# Building a Fiber body EV from scratch: Chassis design



## ftaffy (Mar 13, 2009)

Custom Chassis:
Kit car builders have been doing this for a long time, look at the Lotus 7 (Caterham) replicas that going around.
If you want to go down this route, get one of the lotus 7 replica books (there are 2 by Haynes that well worth a look) and Herb Adams Chassis Engineering book. You will get the idea what is needed from those.

If you already have a wheel base and suspension set up - look at including those as bolt on items.
Set the same geometry (wheel track, wheel base etc) as the original car but you can change the same around those points, will make life much easier. Designing suspension is an art that requires some knowledge in this area. The old VW body will go in the bin, i do not like the idea of hacking into the chassis of a current car as i have no idea the load paths that the designer had in mind. If u can find someone who did this before and it worked, copy them!

Post this all you need to know is how how measure, cut and weld essentially. I suggest you make a gate or a table before starting the chassis so you learn what works with welding, cutting and keeping things square.

In terms of ladder vs space frame... the debate goes on but i like space frames as you dont have this massive floor structure.

Cheers
Taffy


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## Bowser330 (Jun 15, 2008)

frk2 said:


> Dear all,
> I have truly fallen in love with the EV and a local manufacturer has asked me to prototype a fiber body EV which would be even lighter (and possibily cheaper in low volume production) to produce. If luck has it we might be making cheap EVs!!! how much they sell is a different problem altogether
> 
> My current car is the Suzuki Mehran which you can see at:
> ...


It's a great idea and if you have the skills to make the fiberglass bodies you are halfway there! I would definitely try to make the body as aerodynamic as possible for extra efficiency gains..examples: no front grill opening, no front bumper opening, underbody flat panel, rear wheel covers, smaller side mirrors)

as for the frame...

http://www.bolesmotorsports.com/html/tube.htm

http://www.mcsorley.net/locost/

www.factoryfive.com/hotrod/design/1.jpg


google search tube frame and check out the images...tons of custom frames which can take you to websites about how they made them etc..


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## frk2 (Jan 2, 2009)

The problem with tube frame design is that there is too much 'pipe' that gets in your way - not roomy enough. The fiberglass body kits that I am talking about are actually COMPLETE bodies - include the floor/tub , doors, etc. All I need is a simple ladder / box chassis (like the old jeeps / cars) on which to mount the body. Jeeps were made of rectangular beams of structural steel. I can do the same - but where to start?


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## frk2 (Jan 2, 2009)

ftaffy said:


> Custom Chassis:
> Kit car builders have been doing this for a long time, look at the Lotus 7 (Caterham) replicas that going around.
> If you want to go down this route, get one of the lotus 7 replica books (there are 2 by Haynes that well worth a look) and Herb Adams Chassis Engineering book. You will get the idea what is needed from those.
> Taffy


I checked out that book preview on google books- do you have it? Can you tell me if it will explain tubular frame and/or ladder/box chassis designs in detail in the 'frame design' section (it was not part of the preview). If it does I'll simply buy it and read it.

Thanks!!


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## Drew (Jul 26, 2009)

I posted a number of books in the Kit Car thread a little while ago, probably the best of those is the Race and Rally car source book by Allan Staniforth. 

BTW I would recommend against a ladder frame, they are heavy, lack torsional rigidity (the key to good handling from a chassis perspective) and are problematic when it comes to mounting anything structural which is not in the base plane (think upper seat belt mounts and the like.

Tubular space frames are about as good as it gets for somebody without the backing of a large international race team or a vehicle OEM, they are light, strong and rigid.

The next big problem you're going to have is that when you design your own chassis you have to design your own suspension as well, which is a big job in its self, I'd recommend a program called Susprog which allows proper analysis of suspension behaviour, but to understand what its talking about you'll have to do some more reading through similar books to the above reommended.

As a side note you won't be able to directly copy the suspension off your existing car, the suspension behaviour of a vehicle is intrinsically related to the cars mass, CoG and a large number of other interrelated parameters.


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## Dan Frederiksen (Jul 26, 2007)

take a look at the solectria sunrise. and the current adaptation EV2 by Lee Hart. it's a fiber monocoque.
and although it's likely more aggressive than what you had in mind, maybe look at this too www.zev.dk


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## frk2 (Jan 2, 2009)

@dan: the solectria EV2 looks amazing. I think I am more and more inclined towards the VW-beetle like RWD configuration with direct drive. What RPM would be required to say achieve 100Kmph? I think the VW is a 4.4 / 4.5 diff ?? 

if I go with smaller 12" tires that should reduce the overall gearing. 

According to my calculations. At 5500 rpm (with a 8" Netgain):

Axle revolutions = 5500 / 4.4 = 1250

distance travelled with 12" wheels / min = 1250 *2 * 3.14 * 12 * 0.0254 (converting to meters!)

1993 meters per minute = 120Kmph 

Is this calculation correct? This is way more than enough speed for this country 

How smart an idea is a direct drive? I'm thinking of making this car from scratch - using a transmission (in a FWD) and the CV joints up front really stack up the cost. I am thinking of a standard 'live axle' rear wheel drive with diff and VW-like simple front suspension. Dont know where i'll get the live axle from- most likely a old corolla.


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## Dan Frederiksen (Jul 26, 2007)

a wheel with 12 inch outer diameter is extremely small. a typical car tire is around 23 inches. if you have a rim that's called 12 inches then that's just the rim size and the tire would be bigger. you'd need to find the tire size to make accurate predictions on speed. let's say a small wheel is around 21 inches or 53.34 cm. circumference is 2*PI*radius or PI*0.5334m = 1.6757m.

at 5500rpm or 91.66Hz and 4.4 ratio that's 20.833Hz at the wheels or 34.91m/s or 125.68km/h

In general the formula is: Speed (km/h) = 3.6*Diameter*PI*motorrpm/(60*ratio) = 0.1885*Diameter*rpm/ratio (Diameter is in meter, rpm is motor rpm)


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## frk2 (Jan 2, 2009)

but its doable right? I mean my motor conks out past 2500rpm or something. Is there a HP-RPM curve for these motors at 120v ? I only have been able to get one for 72v from go-ev.com


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## ftaffy (Mar 13, 2009)

Welcome to our endless dramas of lack of info in planning and getting really close to the edge. 
I estimated that you can uprate the power graphs by 100/32 (that is peak 100hp/peak on the graph of 32hp) if going to the limits of the motor. So for a Warp 9 you will end up with 3000RPM=55KW. 
** This is just how i am doing it for estimating and looking at from a design point of view. 
I cant post pictures at work but can SS the chart of the scaling for 0-4000rpm if anyone would like. - Not saying its correct.

Re: The design book - It explains the basics but there is no how to guide for the simple reason that there are too many variations. It covers all aspects of chassis design including suspension, air flow etc.


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## Dan Frederiksen (Jul 26, 2007)

what do you mean it conks out?


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## frk2 (Jan 2, 2009)

my top speed in second gear is 35kmph. That roughly means my RPMS dont go over 2500/2800 - after that I have no power (even with the accelerator floored)


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## Dan Frederiksen (Jul 26, 2007)

ah, I see. that's voltage limited then. in electric motors there is a strict correlation between voltage and rpm. in other words at a given voltage and a given motor, even without load the motor will only spin up to a rate that corresponds to that voltage. if you double the voltage it will spin up to double rpm. what's your system voltage now?


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## frk2 (Jan 2, 2009)

also I am thinking of using a solid rear axle (far,far cheaper!) and sticking the motor directly to it. A huge problem with this design is that the motor will move with the suspension - is this a huge problem? 

I can always bolt the motor to the chassis and use a small driveshaft but I really dont wanna do that. Is sticking the motor into the live axle a bad idea?


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## frk2 (Jan 2, 2009)

a measly 72v. Hence am thinking of going up to 120v and see if I can make a rocket out of this car


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## Dan Frederiksen (Jul 26, 2007)

if it's a typical heavy forklift DC motor then that might be 50+kg and I think that's unfortunately too much to have directly coupled to a solid shaft. you can find a video showing a car going over copple stone road and see how rapidly the wheels bounce. I think it would put great load on the shaft and it might shake itself apart. the solectria sunrise was originally based on geo metro parts. maybe you could do the same and it wouldn't be terribly expensive.

72V is not weak in an absolute sense. the power of the car is the product of current and voltage, not voltage alone. it depends on the motor constant, often stated as rpm/V or V/rpm. let's say yours go 2500rpm at 72V so its constant would be around 35rpm/V. this constant can be engineered simply by having fewer windings with thicker wire or vice versa. you can think of it like gearing. if you gear up you will need fewer motor rpm to go the same speed but you'd need greater torque. volt is closely related to rpm and current is directly related to torque. so while your motor stops at 2500rpm with your 72V it would actually be possible to make the same size motor go 10000rpm at 36V. so you pick a motor and a system voltage to match reasonably with the target rpm you'd want. your motor would need something like 150V to go the 5500rpm. if that's more voltage than you'd like you could use a diff with maybe 1:4 ratio instead but keep in mind that the tougher the ratio the more torque it will need to deliver and you have to make sure it can do that even up hill


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## frk2 (Jan 2, 2009)

The EV2 uses a IRS from a ford thunderbird. That is simply unaffordable / impossible for me. Single gear transaxles are simply unavailable in the market. This leaves me with a choice of somehow altering/developing a live rear axle from a jeep or something like a suzuki jimny (4wd) car and attaching a small double cardan driveshaft with the motor.

Dont they make simple differentials with sprockets separately (like the final gear ones found inside manual transmissions, only I want it as a separate 'unit' outside) so I can plug in my CV axles to it?


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## ftaffy (Mar 13, 2009)

Why not keep the solid axle and use the drive shaft with the motor mounted in place with an adapter on it? Its what i am going to do. The suspension to motor length stays the same if the the suspension using trailing arms.

In terms of gearboxs, if you find a light weight, small (9" diameter) one that has a 1:1 and 1:0.75 ratio i would love to hear about it. - Its a on going problem with EV's. The gearbox's are like everything you can have ratios, weight/size and price but only 2 of them.

Diff ratios can be changed, but only within limits(eg say in a range of 1:2.77 and 1:3.4). So if you can find a diff that used in alot of hot rods or modded cars you could be lucky. Also with independent rear suspension setups the diff stays stationing and the CV joints connect to that. I have seen a build EV with direct drive to an independent rear diff.

I like that post Dan, very understandable. Top marks.


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## frk2 (Jan 2, 2009)

ftaffy - thats the only thing I can think of doing. I was really trying to avoid that driveshaft and everything since this is somewhat of a compact car!I guess I'll have to mid mount the motor and use a drive shaft. Would need to mount the motor at a extreme angle to get some clearance underneath. nasty.


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## Dan Frederiksen (Jul 26, 2007)

have you checked what the original sunrise used?
I'm not an expert on the mechanics layout of various car models but there should be an affordable rear wheel drive car you can steal from. the mazda miata is RWD for instance. parts from a salvage model NA can't be too expensive. and it's a relatively light car


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## Drew (Jul 26, 2009)

frk2 said:


> ftaffy - thats the only thing I can think of doing. I was really trying to avoid that driveshaft and everything since this is somewhat of a compact car!I guess I'll have to mid mount the motor and use a drive shaft. Would need to mount the motor at a extreme angle to get some clearance underneath. nasty.


Use a front wheel drive cars gearbox, that way you get a differential, gears, a compact layout, low losses due to inline gear trains etc etc. 

If you want to run very basic setup then you can simply run a front wheel drive gearbox where the rear axle is and then construct your own dedion rear axle, that way you've got all the problems of live rear axles minus the unsprung weight issue at least =)


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