Anyone seeing rust on V6 rear subframe?

[Eagle7]

I could be wrong, but wasn't there some coverage of why Lotus had to go with a steel rear subframe for Project Eagle (Evora) in the New Dawn video?

Perhaps there have been some (minor) changes, but I have assumed to date that the Emira and Evora rear V6 subframes are identical. If these galvanized steel subframes rust, I would have expected this issue to have impacted V6 Exiges and Evoras over the past decade as well.

I just finished watching A New Dawn again. Hadn't seen it since it first came out. With all that's happened since then, I really appreciate the effort that's gone into getting these cars to be what they are, and into production. There's a good section on the rear subframe with the V6 on it, and there's a lot to it. I can see why they didn't bother redesigning a new one. Wasn't really necessary for their goals and purpose for this car.

A carbon fiber rear hatch with thin, light-weight glass would be a good spot to save weight, and a good location to save it. That would help lower the COG of the car. That would be more cost-effective than a new subframe.

In days past I might be more enthusiastic about reducing weight, but right now, as I watched that video and look at the car, I'm not so gung ho to do much to it. More than anything, I want to just drive it. After the first 1,000 miles I expect I'll have a better idea what I think might need improving, if anything.

 

[Porter]

When was it originally designed? Metals and metallurgy techniques have come a long way since then. Lotus pre-Geely didn't have billions to throw at designing their cars. I believe they kept it for the reason you mentioned "tried and true design" and put the development budget into the custom double-wishbone suspension, body and interior which both the V6 and i4 would be able to use.

I don't know what the V6 subframe looks like by itself, so I can't make any kind of a guess as to what would be required to replace it with a high-strength aluminum version, but I don't doubt it could be done. Just depends on the cost and what the actual benefit would be as to whether it would be worth doing.

Aluminum metallurgy hasn't changed significantly in the last decade. I suspect the rear subframe for most Lotus cars has been steel because it's significantly stronger and more durable (toughness) compared to aluminum, at least within the weight and space constraints that they were working with.

Also I believe the objective was to make the rear subframe and drivetrain a single built assembly that could be attached to the body as a unit, and it probably needs to be quite strong without the cross-bracing of the bulkhead in order to carry the engine securely before it's attached to the car. Steel makes sense. To do the same job with aluminum it would probably need to be either cast or be quite thick, which in either case would have added weight.

 

[Nova]

The new aluminum subframe is casted - that's certainly something that only specialized tooling and processing can accomplish for a piece that large. It would allow the subframe to be much more rigid - to better handle the prodigious torque output of the M139 in full output configuration, perhaps?

 

[Porter]

The new aluminum subframe is casted - that's certainly something that only specialized tooling and processing can accomplish for a piece that large. It would allow the subframe to be much more rigid - to better handle the prodigious torque output of the M139 in full output configuration, perhaps?

Absolutely, I assumed that it would be a cast piece. The German brands mostly do cast aluminum for everything nowadays... engine cradles, subframes, and various other kinds of mounts.

 

[Pro-Eng]

Absolutely, I assumed that it would be a cast piece. The German brands mostly do cast aluminum for everything nowadays... engine cradles, subframes, and various other kinds of mounts.

The Steel vs ally rear subframe debate started with the Elise. Extensive testing was done with Ally on the Elise but cracks appeared on Durability testing and the decision was taken to go with Steel for that reason. It’s all documented in the Elise development documentary. Evora followed suit. Cast subframes are a different animal to fabricated/extruded items, particularly around the suspension pick-up points and are commonplace on exotics and runarounds alike as you say. No doubt Lotus balked at spending the money on cast tooling for Elise and Evora as both were developed on a shoestring. Replacing a steel subframe with Ally is hugely challenging because Ally is circa 3x less dense, but 3x weaker, so all the material thicknesses in high stress areas need to be trebled and generally there isn’t the room to do that. One thought from personal experience, the Evora/Emira rear subframe is a very well designed and efficient structure, so celebrate it! Steel is not a poor material choice made based only on cost, it is very often the best material for the job and imho, that applies in this case.

 

[Eagle7]

The Steel vs ally rear subframe debate started with the Elise. Extensive testing was done with Ally on the Elise but cracks appeared on Durability testing and the decision was taken to go with Steel for that reason. It’s all documented in the Elise development documentary. Evora followed suit. Cast subframes are a different animal to fabricated/extruded items, particularly around the suspension pick-up points and are commonplace on exotics and runarounds alike as you say. No doubt Lotus balked at spending the money on cast tooling for Elise and Evora as both were developed on a shoestring. Replacing a steel subframe with Ally is hugely challenging because Ally is circa 3x less dense, but 3x weaker, so all the material thicknesses in high stress areas need to be trebled and generally there isn’t the room to do that. One thought from personal experience, the Evora/Emira rear subframe is a very well designed and efficient structure, so celebrate it! Steel is not a poor material choice made based only on cost, it is very often the best material for the job and imho, that applies in this case.

Newer improvements in aluminum alloys and fabricating technologies have improved the viability of aluminum in those kinds of situations, but as you say, it's expensive to tool up for something like that. It just didn't make sense business-wise to spend that kind of money for an engine they originally thought they were only going to be able to use until 2026.

 

[EFE]

In answer to the original question, Yes I do have rust on the rear sun frame.
I also have galvanised paint and cement! I have poor welds and poor galvanising

 

[laupd]

I think 'cost', pure and simple. Cost, testing, certification. No good business reason for spending the money to develop a new subframe for the V6 for which there isn't a future. Their last ICE car, budget limitations. They went with what they already had for the V6. They went with an aluminum subframe for the i4 because they had to, there wasn't anything they had they could use. Note they chose aluminum for it, not steel. Weight is ALWAYS a thought concern for Lotus.

Yes the engineering, design and development teams know what they are doing, of course, but they also have to get approval for
proposed expenditures. There's always a balance between what they'd like to do, and what they can do within budget limitations. I personally think they did quite a bit to get the Emira to be what it is. The original proposed price points were fantastic. It's just incredibly unfortunate that the world had other things in the pipeline that almost stopped Lotus cold. If it wasn't for Geely's deep pockets, Lotus probably wouldn't have been able to survive. A lot of companies didn't, including much bigger ones.

Are you an engineer ? Sorry. To to be picky, but I am and I would make the same decision as they have. If you are, please explain why Alloy is better for this application.

 

[EFE]

In answer to the original question, Yes I do have rust on the rear sun frame.
I also have galvanised paint and cement! I have poor welds and poor galvanising

 

[laupd]

In answer to the original question, Yes I do have rust on the rear sun frame.
I also have galvanised paint and cement! I have poor welds and poor galvanising

I would monitor this carefully now you have pics and see how it develops. It doesnt look right, but the galv will generally show spots of rust Ona weld that are eating outwards and this doesn’t look like I’d expect rust to look, it’s more yellow than red.

Questions : Have you driven it on salty roads ? Have you asked your dealer about this ?

 

[Porter]

I would monitor this carefully now you have pics and see how it develops. It doesnt look right, but the galv will generally show spots of rust Ona weld that are eating outwards and this doesn’t look like I’d expect rust to look, it’s more yellow than red.

Questions : Have you driven it on salty roads ? Have you asked your dealer about this ?

Agreed

 

[EFE]

I would monitor this carefully now you have pics and see how it develops. It doesnt look right, but the galv will generally show spots of rust Ona weld that are eating outwards and this doesn’t look like I’d expect rust to look, it’s more yellow than red.

Questions : Have you driven it on salty roads ? Have you asked your dealer about this ?

I would think yellow colour is from the flux on the welds which should have been cleaned off before galvanizing.
I have never driven on salty roads and I did not paint the subframe with galvanized paint.
I have a long list for my dealer and this is not on it yet. I only looked because of this thread. The build quality on my car is quite appalling. I love the car but I am deeply saddened by the build quality and lack of support from Lotus customer care.

 

[ADC]

Some of those welds look like they got the apprentice to do them on Friday afternoon after a few pints for lunch, but presumably they are acceptably strong.

A quick search on the yellow staining turn up this:

Yellow staining on galvanized steel can be caused by chromate passivation. The yellowish stain left after passivation isn't a process defect, but it should not normally be used at such a strength to be green. Staining and discoloration sometimes takes place at or adjacent to welds as a result of weld seepage.

 

[laupd]

I would think yellow colour is from the flux on the welds which should have been cleaned off before galvanizing.
I have never driven on salty roads and I did not paint the subframe with galvanized paint.
I have a long list for my dealer and this is not on it yet. I only looked because of this thread. The build quality on my car is quite appalling. I love the car but I am deeply saddened by the build quality and lack of support from Lotus customer care.

Wondered that, but flux is generally washed off in anything and a galv bath would almost certainly get rid of it. I’d ssay monitor and notify the dealer that there may be an issue asap.

Weld quality : Its hand welding obv. Although not great, Ive seen similar on many luxury cars, it wouldn’t stop me buying.

Customer care however... Thats expected with any purchase of this value. I am lurking here considering a purchase to replace my M3 Heritage (that I have loved). BMW have been excellent on all aspects of customer care.

 

[EFE]

Wondered that, but flux is generally washed off in anything and a galv bath would almost certainly get rid of it. I’d ssay monitor and notify the dealer that there may be an issue asap.

Weld quality : Its hand welding obv. Although not great, Ive seen similar on many luxury cars, it wouldn’t stop me buying.

I am not suggesting anyone should be put off buying an Emira, I am merely responding to the OP. Having read the post I decided to investigate and report back.

 

[Porter]

Wondered that, but flux is generally washed off in anything and a galv bath would almost certainly get rid of it. I’d ssay monitor and notify the dealer that there may be an issue asap.

Weld quality : Its hand welding obv. Although not great, Ive seen similar on many luxury cars, it wouldn’t stop me buying.

Customer care however... Thats expected with any purchase of this value. I am lurking here considering a purchase to replace my M3 Heritage (that I have loved). BMW have been excellent on all aspects of customer care.

From what we are seeing, it doesn't look like they are doing a galv bath on these. Which shocks me. Looks like maybe they are welding together pre-galvanized sheet stock, and then spot-coating the weld locations? Really weird process choice, if so.

 

[EFE]

Another area of my sun frame

 

[EFE]

Whilst I was removing the undertray I a large amount of stones and gravel. There are quite a few places that are worth checking on. In particular there are two pockets without a drain that fill with stones. I have now filled them with foam so they shouldn’t fill up in the future.

 

[Eagle7]

Are you an engineer ? Sorry. To to be picky, but I am and I would make the same decision as they have. If you are, please explain why Alloy is better for this application.

No I'm not, and after re-watching the A New Dawn video where I could get a better look at the rear subframe, I can see why they chose steel. Practically speaking, it was the best choice for them.

Cast alloy aluminum provides some of the "add lightness" that Chapman was famous for. It's why Lotus spent the money to make cast alloy double-wishbones for the suspension, and most likely anything else they could cost-justify. The chassis itself is bonded aluminum. The Evija has an all-carbon fiber chassis from front to back, so that indicates where their minds are at for saving weight, albeit that's a rather expensive option which is partially why the Evija costs so much.

It will be interesting to see what they choose for the Type 135 chassis. Oh, and welcome to the forum!

 

[Andy H]

Pardon my ignorance in all matters welding but could this problem also be present on the I4 with it's aluminum subframe? Not sure if all these issues are specific to the steel process or could be equally replicated with aluminum.