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Making the engine a "stressed member" talk to me
jon200 - 2/12/12 at 09:01 PM

I am changing the engine in the Locost and someone mentioned I could make it a stressed member. Now I know it means making it part of the chassis but what benefits would there be? The engine will be a 2l turbo. Stiffer chassis being one, anything else?

Any negatives?

Had anyone done this and are there any specific points to mount it to on the chassis (triangulation) It will depend on space around the engine bay mainly I can't really find much info on it. I was going to use the normal mounts but I like a challenge!

Jon

v8kid - 2/12/12 at 09:14 PM

Hi Jon,

I did this on mine with no problems at all.

It also makes the gearchange easier to make more positive as you do not have to allow for the engine wobbling about.

Some say that it will cause the chassis to crack or the block to warp but mine has done none of these. And that's with an alloy block and a very vibration prone v8.

Course they could be right eventually but I'll probably be past caring then

Cheers!

franky - 2/12/12 at 09:16 PM

I wouldn't listen to that person again.

Only engines designed to be used as stressed parts of the chassis should be used, and only in the chassis they were designed too. I know of some high profile single seat cars that kept nipping cranks as the engine blocks were twisting.

scootz - 2/12/12 at 09:24 PM

It's a 'suck and see' issue. Some engines will be up to it... others won't! Take your lead from what the single-seater guru's get up to!

I'd guess you would also want a 'stressed-member' engine to be hard-mounted to the chassis. Those vibrations will be a (literal) pain in the ass!

jon200 - 2/12/12 at 09:34 PM

I just read about damaging the block, would this be more for single seat guys having the chassis bolted to the engine rather than the engine bolted to the chassis though? Cracking welds etc is a concern.

By block is ally too.

Confused but excited. - 2/12/12 at 09:47 PM

Just my humble two penneth, but wouldn't all those skinflint car manufacturers do this to save the price of a bit of rubber, if it was viable?

franky - 2/12/12 at 09:56 PM

quote:
Originally posted by jon200
I just read about damaging the block, would this be more for single seat guys having the chassis bolted to the engine rather than the engine bolted to the chassis though? Cracking welds etc is a concern.

By block is ally too.


When you say stressed member that to me means that you'll be using the block as part of the chassis(for extra strength)?

blakep82 - 2/12/12 at 10:02 PM

quote:
Originally posted by franky
quote:
Originally posted by jon200
I just read about damaging the block, would this be more for single seat guys having the chassis bolted to the engine rather than the engine bolted to the chassis though? Cracking welds etc is a concern.

By block is ally too.


When you say stressed member that to me means that you'll be using the block as part of the chassis(for extra strength)?


yep, like what tractors do really. they don't actually have a chassis most of them i think, most have everything mounted to the engine, gearbox and axle

coyoteboy - 2/12/12 at 10:06 PM

Its only a stressed member if it IS solid mounted. Most engines are not designed to cope with it. Shouldn't make shifting any easier, unless your engine and box are separately mounted!

Bluemoon - 2/12/12 at 10:06 PM

I think the chassis would really need to be designed to do this. Not sure you will gain much as the engine on the locost in the cente of a void in the space frame; you will need to design the mounts carfully gain any thing I suspect... Personaly I would rule it out on the grounds of vibration if nothing else; might consider harder mounts, but not directily bolting it to the chassis.

Dan

[Edited on 2/12/12 by Bluemoon]

britishtrident - 2/12/12 at 10:39 PM

Solid mounting the engine (which was general practice in the 1920s) iisn't quite the same as per using the engine as part of the main structure as per Lotus 49 and Ferguson Te20 tractor.
Personally I would not do as you get it right it will open the way to lots of problems.

If you want a stiffer chassis look at the Australian mods.

twybrow - 2/12/12 at 11:09 PM

Get a bike engine - they are designed for it (in original application)....

stevebubs - 3/12/12 at 12:27 AM

quote:
Originally posted by twybrow
Get a bike engine - they are designed for it (in original application)....


But not for a car where the loads are (most likely) significantly higher and in different planes

ETA: most likely

[Edited on 3/12/12 by stevebubs]

MRLuke - 3/12/12 at 01:15 AM

quote:
Originally posted by britishtrident
Solid mounting the engine (which was general practice in the 1920s) iisn't quite the same as per using the engine as part of the main structure as per Lotus 49 and Ferguson Te20 tractor.
Personally I would not do as you get it right it will open the way to lots of problems.

If you want a stiffer chassis look at the Australian mods.


This.

Bolting the engine to the chassis is completely different to having suspension pickup points on the engine / gearbox.

The benefits are that your chassis only has to go to the start of the engine not all the way to the front (or rear depending on placement) of the car. So basically its a weight saving.

The downsides are that you can twist your engine or that your mounting points are not sufficient as there are no designed pickup points on your engine / gearbox. Id imagine it probably affects crash performance as you have lost a deformable structure that would normally take the impact.

[Edited on 3/12/12 by MRLuke]

Canada EH! - 3/12/12 at 02:47 AM

As I recall the formula A and Can Am cars I worked on in the 60's and 70's used the engine as part of the chassis.
The front of the engine was bolted to the the rear of the tub which contained the drivers compartment and front suspension and the rear suspension was bolted to bell housing and Hewland diff transmission unit.
I am sure that General Motors never intended their small and large block engines to be used in this manner.
The front motor mount was a flat magnesium plate sandwiched between the water pump and the block. The rear suspension was bolted to a cross member that was affixed to the bell housing.
The last Mclarens had all ali big blocks of 500 c i displacement .

coyoteboy - 3/12/12 at 09:26 AM

quote:

Solid mounting the engine (which was general practice in the 1920s) iisn't quite the same as per using the engine as part of the main structure as per Lotus 49 and Ferguson Te20 tractor.


Indeed, though to be a stressed member it can't be soft mounted. It can be a semi-stressed member if it just stiffens and adds something, but as you say BT, there's a whole different world between bolting the engine into a space in the chassis solidly and using it to replace a large part of the chassis.

quote:
As I recall the formula A and Can Am cars I worked on in the 60's and 70's used the engine as part of the chassis.
The front of the engine was bolted to the the rear of the tub which contained the drivers compartment and front suspension and the rear suspension was bolted to bell housing and Hewland diff transmission unit.


A) I don't know that they used the same block so I can't answer that either way, and also can't comment whether they were initially designed to accommodate that use later on, just not in mass production.
B) It's possible to modify a currently-strong/stiff engine to deal with the problems - you need to use dowels (usually mounted cross-plane rather than perpendicular to plane I'm led to believe for this use) between most mating surfaces to pass the stresses from one part to another. So between head(s) and block and between block and sump, and between bell-housing and block. And not your piffling alignment dowels used on production engines, we're talking tight tolerance solid dowels.

phelpsa - 3/12/12 at 10:04 AM

If your external loads are small compared to your internal loads then all is good. How do you know? Try it!

jon200 - 3/12/12 at 12:56 PM

Ok so I wasn't looking for a fully stressed member but semi stressed. Just to add some rigidity if it will? Are the top tubes in the bay worth strengthening then? I'm not about to chop the car in half to then join it with the engine.i think It would be asking for trouble too.

mcerd1 - 3/12/12 at 01:09 PM

^^ the most worth while looking mods to the engine bay that I've seen involved a bolt-on X-brace over the top of the engine

I think a full roll cage would add even more though

[Edited on 3/12/2012 by mcerd1]

jon200 - 4/12/12 at 06:30 PM

I am not going to be able to go over the engine as its high anyway but what about in front and the rear of the engine?

coyoteboy - 4/12/12 at 07:32 PM

Question is what's your purpose and how much difference can you actually make, and finally will you be able to quantify the difference or is it like adding a strut brace to the top of a hot hatch and assuming it must be better?

I'm not saying don't do it, just check it's worth while.

umgrybab - 4/12/12 at 09:14 PM

quote:
Originally posted by jon200
Ok so I wasn't looking for a fully stressed member but semi stressed. Just to add some rigidity if it will? Are the top tubes in the bay worth strengthening then? I'm not about to chop the car in half to then join it with the engine.i think It would be asking for trouble too.


If you want to add rigidity, look at some of caterham's bracing techniques, or the roll cage as mcerd1 suggested. With both fully stressed and solid mounted engines, the vibration can cause problems. Especially with inexperience.

jon200 - 5/12/12 at 07:56 PM

quote:
Originally posted by coyoteboy
Question is what's your purpose and how much difference can you actually make, and finally will you be able to quantify the difference or is it like adding a strut brace to the top of a hot hatch and assuming it must be better?

I'm not saying don't do it, just check it's worth while.


I wouldn't be spending loads so if it makes it stiffer then its a bonus surely. If not then I tried.

jon200 - 5/12/12 at 08:00 PM

quote:
Originally posted by umgrybab
quote:
Originally posted by jon200
Ok so I wasn't looking for a fully stressed member but semi stressed. Just to add some rigidity if it will? Are the top tubes in the bay worth strengthening then? I'm not about to chop the car in half to then join it with the engine.i think It would be asking for trouble too.


If you want to add rigidity, look at some of caterham's bracing techniques, or the roll cage as mcerd1 suggested. With both fully stressed and solid mounted engines, the vibration can cause problems. Especially with inexperience.


What do you mean with inexperience? Is it possible to add a roll cage to an existing roll bar?

chillis - 5/12/12 at 09:56 PM

When an engine is used as a stressed member it is to replace part of the chassis to reduce weight. HOWEVER normal production car engines are not designed as stressed members and the weight that would need to be added to the block may well exceed the weight of the chassis structure it replaces. As already mentioned elsewhere in this thread if the engine could be part of the structure I'm sure the car manufacturers would already have done so - especially as weight has become so important now.
Quite some stress analasys goes into just the engine mounting points and belhousing flange. Putting the structural integrity of the whole car through something thats designed to just keep itself together could well prove to be a disaster. All modern prod engines are designed down to the lowest weight these days so there'll be little 'spare' for holding the rest of the car up
Having said that back in the '80's we used the Aston Martin 5.3L V8 as a stressed part of both the EMKA and Cheetah Group C racers without issues with the block, but then this engine originally designed in the early '60's so may well have had strength to spare.
What engine are you planning to use? are there any lighter engines that could provide the performance you need without cutting the structure? For a road car a cut of the chassis and use of the engine in this way could see you being sent to see the IVA inspector

v8kid - 5/12/12 at 10:09 PM

Ah well!

Lots of opinions to say I'm wrong - one quite insulting really - but I've actually done it and it works.

Make of that what you will.

Cheers!

PS With a mid engined car and the gearbox at the rear the gearstick end of the linkage is bolted to the chassis and the gearbox end is on a gearbox which wobbles about if its on rubbers - so solid mounting makes a big difference. Like I said I've done it and I learnt from someone else who did it.

Now I see why some other forum members get so grumpy.

coyoteboy - 6/12/12 at 11:41 AM

v8kid - I wouldn't have thought you'd have enough flex in rubber mounts to cause THAT much wobbling, surely you pick your mount stiffness to suit?! I guess if you're going to use linkages instead of cables you will have that issue though!

Do you have images showing how you mounted it as a stressed member? I'm curious. What generally causes problems is twisting of the block and heads can cause separation at the interfaces, but also slight misalignment at the bearings and piston bores, which means increased wear and warp over extended periods.

mcerd1 - 6/12/12 at 11:58 AM

quote:
Originally posted by v8kid
Lots of opinions to say I'm wrong - one quite insulting really - but I've actually done it and it works.

I don't think most of us are saying your wrong, just that it makes things a bit more complex and there is a chance that the engine won't like it much....

as chillis said allot of older engines may well have the spare capacity in the block design - but its not like there are figures avalible for these


if you had fancy FE software and/or loads of cash to spend on analysing the design you'd get a better idea what the engine was capable of
but the only way to find out for sure is to try it and see what happens

[Edited on 6/12/2012 by mcerd1]

coyoteboy - 6/12/12 at 01:01 PM

I had considered making the engine a stressed member in mine (Audi ABZ V8) due to the nice availability of large attachment points on the heads and many attachment points on the block, plus extensive casting braces around the block. I decided against it as I think I can make a stiffer chassis *around* it and I've very little to gain(lose) weightwise.

v8kid - 7/12/12 at 02:04 PM

Here is photo I've circled the mounts in red. semi stressed engine
semi stressed engine

The bottom two bolt through the gearbox. The top two bolt on to the bellhousing bolts and there are another two that bolt the siderails on to the block at the usual mounting points.

Argh! you can't see them try this Description
Description

jon200 - 7/12/12 at 04:08 PM

quote:
Originally posted by v8kid
Here is photo I've circled the mounts in red. semi stressed engine
semi stressed engine

The bottom two bolt through the gearbox. The top two bolt on to the bellhousing bolts and there are another two that bolt the siderails on to the block at the usual mounting points.

Argh! you can't see them try this Description
Description


Is that solid mounted then?

Anyone know how rubber is graded from soft to hard? With examples?

Alan B - 7/12/12 at 04:50 PM

quote:
Anyone know how rubber is graded from soft to hard? With examples?


Good reference here...

http://www.mcmaster.com/#about-firmness-ratings-for-rubber/=khnu9z

coyoteboy - 7/12/12 at 05:30 PM

That's not using the engine as a stressed member though, that's semi-stressed at best, but looks like it's just solid mounted (i.e. you have framework around that contributes to sharing stresses, the box/engine are just helping out with triangulation and a bit of rigidity?). That is MUCH easier on the block/box, unless I can't quite make something out in the picture (possible).

A properly stressed member looks like:


Where the whole transmission is used to replace the rear chassis structure, supporting all of the rear suspension pickup points etc.

[Edited on 7/12/12 by coyoteboy]

v8kid - 7/12/12 at 06:34 PM

quote:
Originally posted by coyoteboy
That's not using the engine as a stressed member though, that's semi-stressed at best, but looks like it's just solid mounted (i.e. you have framework around that contributes to sharing stresses, the box/engine are just helping out with triangulation and a bit of rigidity?). That is MUCH easier on the block/box, unless I can't quite make something out in the picture (possible).

A properly stressed member looks like:


Where the whole transmission is used to replace the rear chassis structure, supporting all of the rear suspension pickup points etc.

[Edited on 7/12/12 by coyoteboy]


Aye OK whatever

coyoteboy - 7/12/12 at 06:57 PM

Don't you whatever me If you're going to claim it's a stressed member and tell people they're wrong because you've done it, you need to be right!

jon200 - 7/12/12 at 07:15 PM

quote:
Originally posted by Alan B
quote:
Anyone know how rubber is graded from soft to hard? With examples?


Good reference here...

http://www.mcmaster.com/#about-firmness-ratings-for-rubber/=khnu9z


Thanks Alan, my problem now is that they don't state what shore grade on many ads. oo a or c