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I'm building a 7 clone based on Lotus 7 Series 2 plans.

Soon I will be working on motor mounts. Is there a distinct advantage to the Caterham style mount (fastener in tension, using the equivalent of a Jag XK motor mount) or to the birkin mount (fastener in shear, poly bushing)?

Caterham:



Birkin (sorry for the large hot-linked pic):



Thanks in advance.

Maybe it depends on what your frame looks like. The Birkin mount looks a lot like the one on my Formula Ford. The bolt goes to some diagonals and directly into the mount like that. There is just a bit of some type of rubber in the frame tube where the bolt goes through. It is very strong and has withstood some abuse. The Caterham bolt to the frame doesn't look in tension, perhaps I misunderstand the picture. I have seen those mounts up close and they are very elegant. Couldn't say what gauge tubing they use though.

Thank you for the input - I like to hear detailed answers like this.

I'm focusing more on the actual rubber bit on the frame. Both the Cat and the Birk connect to the chassis in about the same location. The frames are pretty much the same.

The Caterham rubber mount bolts vertically into the frame, with a vertical bolt attaching to the engine bracket, whereas the Birkin mounts sort of sideways - a horizontal bolt supports the engine rubber mount in single shear. I guess the Caterham one isn't so much tension, but the securing bolt would be loaded in tension/compression (if my mind is picturing engine motion and loading right).

The Caterham mounts have been historically weak, and they have a revised design that still holds itself together when it fails.

The Birkin chassis mount.... I haven't heard anything positive or negative about them.

A quick seach came up with one failure for the Birkin style mounts.

http://californiacaterhamclub.com/chat/showthread.php?1775-Sheared-right-side-engine-mount-bolts

From a layout and fabrication point of view, it seems the Birkin design lends itself to the motor bolts being well above the poly bushings.







I don't know the gauge, but it seems that Caterham changed from round tubing to rectangular tubing at some point, probably for better torsional strenth.

At the block:
I can see why the Birkin failed. the block attachment points fall on a single line, basically a bend line for the bolts to eventually fail, just like the pic. This block needs 3 bolts that do not line up.

At the chassis:
The Caterham mount probably doesn't have a safety stop built into it for when the rubber tears from stretching the mount under full load. Use a strap or a better mount.

Bushings are typically rated for greater thrust load, so that is the way the load of the static weight of the engine and trans should be placed. Caterham yes, Birkin no. OEM heavy loads such as body on frame, and cylindrical engine mounts are arranged this way. Torque links are not, but they are not supporting the engine weight.

So it appears as if the Birkin solution has a successful history, on the chassis side. Still, given a choice between placing a fastener in single shear or in it's as-designed condition, I'd choose the latter and enjoy imagining Carroll Smith nodding in approval. On the engine side, if single shear is the only option, it would be important to make certain that the last thread of the fastener (the stress riser plane) is well outside the shear plane. If they are coplanar, it's a classic failure mode.

I know that a number of solutions on Sevens are not optimal and yet work just fine (single shear lower wishbone pick-ups anyone?), but I'm just sayin'... given the choice...


-Pete

It seems like both have their pros and cons. Any thought of doing a hybrid version by using three holes on the block side like MV8 suggested but use a rubber bushing in double shear at the chassis?

I don't know exactly what the bottom of the Caterham mount looks like, but I expect it is similar to my doodle. A spacer and washer on just the left side mount should prevent pull through and tearing of the rubber mount.

There some photos of the Jaguar XKE motor mounts here:

http://www.locostusa.com/forums/viewtopic.php?f=4&t=11835

I also went out to the garage and snapped these:





From the Pat Prince S2 drawings, the mounts are bolted through a 1/2" OD x 5/16" tube passing through the 1" square frame tubing.

I am trying to find a parts diagram showing it, but I am fairly sure the 1/2" bolt in the center of the mount is threaded in blind.

I think I feel happier with the weight and torque of the engine affecting the lower longeron in beam by the Caterham mount, versus affecting in torsion as per the Birkin mount.

It's not the attachment to the block that concerns me (and dude, two bolts? Seriously?), I'm just trying to decide on what to use at the frame. What's best for longevity, in other words.

Thanks for the insights - any others?

The Caterham mounts could allow some adjustment in engine height relative to the frame if ground clearance becomes as issue.

Ok, I ordered the Caterham/Jaguar mounts. No matter how I envisioned it, I wasn't happy with how I'd make the Birkin mounts fit the way I'd triangulated the bottom of the engine bay, plus the location of the frame mount and engine block mounting bosses - more like current Duratec Caterhams. The Birkin works best if the frame mount is below and close to the mounting bosses on the block. The Caterham style allows more fore-aft placement flexibility.

Still, I think the Birkin style would be a LOT more fail-proof. Plus the polyurethane would restrict engine movement nicely.

Perhaps, if one of my mounts fail, I will burn out the rubber and re-cast them with a polyurethane mix..... Only the future will decide.

You can add a 6" strap with one end under the vertical top bolt and the other laterally through the frame rail beneath the mount.

Another option is to use a small square U-bolt (available at hardware stores) vertically, around the top of the mount bracket a couple inches inboard of the vertical bolt through the mount. A short piece of fuel line split lengthwise slips over the u-bolt where it would make contact with the mount bracket.

The u-bolt engages a steel plate, sandwiched between the mount and the frame, extending inboard as necessary. The u-bolt has a lot of threads, so the engagement height is adjustable, so contact between the u-bolt, fuel line, and mount bracket only occurs under maximum load, preventing the caterham mount from being overextended and tearing away.

Need a pic?

No, I can picture this. I had a similar plan up my sleeve.

Just like I used a piece of chain on my (now late) V8 Pinto to keep the engine-mounted cooling fan from sawing through the hood on acceleration....

Little late, but I've always thought the way I did my mounts was super easy, and effective.
viewtopic.php?f=4&t=11912

You basically just bolt a plate to your stock engine mount locations, then weld ears to it. Then whatever frame attach mount you want. Or you can bolt the mount ass'y to the engine, then the ears on the frame. Some flat stock and a couple hours and your mounts are done. Mine is pretty heavy, but so is my engine so...... yeah.

Cheers.