Project V6 Elise

[Ferg]

Ha, is that not the selector shaft assembly? Slave cylinder's not in any of the pics.

Ah yes...I'll stick to reading..

[Woodsport]

Where the subframe is marked on the left hand side in the gear linkage photo... is that marked to be cut out for clearance?

During my initial measuring i thought i needed to remove that area, but decided to leave it to see how close it ended up being, turns out i don't need to remove it after all.

[Woodsport]

Post #1 updated

[robin]

Hi Paul,

As you'll no doubt remember from when you removed it, the Rover torque mount connects to the sub-frame on the right hand side (near where what looks like the hanger bearing for the right hand shaft of the new engine/box now sits). The torque load was being returned to the engine mounts via the bolted joint between subframe and chassis just where the two roll-over-bar stays land on the very ends of the chassis arms. The original engine mountings were designed to pivot along an axis parallel to the drive shafts so the engine could rock back and forth, stretching and compressing the torque mount.

With the torque mount now connected directly to the chassis, there is no such path for the torque load to return to the engine mounts; instead it will travel through the chassis rail where your new bracket is mounted, then along that rail until the point where the rail meets the rear chassis arms and from there back to the left/right engine/gearbox mounts. Your new engine mount allows for very little rocking, so your new torque mount needs to be fairly stiff I think. In reality both the new torque mount and the new engine mount provide resistance to torque; the gearbox mount looks like it preserves the original rocking design, though you could easily stiffen that up too to provide a third torque mount if you went for a bush design much like the engine side one. I know a lot of the earlier Honda conversions had problems with the gearbox mount so you might want to dig about to see what they started with and what they ended up fitting.

Anyway, I think that the sub-frame is almost not loaded by the engine in your design; put another way, ignoring the lack of working suspension, the car would accelerate and slow down with the sub-frame removed and without the engine flailing around. This might well be a good thing, by the way - I'm not saying it's a problem, just making the observation. I suspect Lotus attached the engine torque mount to the sub-frame simply because that's where Rover put the mount on the engine and not for any clever reason; in a FWD application, the torque mount would connect to the chassis (or more likely the reinforced bulkhead in some tinny little car) much like you have it. Also I'm pretty sure some of the Honda conversions adopt a forward torque mount not dissimilar to the one you've made.

But I don't understand the point of the brace bar in this? Worse I think it could do some harm.

Lotus advise against running the car with the shear panel off (you'll have seen the sticker on it, no doubt). To me this implies that there are some reasonable shear/twist loads on the rear frame of the car. I suspect that the rear chassis arms and the sub-frame all sway/flex/twist a little when the car is in action. The more-or-less rectangular arrangement of the three chassis rails and sub-frame around the engine bay allow this to happen in that area while the shear panel stiffens up the next section of the chassis to prevent the same flex happening around the fuel tank, I suspect.

During cornering or travel over very uneven surfaces, the flat rectangle formed by the chassis arms and sub-frame around the engine bay simply becomes a rhombus with slightly curvy sides or similar. By bracing across this rectangle, I think you create the possibility of gearing up these sway/shear/twist forces and focusing them on specific points in the ensemble. Although the spherical bearings will allow the bar to adopt small angular changes (left/right and up/down) and thus "go with the flow" of shear and twist, the distance between the anchor points must remain fixed as your tie rod won't compress/stretch by much if anything at all. I'm pretty sure that this will create a lever that will put more force into the joints near the rod ends.

So unless there is a very good reason for this brace to exist, I wouldn't have it in there.

On the subject of reinstating the chopped sub-frame, I cannot quite get my head around the original cross section of the sub-frame - I think it's more-or-less triangular tube (like a long Toblerone bar), with each of the panels being stamped with a series of large holes to save weight. I believe that it is this tubular shape not only to provide effective resistance to compression/extension but also to provide a controlled resistance to twisting (torsional rigidity). So whatever you do in terms of reinforcing the remainder of the sub-frame, I think you should aim to reinstate the original torsional rigidity - it might be counter productive to substantially increase the rigidity as this may simply transfer load onto other, weaker, areas (e.g. the four bolts that hold the sub-frame to the chassis).

With more-or-less two out of three panels of the tube removed, the middle section of the sub-frame will now be much easier to twist as it's nearly just a flat sheet - it will twist further for the same applied force and so might now fatigue and then fail. You've added another tube left-right at the bottom and I'm sure this will go a long way towards reinstating the compression strength of the frame and probably acts like the fancy brace bars that the newer Exiges have fitted, but I don't see it doing much for the torsional rigidity. You either need to construct something approximately tubular from similar material to and with the same cross sectional area as the original triangular section, or you need to make something with a smaller cross section out of thicker material and then anchor the ends of that new structure back to where the wishbone mounts are. There's no point putting in a strong torsion beam across the middle of the sub-frame if the material on either end isn't strong enough to hang on to it - it will just fatigue/fail there instead.

Worst case you might end up chopping out the whole middle section of the original sub-frame and building a "space frame" that attaches to the original left/right ends and preserves the exact positions of the upper/lower wishbone mounts.

Anyway, I know nothing about mechanical engineering, so I'll leave it to you to ponder, but I thought it would be interesting to mull over some of these issues.

Cheers,
Robin

[campbell]

You gotta love him...FBF lobs in a (well thought-out) grenade and stands back to watch you then throwing it around like a hot potater at a pass-the-parcel party

We were kicking some of this around briefly during UJI's spell in "hospital" and joking aside I'm also sure it's well worth thinking about. I believe Robin is picking up on some of the "long-term test" issues that have now begun to emerge from Honda-Elise conversions. Before anyone jumps in to defend those conversions, I am not attacking them. It's just that the boys at Lotus will surely have thought about what they needed to spec in the structural capabilities of the Elise, and presumably they never really expected it to be taking more than about 190bhp or the relatively measly torque that provides from the K series. Presuming that nice Yota V6 lump offers much better torque than any K, then my simplistic mind says there is now much to think about.

If only you could get hold of Richard Rackham. Tut...you got his number?!

Campbell

[Woodsport]

With the torque mount now connected directly to the chassis, there is no such path for the torque load to return to the engine mounts; instead it will travel through the chassis rail where your new bracket is mounted, then along that rail until the point where the rail meets the rear chassis arms and from there back to the left/right engine/gearbox mounts. Your new engine mount allows for very little rocking, so your new torque mount needs to be fairly stiff I think. In reality both the new torque mount and the new engine mount provide resistance to torque; the gearbox mount looks like it preserves the original rocking design, though you could easily stiffen that up too to provide a third torque mount if you went for a bush design much like the engine side one. I know a lot of the earlier Honda conversions had problems with the gearbox mount so you might want to dig about to see what they started with and what they ended up fitting.

Hi Robin, thankyou for taking an obviously considerable amount of time to reply and comment on possible issues, all taken on board, however i don't think you have grasped how the torque mounts work, my design allows the engine to pivot in the same way it did before, it directly copies Toyotas way of doing this, You also have to understand i haven't finished, i need to add a rear torque mount too, these V6 engines are all about the torque and they twist very hard under acceleration, so the Rover system needs throwing away, it won't work for this engine.

Anyway, I think that the sub-frame is almost not loaded by the engine in your design; put another way, ignoring the lack of working suspension, the car would accelerate and slow down with the sub-frame removed and without the engine flailing around. This might well be a good thing, by the way - I'm not saying it's a problem, just making the observation. I suspect Lotus attached the engine torque mount to the sub-frame simply because that's where Rover put the mount on the engine and not for any clever reason; in a FWD application, the torque mount would connect to the chassis (or more likely the reinforced bulkhead in some tinny little car) much like you have it. Also I'm pretty sure some of the Honda conversions adopt a forward torque mount not dissimilar to the one you've made.

Like i said, it's not finished, you are getting to see progress as it happens rather than the finished conversion, plus Lotus didn't build a Camry V6 Elise so i'm kinda out on my own here forging an unbeaten path, i may make mistakes with the design but that is after all what a test mule car is for. I have some other upgrades coming that you may well like, some not, who is to say what is the correct way to build a V6 Elise? I can only draw on my ten years of engine conversions and apply all of the experience and knowledge we have to the issues at hand, consult my engineer as much as possible, and of course listen to peoples comments, that is half the reason for my build threads, i am far from blinkered to think i can just build a V6 Elise straight off the bat and hit a homer.

I have also thought about a "two rear torque mount" design, one on my brace and the other in the original position that ties into the rear axle carrier bracket, exactly how Toyota do it in the Camry, that would leave the front member untouched with no vibration going through it at all.

But I don't understand the point of the brace bar in this? Worse I think it could do some harm.

Damned if i do and damned if i don't it seems, i got a flaming on Seloc for taking bracing away from the subframe. Simply put the current chassis/subframe is a lower case "n" by basic design, the subframe being the left leg, the gap being the engine bay... now under acceleration the engine is trying to pull the subframe at the bottom of that left leg, my tie bar braces the bottom of that "n".... currently the only thing doing so is the suspension lower wishbone, that ties the subframe lower point to the front chassis in a fashion, my tie bar is preventing the subframe from flexing, my engineer says it is a sound modification so i'll be keeping that and adding two more.

Lotus advise against running the car with the shear panel off (you'll have seen the sticker on it, no doubt). To me this implies that there are some reasonable shear/twist loads on the rear frame of the car. I suspect that the rear chassis arms and the sub-frame all sway/flex/twist a little when the car is in action. The more-or-less rectangular arrangement of the three chassis rails and sub-frame around the engine bay allow this to happen in that area while the shear panel stiffens up the next section of the chassis to prevent the same flex happening around the fuel tank, I suspect.

I agree, and it's this shear/twist i am trying to negate with the extra loads being applied by the V6.

During cornering or travel over very uneven surfaces, the flat rectangle formed by the chassis arms and sub-frame around the engine bay simply becomes a rhombus with slightly curvy sides or similar. By bracing across this rectangle, I think you create the possibility of gearing up these sway/shear/twist forces and focusing them on specific points in the ensemble. Although the spherical bearings will allow the bar to adopt small angular changes (left/right and up/down) and thus "go with the flow" of shear and twist, the distance between the anchor points must remain fixed as your tie rod won't compress/stretch by much if anything at all. I'm pretty sure that this will create a lever that will put more force into the joints near the rod ends.

So unless there is a very good reason for this brace to exist, I wouldn't have it in there.

Surely by that reasoning the lower wishbones are doing the same thing? How much movement of the chassis do you think exists? If it's more than a rose joint will allow then i'm in trouble! Honestly don't think this is even a factor although i appreciate your comments.

On the subject of reinstating the chopped sub-frame, I cannot quite get my head around the original cross section of the sub-frame - I think it's more-or-less triangular tube (like a long Toblerone bar), with each of the panels being stamped with a series of large holes to save weight. I believe that it is this tubular shape not only to provide effective resistance to compression/extension but also to provide a controlled resistance to twisting (torsional rigidity). So whatever you do in terms of reinforcing the remainder of the sub-frame, I think you should aim to reinstate the original torsional rigidity - it might be counter productive to substantially increase the rigidity as this may simply transfer load onto other, weaker, areas (e.g. the four bolts that hold the sub-frame to the chassis).

With more-or-less two out of three panels of the tube removed, the middle section of the sub-frame will now be much easier to twist as it's nearly just a flat sheet - it will twist further for the same applied force and so might now fatigue and then fail. You've added another tube left-right at the bottom and I'm sure this will go a long way towards reinstating the compression strength of the frame and probably acts like the fancy brace bars that the newer Exiges have fitted, but I don't see it doing much for the torsional rigidity. You either need to construct something approximately tubular from similar material to and with the same cross sectional area as the original triangular section, or you need to make something with a smaller cross section out of thicker material and then anchor the ends of that new structure back to where the wishbone mounts are. There's no point putting in a strong torsion beam across the middle of the sub-frame if the material on either end isn't strong enough to hang on to it - it will just fatigue/fail there instead.

Hence the need for the tie rod to brace the rear member and transfer some of it's load forward, i think you're over analysing this too much.

Your comments have all been taken onboard, feedback is important, it helps me make the correct decision regardless of my knowledge and experience, and is half the reason i post such detailed build threads.

Thanks Paul.

[campbell]

Paul - I hope you are continuing to get positive value out of the feedback here on SE. As I think you have already identified, we're a very different bunch from SELOC (which has its own shining stars btw so I am not slanging a different Lotus enthusiasts' faction, btw!). Here's hoping you will keep sharing, warts and all.

Great stuff. Makes swappping out my steering rack look like a quick snack!!

[Woodsport]

Absolutely, i value posts like that from Robin, it serves to highlight something i may have overlooked and helps create a better conversion.

We are still very much in the "warts" stage, but i'm hoping as i progress people get to see the overall package emerge.

[Woodsport]

Post #1 updated

[CubanGav]

Pic links not showing the image fella.

[Woodsport]

Apologies, it was pasted from Seloc and they have an image resize code built into the link, sorted now.

[CubanGav]

[Woodsport]

Post #1 updated

[ikarl]

Ok so you know i am detailing every part of this conversion but there is one bit that i am afraid i'm keeping to myself, and that is exactly what i am doing about the driveshafts. I will say they are Toyota based, and i am having a very nice supported shaft set up that gives an equal length halfshaft arrangement (something that's missing from the Elise K setup).

My reasons for witholding that part of the conversion is that i have put more research into that bit than any other part of the build and i have been all too often copied in the past by some plageristic twat who decides to offer the very same conversion using my build thread as a blueprint for "their" swap, so it's a little bit of protection on my part. Likewise the wiring will not be detailed either, it will just be "today i wired the whole thing up" wiring done, i hope you chaps understand my reasons for not detailing certain aspects, it's not funny watching all of your R&D work pop up on another garages menu.

seems fair enough to me!!

[Shug]

Did you drill a hole in the roll over bar stay for a water line? These are a weak point in the rollover protection at the best of times.