# Help Required MX5 Conversion



## Plymouth60 (Sep 4, 2012)

Hi All,
I am currently trying to get my 1993 Mazda MX5 conversion under way and am having trouble with the GVM. The GVM of my car is listed on my registration papers as 1050kg. The car with hard top on and 1/4 of a tank of gas weighs 1017kg. With a full tank it would weigh close to the GVM of 1050kg. I have been talking to the EV certifier in Dunedin about it and he requires me to remain under the GVM. It seems that the GVM of my car is obviously incorrect on the registration papers, but that isn't necessarily a problem if I can find out what it should be.

My desired design would be an AC50 motor and Curtis controller, with 36 CA180FI cells. The total battery weight is 205kg. I would like to retain the transmission as I live on a hill. My estimated final weight is 1149kg + 2 people at 80kg = 1309kg

I have approached Mazda here in Christchurch and Head office in Auckland, but all they have been able to supply is the curb weight. I have also done extensive internet searches for the gross weight, but not found any information for the series 1 (NA) model. 
I found some Japanese documents that related to a later model, that gave a gross vehicle weight of 110kg greater than the curb weight. I suspect that if I ever find the data for my model it would be similar. 

Has anyone converted a MX5 in New Zealand?

Does anyone have any suggestions or experience working through the increase in weight that will likely put me over the GVM?

Your help would be much appreciated.
John


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## Nick Smith (Oct 26, 2007)

This issue seems to be causing a few concerns all round. I have noticed much confusion regarding GVM and kerb weights and no consistent numbers for any. The Japanese (being a smaller people) tend to rate the occupants at 55kg only. Having said that, Wikipedia seems to indicate that the kerb weight of the car varies according to the option packages and can be as high as 980kg. Adding 2 passengers brings this up to 1090kg which, as you state, is actually in excess of the number on your registration, and does not allow for even a handbag or laptop for luggage in the trunk. Based on the suspension, tyres, and chassis design of the car, I would expect the vehicle to be good for about 1450kg all up with no effort. You MAY find a label inside your door frame that might indicate otherwise if you are lucky. I note that the current model has a kerb weight of 1167/1177 (man/auto) for the roadster model (hardtop).

I honestly feel that the registration papers are in error! These specs for a Yaris would have to give you some hope.
http://www.toyota.com.au/yaris/spec...manual?WT.ac=VHYarisRangeSpecsHatYR3DoorSpecs



Albo2 (on the forums here) if just finishing up his conversion with much the same setup that you propose (only 32 cells though) and his vehicle weight is looking pretty similar to the original, although the aircon, power steering have also been removed and he is leaving the charger off-board. My suggestion would be to get on one of the MX5/Miata forums and see who can help, or even write to Mazda Japan for assistance. You were wise to contact the certifier before you started. Please keep us all updated...


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## Ziggythewiz (May 16, 2010)

Plymouth60 said:


> Does anyone have any suggestions or experience working through the increase in weight that will likely put me over the GVM?


Haven't had an issue but out of paranoia I've removed half the pack and replaced the back seat to take it in for inspection, just in case they're out to get me.


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## Nick Smith (Oct 26, 2007)

Been doing some looking myself. It would seem that there should be a metal tag in the door sill somewhere with the max GVM listed. Looks like you will be in the region of 1275kg. Let us know what you find.

Cheers, Nick


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## morse90 (Mar 23, 2010)

GVM seems to be very tricky here in New Zealand. I know as I have just been through the process with a Toyota Sera Nick worked on with me. My car was an import and I was told by certifier and Toyota the GVM was based on 55kg person. The trick is though in New Zealand my car was going to be sold in NZ at one point which means it must have been able to hold 80 kg per person per seat but could not find a the specs anywhere. My conversion was based on an Australian conversion where the final curb weight was 1115kg at end of conversion which is 5 kg of the GVM. In Australia apparently all converters needed to do was have car checked over by engineer before going to certifier to get the ok to carry addition weight.


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## Plymouth60 (Sep 4, 2012)

I have yet to get an answer back from Mazda Japan, but I did find some figures from a US website. GVM 1190 kg for my 1.6l model & 1230 kg for the 1.8l.
Option 1 
Remove hardtop
AC 50 Motor coupled to the transmission
205 kg batteries
curb weight 1151kg
2 passengers 160kg
Final Weight 1311kg (121kg over the GVM)

I came up with an alternate option based on the soon to be released AC75 motor.
Option 2
Remove hardtop
AC 75 Motor direct drive to the diff
205 kg batteries
curb weight 1091kg
2 passengers 160kg
Final Weight 1251kg (61kg over the GVM)
Excluding losses in the diff and tyres it should be capable of a 17.5 degree incline. Not enough to go up baldwin street (steepest street in the world), but should be acceptable.

I have researched other MX5s on the net where curb weights were given after conversion. The values excluding passengers are below:
1157kg
1383kg
1133kg
1496kg
1240kg
1451kg
1360kg
1315kg
1297kg
1100kg

I did have a play with the numbers of a number of other vehicles, like the yaris above, but you need to allow for passengers as well. The yaris is a 5 seater, therefore you need to add 5 x 80 = 400kg which puts it very close to the limit again without any additional weight. It would work if you made it a 2 seater instead. Unfortunately with the MX5 I only have 2 seats to start with .


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## Duncan (Dec 8, 2008)

Hi Plymouth
How much weight are you removing with the IC junk?

After you have ditched the fuel system, radiator, engine

Then you can start 
Removing carpets/sound proofing
Remove the seats and get lightweight ones - (seats can be very heavy)
Spare wheel - use a can of instant fill - or just phone a friend

Air conditioning - you live in South Island!! - with a convertible!! - dump it

Next - how much range do you need? - have a look at your driving patterns
is it worth while having an expensive heavy battery for a trip you make once a month??

My battery pack is 50Kg - bit small 
but a 100Kg pack may meet 95% of your needs
Maybe the rest of the batteries go on a small trailer - just for when you do need more miles


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## Plymouth60 (Sep 4, 2012)

I have an update on the GVM issue. The value I was using form the US website appears to be incorrect. It gave a curb weight of 950kg as well as the GVM. There is no way the curb weight could be correct, so am now going to rely on information from Mazda only.
Mada here in Christchurch and head office in Auckland were able to give me a curb weight of 983kg. They were unable to provide a GVM. The 983kg curb weight would be correct for the New Zealand new models which didn't have hard tops, power steering and AC.
I had an email back from Mazda Japan last night stating the GVWR for my Chassis number is 1060kg. This is about 10kg over the curb weight of my car with all its options, new tyres, and a full tank of fuel. 
I am now going to assume that there is no GVM for this vehicle, as per Mazda, and talk to the certifier again.


For reference:
The *gross vehicle weight rating* (also *gross vehicle mass*, *GVWR*, *GVM*) is the maximum operating weight/mass of a vehicle as specified by the manufacturerincluding the vehicle's chassis, body, engine, engine fluids, fuel, accessories, driver, passengers and cargo but excluding that of any trailers


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## John (Sep 11, 2007)

Some vehicle manufacturers seem to take the curb weight of the vehicle and add a passenger allowance per seat to get the GVWR rather than this rating giving a true indication of what weight shouldn't be exceeded for that chassis. Less seats automatically means a lower margin or payload between curb and GVWR. To my mind the GVWR shouldn't vary with the curb weight but with changes to the underpinnings of the vehicle which seems to be the way it worked with my Hyundai Accent. Various models and configurations all had the same GVWR because they all shared the same floor pan with the same brakes, hubs, wheels, and structural elements inspite of variations in curb weight. This said do the 1.8l and 1.6l MX5 share the same brakes. I'm picking that the 1.8 is actually a latter model. http://www.internetautoguide.com/13-12-1995-50-912/1995-mazda-miata-specifications.html This web site gives the GVWR for the 95 model 1.8l as 1225kg. You could look for the heaviest configuration of the vehicle with the same underpinnings and try and find out the GVM for it and see if your certifier would buy the logic for using that instead of strictly as originally configured.


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## John (Sep 11, 2007)

PS. The 1st generation were produced from 1989 to 1997 and the 2nd generation from 1998 to 2005. The 1.6 gave way to the 1.8 at the end of 1993 and the beginning of 1994. Unfortunately at this time they also added extra bracing to the car. To quote Wikipedia "The chassis was substantially braced to meet new side-impact standards, most visibly by adding a "track bar" between the seatbelt towers inside the car, but also to the front and rear subframes." This could have changed the Gross load carrying capacity of the frame from model year 1994 on though it doesn't sound like that is why the bracing was added. The underpinnings should be the same from 1989 to 1993.


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## albo2 (Oct 4, 2011)

the AC50 is designed to run on 90Volts, just Sayin


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## albo2 (Oct 4, 2011)

I contacted Mazda and have been supplied a sheet mostly in Japanese, but basically all they have done is add 110kg to the curb weight and spread it between the 2 axles so we are left with having to actually be lighter than the car was before the conversion, this seems a little mental to me but we'll see how we go I'm going to weigh mine tomorrow, I think I will be a bit lighter I took a heap of heavy smelly stuff out of the car, like I said it will still run on 96 volts that only gives me 2 batteries to play with at 5.79kg each.


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## Plymouth60 (Sep 4, 2012)

Thanks for the help everyone.

I spoke to the certifier today. He is happy to accept the weight of the vehicle from Mazda Japan as the curb weight and that they didn't have a GVM for that model.
This means we will estimate what the GVM would be, probably by comparing the curb and GVM of a future model.

What this means for me is; by reducing my batteries slightly I can meet the current curb weight with the AC75, but will not need to remove so many dependent on the estimated GVM. 
Alternatively I could retain the transmission and run the AC50. I will need to get a more accurate weight for the transmission and work harder at removing excess weight before I can be sure of this solution, as overall it's a bit heavier.

Overjoyed at being able to make this project work. Can start ordering parts soon.


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## Plymouth60 (Sep 4, 2012)

Hi Albo2,

After a bit of investigation, I discovered the 110kg on a number of vehicles comes from the Japanese practice of allowing 55kg per person vs our 80kg.

Which model are you working on? It would be great to hear about your progress.

Regards
John


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## albo2 (Oct 4, 2011)

Plymouth60 said:


> Hi Albo2,
> 
> After a bit of investigation, I discovered the 110kg on a number of vehicles comes from the Japanese practice of allowing 55kg per person vs our 80kg.
> 
> ...


Hi John I have nearly finished a 1990 you can check it out just go to all builds mine is titled "finnaly starting my mx5" 
good news just adding 55kg to the curb weight is a crazy way of finding how much the car can safely carry our cars are made of the same metal other small jap cars that have 4 seats and we have 4 wheel disc brakes.
could you send what info you have as it looks like im having to actually be 50 kg lighter than curb. im going thursday lunchtime to get checked my email is [email protected]


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## Richard Wood (Jun 27, 2008)

Hi
I'm keen to understand the suggestion that an AC75 would any good for direct drive. A look at the charts would suggest that its sweet spot for torque runs out at just 2000 rpm, and only holds up close to the AC50 at 3000 rpm. The AC50 has better torque out to 6000 rpm (I realise these charts are at slightly different voltages).
http://hpevs.com/Site/images/jpeg/power-charts/pdf/ac50_102v_650a_metric.pdf
http://hpevs.com/Site/images/jpeg/power-charts/pdf/AC75_96V_650 Metric.pdf
A direct drive motor will need higher range of RPM capability, right? 
The AC75 to my uneducated eye looks like a motor designed to work with a regular transmission for heavier cars than the AC50 has been used for. It seems to more closely match a petrol motor, I wonder if it would even work well with an existing automatic tranny's shift points. Looks like a competitor to some water cooled solutions that are around for the 1200kg+ market. Being air cooled means weight advantage in that market.
Happy to be told I've got it all wrong of course.


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## Plymouth60 (Sep 4, 2012)

Hi Richard,
I played with the idea of direct driving the mx5 with the AC75. I live on a hill and eventually came to the conclusion that there wasn't enough torque to deal with a section of the road to my house. There is an aftermarket diff ratio available that would have done the trick, but with an adverse affect on higher speed acceleration and top speed. If a new controller with a higher voltage comes out for this motor, I think it would be an excellent candidate for direct driving a small car (it would maintain the torque to a higher rpm). By the way I had an email from HPEVs a couple of weeks ago to say that the AC75 was now available.
I have been investigating 2 speed manual transmissions and there are some interesting speedway transmissions with ratios of 1:1 and 2:1. These would be ideal to reduce weight and simplify operation, but they seem to all have straight cut gears rather than helical, so would be very noisy in low ratio.
My final decision is to retain the standard transmission and go with the AC50.
Cheers
John


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## Richard Wood (Jun 27, 2008)

I can see your point in a direct drive scenario. Working on the basis of some sort of transmission for my car at this point I'm seeing the AC75 in a different light. I don't need to go above 4000 rpm as at 1-1 through the gearbox I'd be just over our 100km/h speed limit and not poorly off compared to the AC50. At 3000 rpm I'd be doing 80, where the torque looks about the same on either motor. Leading up to 80km/h is where I need the power and that is where the AC75 looks superior.
Now I just need to determine whether I would be be better off going for a water-cooled system instead with the added weight and less space - it's going to be tight in the front of my car. This may come down to some finer calculations as to whether the AC75 will get my car up the hill I need it to at the speed I want.
Richard


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