# Slingshot



## Functional Artist (Aug 8, 2016)

I'm building a go kart sized Slingshot
...or at least "my version" of one

Most of the "main frame" is now complete

I also, designed & built a chain drive steering rack
...just had an idea & ran with it 

Here is a demo video of the steering rack


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## Functional Artist (Aug 8, 2016)

Um...
Well, here is the video


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## Functional Artist (Aug 8, 2016)

Um...
Here is a drawing
...& some pics of the rack


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## Functional Artist (Aug 8, 2016)

Here is a quick video demonstrating the brake system, that I've installed on the Slingshot


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## ricbarbour (Jul 28, 2021)

Looks great---keep going!


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## Functional Artist (Aug 8, 2016)

ricbarbour said:


> Looks great---keep going!


Thanks!
...it's a "work in progress"  
Here is a quick "bench test", demonstration video, of the electrical propulsion system


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## Functional Artist (Aug 8, 2016)

I mounted most of the electrical system on the Slingshot 
...& then, did a system check video


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## Functional Artist (Aug 8, 2016)

I've been working on a fiberglass hood with dash board 





Now, I'm working on getting it wired up. 
It'll just have an On/Off switch
...a Reverse switch
...a PZEM-051 power meter PZEM-051 100A DC LCD Digital Panel Power Meter KWh Voltmeter Ammeter With Shunt | eBay
...& a small GPS speedometer Car Digital GPS Speedometer Odometer Head Up Display Overspeed Warning Alarm HUD | eBay

The power meter can be/is "powered" by up to 100V (this vehicle has a 48V system/~54V top charge)
...but, the speedometer can only be powered by ~5V (like from a USB port)

So to "power it", I got the bright idea to build a voltage divider (to reduce the 48V (pack voltage) down to 5V)

I did some calculations & it looked like, if I used a 2,000 Ohm 2w resistor for (R1) & a 240 Ohm 2w resistor for (R2) 
...the combination would reduce 48V down to ~5V

So, I assembled one then tested it with a MM
...& there was ~50V going in & 5V coming out
...but, when I connected the speedo it wouldn't/couldn't "power" it

So, I did some investigating, here are the specs
Specifications:
Model: C60S
Screen size: 3.0 inch
Product material: ABS+PC
Input voltage: DC 5V
Working temperature: -30~80°C
Input current: 50~120mA, can be powered by power banks (It has no built-in lithium battery and only saves the total mileage data when power is off, other data is not saved)
Satellite positioning speed: 1 to 3 minutes in open space; no satellite signal when indoor. (rainy days affect satellite positioning speed)

So, it looks like the input current is 50~120mA (.05A-.12A)
...& with the (2) 2W resistors that I used "in series" (I believe) the math would be (2W/48V = .04A)

Then, I tried using (2) of the 2,000 Ohm resistors & (2) of the 240 Ohm resistors (4W/48V = .08A)
...but, the speedo still wouldn't come on or "light up"

So then, I said, "what the hell" & tried a (4) 2K Ohm & (4) 240 Ohm set up
...& Boom! it came on or "fired up" 

But, now I've noticed that the (4) 2K resistors warm up & get kinda hot
...even when NOT "in use" or "powering" the speedo (parasitic draw?)

I understand resistors sometimes "warm up" during use
...but, how warm or hot is "too hot"?


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## Functional Artist (Aug 8, 2016)

I got the Slingshot mostly assembled
...& took it for a test ride


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## remy_martian (Feb 4, 2019)

Where'd you source the springs/struts?


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## Functional Artist (Aug 8, 2016)

The little coil-over shocks are for mountain bikes (~$10.00 ea.)
2Pcs 1200LBS/in MTB Bicycle Mountain Bike Rear Suspension Spring Shock Absorber | eBay

The rest is custom made


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## brian_ (Feb 7, 2017)

Functional Artist said:


> The little coil-over shocks are for mountain bikes (~$10.00 ea.)
> 2Pcs 1200LBS/in MTB Bicycle Mountain Bike Rear Suspension Spring Shock Absorber | eBay


That's an excellent source for this type of vehicle. For larger vehicles (still smaller than a typical car), coilover shock units from the rear suspensions of motorcycles are available from motorcycle salvage businesses.

That eBay listing specifies a spring rate (stiffness) of 1200 pounds per inch - if it is really that high, it's no wonder that cheap mountain bike suspensions don't actually do anything, as they are far too stiff to be useful. Hopefully other spring rates are available. For sensible rates, Google search for "mountain bike spring rate" to get a calculating tool; the first one that I checked is from Fox (a major shock manufacturer for both motor vehicles and bikes) and was specifically for a rear shock, but would work for the fronts as well.


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## remy_martian (Feb 4, 2019)

You don't understand, Brian....the spring rate is sized for Walmart/Costco fat-arsed Americans


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## brian_ (Feb 7, 2017)

remy_martian said:


> You don't understand, Brian....the spring rate is sized for Walmart/Costco fat-arsed Americans


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## Functional Artist (Aug 8, 2016)

brian_ said:


> That's an excellent source for this type of vehicle. For larger vehicles (still smaller than a typical car), coilover shock units from the rear suspensions of motorcycles are available from motorcycle salvage businesses.
> 
> That eBay listing specifies a spring rate (stiffness) of 1200 pounds per inch - if it is really that high, it's no wonder that cheap mountain bike suspensions don't actually do anything, as they are far too stiff to be useful. Hopefully other spring rates are available. For sensible rates, Google search for "mountain bike spring rate" to get a calculating tool; the first one that I checked is from Fox (a major shock manufacturer for both motor vehicles and bikes) and was specifically for a rear shock, but would work for the fronts as well.


Thanks for the info
...but, I don't think that 1,200 lb. spring rate is correct 

Chinese manufacturers seem to "flub" many specs so, why not spring rates too

The label on these springs have a company name
...but, they don't have any model number or part numbers 
...& they don't have any kind of rating on the spring itself, either

More info:
I can feel these "springs", that I have on the kart, "give" while riding
...& I can even get them to "give" a bit, by just applying some downward force, by hand (just leaning on it) just above each of the springs.

Also, I had just (1) of these same "springs" on the rear 
...& I could feel it "give" a lot (~1/2", easily) just from me getting in the kart (making the rear "sag" a bit)

So, I (over-compensated) by trying a "bigger shock" 100-150mm Bicycle MTB Rear Suspension Spring Shock Absorber 500lbs-1500lbs | eBay 
(Also, with no rate or part number on it)
It is (~1") longer but, it actually has a lower spring rate (supposedly 750 lbs.) 

Now, the ride "feels" about the same
...it's just that the rear rides a little higher 
...& stays more level, when I get in it


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## MesquiteTim (Sep 30, 2021)

You can spin the ride height ring to lower it or raise it. Once the shock is fully extended, it becomes preloaded. If you are really adventurous, you can cut the smaller spring and stack it on the shock with the larger spring. It becomes a tender spring. tender springs


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## brian_ (Feb 7, 2017)

Functional Artist said:


> Thanks for the info
> ...but, I don't think that 1,200 lb. spring rate is correct
> 
> Chinese manufacturers seem to "flub" many specs so, why not spring rates too
> ...


I suspect that you're right - the 1200 lb/in value is just nonsense.



Functional Artist said:


> More info:
> I can feel these "springs", that I have on the kart, "give" while riding
> ...& I can even get them to "give" a bit, by just applying some downward force, by hand (just leaning on it) just above each of the springs.
> 
> ...


The larger spring has much thicker wire and fewer free turns (four instead of five) of about the same coil diameter, so it should be much stiffer. You can test them for actual rate if you're curious, but it doesn't matter as long as they work. 

I think that you actually want it to sag when you get in, because the load carried is substantial compared to the vehicle weight. The shock should be significantly compressed (Fox recommends one-quarter of the stroke length) when you're in and stationary. Select the spring stiffness for best ride, and set the height with the preload adjuster.


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## ricbarbour (Jul 28, 2021)

FA:
Your vehicle looks great. Very solid design, wise to use off-shelf motorcycle and go-kart components. The shocks are fine--I've been experimenting with similar ones and they work very well for small autocycles. Can't really expect a "pillowy soft" ride on a small autocycle anyway, it might even cause handling problems.

The 1kW motor is a bit on the small side but I take it you weren't planning to race this or drive it on freeways. Top speed will likely be around 20-25 mph.

Today is like a "Golden Age" for small DIY EVs. Instead of being forced to scour junkyards or pay full retail for OEM motorcycle or auto components, there is a large "underground" of sellers on eBay, Aliexpress and Amazon with suspension, wheels, drivetrain parts, electrical and so on. Motors and digital controllers were usually very costly before 2010 but now are almost commodity items. So are lithium batteries. I suspect it's literally much cheaper/easier to scratchbuild an EV now than it EVER was in the past.


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## Functional Artist (Aug 8, 2016)

My Slingshot kart started "acting up" over the weekend.
It would still "go" buy it was very "lacidasical" (no spunk)

The power meter showed ~54V in the batt pack
...but, when the throttle was applied, there was no substantial change in the amp draw.

My first thought was the battery (Chinese Lithium battery with built-in BMS) 
(did a cell die or maybe the BMS wasn't letting the "power" come thru)
...but, I tested it on another kart & it seemed to work fine (had & delivered, plenty of power)

Second thought was maybe the motor got "overworked"
...but, it seemed to check out OK too

* About (1) month ago, I switched the driven sprocket from a 72T down to a 60T, to increase the top speed a bit.
(72T = ~16MPH & 60T = ~21MPH)





Third thought (I'm running out of thoughts) must be the speed controller

So, I switched the speed controller 
...& she's "Back in Business" (seems to be fine now)

Investigating, I opened up the SC, to see what I could see
...& it looks like a capacitator "blew out" (it even pretty much "desoldered itself from the board)








Here is a comparison pic the (2) capacitators that were in this SC








I checked it after several "runs" 
...& the SC would get warm 
...but, never really hot

Hmmm...was it overworked because of the gear ratio change?
* It never seemed to be "struggling or straining"


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## Functional Artist (Aug 8, 2016)

I was just thinking, could this be a "classic case" of capacitator "blow out" by the "in rush"?

These little 48V motors/controllers, that they sell for go karts, don't usually "mention" a pre-charge circuit.
(not, even the 60V models like I have on my Aerial Atom kart)

Is this what happens when a capacitator "blows out" from a huge "inrush" of current?

Here is a more close-up pic
...& pointing to where the "blown out" capacitator was mounted
Those (2) resistors (in the middle) looks like they may have "took a hit" too


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## ricbarbour (Jul 28, 2021)

Not your fault. Those Aliexpress-type Chinese PWM motor controllers are usually rather poor quality, made with the cheapest MOSFETs and capacitors available that will "meet the ratings". Meant for bicycles and apparently not meant for larger loads. "How dare you use them for anything larger." The ratings "look okay on paper" but the actual hardware does not really make it for long-term use. I see identical complaints on electric-bike forums. Exploded capacitors are probably due to back-EMF from the motor, and/or too much heat.

Either buy a better/larger controller, or upgrade the one you have with higher-voltage rated 105-degree-C capacitors and better driver MOSFETs. Also attach it to a larger heatsink or use a cooling fan. 48v equipment fans are easy to get.

I'll say it again: your build is EXCELLENT. You could make copies and sell as kits to others. Thanks to the modern attitude of "I don't need to build a kit or DIY anything, you can get anything you want online", there are very few options for this other than learning to weld and putting hundreds of hours into frame and body fabrication.


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## bchk (5 mo ago)

MesquiteTim said:


> You can spin the ride height ring to lower it or raise it. Once the shock is fully extended, it becomes preloaded. If you are really adventurous, you can cut the smaller spring and stack it on the shock with the larger spring. It becomes a tender spring. tender springs


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## bchk (5 mo ago)

Functional Artist said:


> Um...
> Here is a drawing
> ...& some pics of the rack
> View attachment 123348


looks great


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## Duncan (Dec 8, 2008)

I know its very late in this conversation but its worth measuring the springs and using one of the spring rate calculators on the internet - just use steel as the material


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## Functional Artist (Aug 8, 2016)

We had a couple of my karts out this weekend, zipping around & having some "fall fun"
...but we ended up "smoking" the motor on the Slingshot.

It still functions (after cooling down) I drove it around back, to put it away
...but, it definitely let some of its "magic smoke" out.






After burning out the speed controller, back in July, I switched the gear ratio back to the 7.2:1 
(I figured the lower GR would be "easier" on the drive system (motor, controller, connectors, wiring & batt pack)
&
We "usually" ride it around on paved roads
...so, I think the higher rolling resistance, of riding it "off road", was a bit too much for it.

* I also, added some Slingshot inspired graphics, to the "hood"

The "stripes & louvers" are "hand-cut" out of some Satin Black vinyl


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