[goldtop]

In doing a 'reality check' on my EV conversion idea, I've suddenly had a realisation: the lighter the car, the less likely it is that an EV conversion will fit within the limitation of the gross vehicle weight (as specified by the original manufacturer for the chassis).

For example, my weekend fun car is a Honda NSX and that has a kerb weight of 1370kg (3020 pounds) and a gross vehicle weight of 1520kg (3351 pounds). And that is accounted for by just driver + passenger - at 2 x 75kg (hey, no chubbies!)

So it occurs to me that pretty much any two-seater must be a poor choice for EV conversion. Unless using small battery pack and putting up with restricted range.

Or is there something I am missing?

 

[cemyilmaz]

I'm going to follow this thread.

 

[Duncan]

In doing a 'reality check' on my EV conversion idea, I've suddenly had a realisation: the lighter the car, the less likely it is that an EV conversion will fit within the limitation of the gross vehicle weight (as specified by the original manufacturer for the chassis).

For example, my weekend fun car is a Honda NSX and that has a kerb weight of 1370kg (3020 pounds) and a gross vehicle weight of 1520kg (3351 pounds). And that is accounted for by just driver + passenger - at 2 x 75kg (hey, no chubbies!)

So it occurs to me that pretty much any two-seater must be a poor choice for EV conversion. Unless using small battery pack and putting up with restricted range.

Or is there something I am missing?

There are the "legal limits" and the physical limits - one work around is to get your car approved with a small battery and then swap it for a larger one later - but you will need to make sure that your car is up to it - this may mean stiffer springs for instance

The general "rule" is that if you take out all of the dino burner stuff and replace with electric your car becomes lighter - then when you add batteries at about 60 miles range the weight is the same

 

[brian_]

It isn't so simple: if comparing a lighter car and a heavier car, the lighter car might work better despite having less payload capacity, if it is light enough that the required motor and battery are light enough.

A good measure might be payload as a fraction of curb weight. Any modern light commercial vehicle could carry a lot of battery and still have useful payload, but some pickup trucks would use up their entire payload just carrying the enormous (half ton plus) battery that would be needed for decent range.

The payload of the NSX seems unreasonably low, but I agree that a sports car is likely to be unsuitable in this respect, because they are optimized for performance rather than load capacity.

 

[brian_]

Another factor to consider is the weight of components which are coming out. If two vehicles weigh the same when stock, and have the same GVWR (so they have the same payload), they may still be in different situations after conversion. If you're pulling out a boat anchor of iron-block and iron-head inline-six and iron-cased automatic out, you can add back in a lot more battery than if you're only taking out an aluminum-intensive modern powertrain.

It's hard to get the data, but stripped weight (curb weight minus weight of components to be removed) and GVWR are the relevant values.

 

[goldtop]

Thanks, chaps. It is very hard to find real-world figures for this with the many EV conversions on YT (and here, too). Often there will be a weight given for the removed ICE/etc and then a comment about the weight of the electric motor. But the whole-car before/after weights are rarely quoted, sometimes there's a throwaway remark about weight at the end of the completion video being more or less the same as before.

But I've not seen a single one talk about GVWR and chassis strength. Lots of engineering talk about kW, torque, ratios, etc.

Interesting to hear that "about 60 miles of range" is a rule of thumb for battery+motor weight vs ICE weight. Fine for a local commuter, but not a sportscar.

I need to dive further into some of the Boxster and MX5 conversions. While an NSX conversion is not my plan, it's a starting point for me to think about layout, etc. The added complication is that it's all aluminium, including the suspension and - I suspect - this is less suitable for the 'just weld up some big-enough angle iron' approach to engineering.

 

[Duncan]

Thanks, chaps. It is very hard to find real-world figures for this with the many EV conversions on YT (and here, too). Often there will be a weight given for the removed ICE/etc and then a comment about the weight of the electric motor. But the whole-car before/after weights are rarely quoted, sometimes there's a throwaway remark about weight at the end of the completion video being more or less the same as before.

But I've not seen a single one talk about GVWR and chassis strength. Lots of engineering talk about kW, torque, ratios, etc.

Interesting to hear that "about 60 miles of range" is a rule of thumb for battery+motor weight vs ICE weight. Fine for a local commuter, but not a sportscar.

I need to dive further into some of the Boxster and MX5 conversions. While an NSX conversion is not my plan, it's a starting point for me to think about layout, etc. The added complication is that it's all aluminium, including the suspension and - I suspect - this is less suitable for the 'just weld up some big-enough angle iron' approach to engineering.

The 60 mile estimate is the reason that Tesla is so successful
A really good EV has to be designed from the bottom up as an EV
All of the other car companies have huge "parts bins" full of parts for petrol cars -
Tesla has the "advantage" that it does not have those bins so it has to design from scratch as an EV

Which is a roundabout way of saying that getting a lot of range out of a conversion is bloody difficult

Analyse your driving patterns - I found that I was either doing under 40 km in a day or over 150 km
So I went for the short range with the lighter (and cheaper) battery

 

[goldtop]

For me, I have a daily EV (Ioniq 38kWh on lease for the next two years) so ordinary driving is already covered. The appeal of converting a sportscar is an exciting (say 200bhp available) weekend car, and to make outings worthwhile I need 100+ miles. At some point there will be an Mazda MX5 EV, built from the ground up as an EV. I doubt that Mazda will be able to sell one with a 60 mile range, so it will be interesting to see how the construction accommodates the battery pack weight.

But right now, my surprise is that GVWR limitations don't seem to figure in many EV conversions at all. I want to drill down on it before starting the shopping list.

I haven't even gotten as far as working out what DVLA checks after an EV conversion. (must go looking for that thread next!)

 

[Duncan]

For me, I have a daily EV (Ioniq 38kWh on lease for the next two years) so ordinary driving is already covered. The appeal of converting a sportscar is an exciting (say 200bhp available) weekend car, and to make outings worthwhile I need 100+ miles. At some point there will be an Mazda MX5 EV, built from the ground up as an EV. I doubt that Mazda will be able to sell one with a 60 mile range, so it will be interesting to see how the construction accommodates the battery pack weight.

But right now, my surprise is that GVWR limitations don't seem to figure in many EV conversions at all. I want to drill down on it before starting the shopping list.

I haven't even gotten as far as working out what DVLA checks after an EV conversion. (must go looking for that thread next!)

If you are going to compete at events then you are going to risk smacking or otherwise breaking your car
Which means that trailering your competition car is not a bad idea

My "Device" is lighter than an MX5 with over 400 hp - its great fun on the track

If Mazda do make an electric MX5 - will do eventually - it will have no parts in common with the petrol one

 

[brian_]

If Mazda do make an electric MX5 - will do eventually - it will have no parts in common with the petrol one

Perhaps not, but there would be no reason for the suspension or steering to be different from the current car.

Look at the "designed from the ground up as an EV" Tesla models: their suspension, steering, and brakes are conventional current automotive designs. Some steering and brake components (steering rack, brake master cylinder with booster, calipers, etc) are exactly the same as used in many unrelated engine-driven models.

Of course the specific components will be matched to the (likely higher) weight of the EV, but that's basically just spring rates, damper tuning, and maybe bushings. The previous generation MX-5/Miata used the same chassis components as the RX-8, which was bigger and had an unrelated engine.

 

[Duncan]

Perhaps not, but there would be no reason for the suspension or steering to be different from the current car.

Look at the "designed from the ground up as an EV" Tesla models: their suspension, steering, and brakes are conventional current automotive designs. Some steering and brake components (steering rack, brake master cylinder with booster, calipers, etc) are exactly the same as used in many unrelated engine-driven models.

Of course the specific components will be matched to the (likely higher) weight of the EV, but that's basically just spring rates, damper tuning, and maybe bushings. The previous generation MX-5/Miata used the same chassis components as the RX-8, which was bigger and had an unrelated engine.

Disagree ENTIRELY - the issue is not the function which is the same but the packaging which is completely different
The MX5 front and rear suspension is packaged around a diff and at the front an engine - for an EV version the core will be a transversely mounted motor and final drive - that and the battery will drive a completely different layout

 

[brian_]

Disagree ENTIRELY - the issue is not the function which is the same but the packaging which is completely different
The MX5 front and rear suspension is packaged around a diff and at the front an engine - for an EV version the core will be a transversely mounted motor and final drive - that and the battery will drive a completely different layout

Except that any reasonable MX-5 EV will be rear wheel drive, so the same front suspension will still work well (because it won't need to accomodate any powertrain component), just as the same basic design (double A-arms) works at the front of every Tesla. And the same rear suspension will still work well, just as the same basic design (five link) works at the rear of the Tesla Model 3. Some independent rear suspensions used with final drive units (diffs) don't have room to fit a wider transverse electric drive unit, but the MX-5 has quite a bit of width.

It's mostly the body structure which varies between a car with an engine and an EV, due to the need to accommodate the battery in the EV, and the need to accommodate the engine and sometimes driveshaft in the car with an engine. Of course the details of component mounts (including subframes) vary to suit the components.

 

[Duncan]

Except that any reasonable MX-5 EV will be rear wheel drive, so the same front suspension will still work well (because it won't need to accomodate any powertrain component), just as the same basic design (double A-arms) works at the front of every Tesla. And the same rear suspension will still work well, just as the same basic design (five link) works at the rear of the Tesla Model 3. Some independent rear suspensions used with final drive units (diffs) don't have room to fit a wider transverse electric drive unit, but the MX-5 has quite a bit of width.

It's mostly the body structure which varies between a car with an engine and an EV, due to the need to accommodate the battery in the EV, and the need to accommodate the engine and sometimes driveshaft in the car with an engine. Of course the details of component mounts (including subframes) vary to suit the components.

No you get it wrong again
the front suspension is currently configured BECAUSE there is an engine there - take away that constraint and a different design becomes better
The requirement for the batteries is DIFFICULT - you only get a good car when you start again

Its like converting a Stanly Steamer to petrol

Mazda could make some half arsed conversion - but I think they are far far too smart to fall into that hole

 

[brian_]

... the front suspension is currently configured BECAUSE there is an engine there - take away that constraint and a different design becomes better...

Nope. Car with transverse engines in the front typically have front struts, because the upper arm of a double A-arm configuration would be too far inboard for the engine to fit - the engine matters in that case. Everything else is designed essentially without regard to the engine because there's no conflict, which is why the front suspensions of mid-engine cars (and the world's one remaining rear-engine car) are of the same design as the front suspensions of longitudinal front-engine cars... and designed-for-the-purpose EVs. A hint: they're all double A-arm (some with two lower links acting as a virtual A-arm).

Again, if Tesla is the best-known example of "designed from the bottom up as an EV", Tesla suspension designs illustrate that not having an engine up front doesn't change the most desirable suspension. All Tesla front suspensions are the same as many suspensions found in vehicles with front engines; all Tesla rear suspensions are the same as suspensions found in many rear-wheel-drive engine-driven vehicles.

The current MX-5 (Miata) front suspension is a short hub carrier double A-arm design, like typical mid-engine (no engine constraint on the front suspension) sports cars. The current MX-5 (Miata) rear suspension (which it has had since 2005) is functionally the same as the Tesla Model 3's rear suspension. The suspension of this car is not constrained by the powertrain, so a car for the same purpose with an electric powertrain would not need to have a different suspension.

 

[Duncan]

Nope. Car with transverse engines in the front typically have front struts, because the upper arm of a double A-arm configuration would be too far inboard for the engine to fit - the engine matters in that case. Everything else is designed essentially without regard to the engine because there's no conflict, which is why the front suspensions of mid-engine cars (and the world's one remaining rear-engine car) are of the same design as the front suspensions of longitudinal front-engine cars... and designed-for-the-purpose EVs. A hint: they're all double A-arm (some with two lower links acting as a virtual A-arm).

Again, if Tesla is the best-known example of "designed from the bottom up as an EV", Tesla suspension designs illustrate that not having an engine up front doesn't change the most desirable suspension. All Tesla front suspensions are the same as many suspensions found in vehicles with front engines; all Tesla rear suspensions are the same as suspensions found in many rear-wheel-drive engine-driven vehicles.

The current MX-5 (Miata) front suspension is a short hub carrier double A-arm design, like typical mid-engine (no engine constraint on the front suspension) sports cars. The current MX-5 (Miata) rear suspension (which it has had since 2005) is functionally the same as the Tesla Model 3's rear suspension. The suspension of this car is not constrained by the powertrain, so a car for the same purpose with an electric powertrain would not need to have a different suspension.

The fact that they are "functionally the same" is simply NOT relevant!!
The fact is that the mounting points and all of the rest is set by the requirements of the drive hardware
With an EV that is DIFFERENT
and will drive different positions - positions that cannot be used on a dino burner but are optimal with an EV

If Mazda try and use the old bits they will end up with a half arsed conversion and throw away a lot of the benefits

What they are much more likely to do is to design a new "platform" that will be used for the volume cars and THEN adapted for a sports car

 

[goldtop]

Apologies for mentioning the MX5! When Mazda gets around to it, the first q is whether they move the engine or keep the FR layout and pedigree. Moving the engine to the rear takes it into the MR2/MR-S territory, and the gentlemans' agreement between Japanese car manufacturers probably means that this won't happen.

Anyhoo, what Mazda does with the car that reinvented the whole genre will be interesting, notwithstanding the suspension choice.

As to my sportscar thoughts - I've got no interest in racing. This is a 'high days and holidays' road car for fun on twisty B roads. Being limited to a 30 mile radius around my home would be ... irritating, I think.

So there's the question as to whether adjusting spring rates/braking/etc is enough to address the potential extra weight. And also whatever engineering documentation/testing is required for registering the change of engine/fuel and whether it gets weighed again at that point.

 

[brian_]

Apologies for mentioning the MX5! When Mazda gets around to it, the first q is whether they move the engine or keep the FR layout and pedigree.

No serious production car would put an electric motor in the front to drive the rear wheels. There's no reason to do it, and no reason to accommodate the resulting shaft down the middle of the car. In an EV, the largest mass is the battery - that's what is critical to mass distribution, not the motor. To keep the character of the car, it would need to keep roughly equal front:rear weight distribution and rear wheel drive, not the location of the motor.

Mass distribution, not just the change (normally increase) in vehicle mass, is a major factor in the changes needed to properly accommodate a conversion.

Most builders don't worry about this much, because they are not seriously interested in handling (or stability, or durability)... but of course there are exceptions. With rare exceptions, "performance" in EV conversions usually means just acceleration, or in some cases the ability to burn rubber.

 

[Smiley van Iersel]

The Morris minor truck we are converting is only about 750kg total, we intend on using lithium ion phosphate batteries at about 22kw, by the time we take out the original motor and fuel tank, and replace with electric components and batteries, we add about 80kg to the total weight of the car, weight of a passenger. 22kw on such a light car should comfortably do 100 miles+. It's the batteries that are the major weight, but you are also taking out a large weight of a ice engine. Yes you could go for a heavier car, but the range will be reduced, you need a larger motor etc. It's a balancing act. And as of electric motor in the front driving the rear wheels, that is exactly what we are doing, we are leaving the gearbox and drive shaft in. Since we are using only a 60kw peak, 22kw nominal motor, we will need the manual gear box, and we like the idea of a manual still.

 

[brian_]

And as of electric motor in the front driving the rear wheels, that is exactly what we are doing, we are leaving the gearbox and drive shaft in.

Lots of conversions do this; it can make sense. I was referring to production vehicles: no one serious about production would design a vehicle this way.

Also, your battery would presumably have an energy storage capacity of 22 kWh (kilowatt-hours), not a power rating of 22 kW (kilowatts).

 

[Smiley van Iersel]

Lots of conversions do this; it can make sense. I was referring to production vehicles: no one serious about production would design a vehicle this way.

Also, your battery would presumably have an energy storage capacity of 22 kWh (kilowatt-hours), not a power rating of 22 kW (kilowatts).

Fair point, and yes, 22kWh. (just lazy typing)