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Slingshot

3369 Views 24 Replies 7 Participants Last post by  Functional Artist
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
Tire Wheel Bicycle tire Vehicle Automotive tire

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Discussion Starter · #8 ·
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"?

Handwriting Rectangle Font Writing Parallel
Wood Bumper Electrical wiring Gas Audio equipment
Handwriting Rectangle Font Writing Parallel
Wood Bumper Electrical wiring Gas Audio equipment
Light Circuit component Passive circuit component Electronic engineering Electrical wiring
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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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Discussion Starter · #15 ·
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
Automotive tire Coil spring Suspension Gas Shock absorber
Automotive tire Motor vehicle Hood Coil Rim
 

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

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
View attachment 123851
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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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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Discussion Starter · #19 ·
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)
Circuit component Line Electrical wiring Hardware programmer Electronic component

Here is a comparison pic the (2) capacitators that were in this SC
Circuit component Gas Auto part Cylinder Metal

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" :unsure:
 

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Discussion Starter · #20 ·
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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