use 3 of the bushes pictured below - one at rear, and 2 in front of the engine, can make 4th mount in top of it as well - 85Sh hardness of the PU guess it's hard enough to hold engine stiff and suppress vibs......



I would make up a rectangular plate, weld the tube to the plate then weld the plate to the chassis, top and sides.
Could even have a fillet both sides?