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PostPosted: October 5, 2017, 7:21 am 
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We are Slotus!
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Location: Tallahassee, FL (The Center of the Known Universe)
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You might be wondering what I was smoking when I designed the Tiger 700

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Before I started the design, I read a book called "H-Point: The Fundamentals of Car Design and Packaging.

Read it, then smoked it, right? :rofl:

OK, just kiddin'... Interesting design. A man I know here in TLH is fascinated with the Messerschmitt micro cars. He has several of them, along with some other post-war designs that are similar. They're kinda cool, in a wonky-wacky way.

It'll be interesting to watch this one unfold. Speaking of unfolding, tell Maple I said "Hi!"

:cheers:
JDK

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PostPosted: October 24, 2017, 3:52 pm 
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Now that I have my exterior design model complete, its time to start on the suspension.

Design Approach
I will be using the suspension from a Yamaha Raptor 700.
  • The rear suspension should require very little modification. I will start by simply angling the entire Raptor chassis down in the front to pull out some of the angle in the rear trailing arm. I plan to leave a little bit of angle in the trailing arm to provide some anti-squat characteristics. The driver will be placed well forward in the chassis, so the rear spring load should be about right.
  • The front suspension will use many of the parts from the Raptor, such as the spindles, brakes, ball joints, A-arms, and steering linkage. However, the Raptor suspension will be completely redesigned to provide good road manners.

Tires and Wheels

ATV Wheels and Tires
Suspension experts always say you should start with the tires and work your way back to the wheels, suspension, and then chassis. The Raptor comes with knobby dirt tires on very small 9" and 10" rims. There are ATV street tires available, but the selection is very limited. I wanted to use a DOT auto tire on a 14" to 15" wheel. There are aftermarket ATV 14" & 15" wheels. So I called tech support at one of the major ATV wheel manufacturers (IRC). They told me that a DOT tire will fit the ATV 14" & 15" wheels, but flange that the tire bead seals against is not as wide as a auto rim, so the tire bead could slip off on hard cornering :ack:

That news sent me down the path of using an auto rim and an adapter to fit the Raptor bolt pattern.

Auto Wheels
Since the overall vehicle will be very light, it is even more important to keep your unsuspended and rotating weight to a minimum. I looked many different lightweight racing wheel sets, but they were thousands of US dollars. Then I discovered the Mazda Miata "Hollow Spoke" wheel. 1994-1997 Miatas came with the hollow spoke wheel, and they only weigh about 4.98kg (11lbs)! There are lots of Hollow Spoke wheel sets on ebay at very reasonable prices, so I bought a set. After a good cleaning I mounted the lightest tires I could find from Khumo. This wheel and tire combination weighs only about 7.71kg (17lbs) in the front and 10.43kg (23lbs) in the rear.
Image

Adapters
After an exhaustive search, there is really only one supplier of ATV to Miata wheel adapters "Adapterking.com". The adapters they sent me were one-offs. The Rear adapters fit well, but the front adapters did not. The Raptor front hubs has a wheel centering tab that required some additional machining to fit. See attached photo of hub. All of the adapters were very heavy, so I decided to do even more machining to lighten them.
  • Rear Original: 1.79kg Lightened: 1.16kg Difference: 0.63kg (35% lighter)
  • Front Original: 2.45kg Lightened: 1.36kg Difference: 1.09kg (44% lighter)

Note: I just checked and Adapterking.com webste no longer online.

Wheel backspacing is important when choosing the wheel/adapter combination. I will cover backspacing in my next post as part of the suspension geometry design.


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PostPosted: October 25, 2017, 12:36 pm 
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Is this for street use? If so what sort of track will you use?


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PostPosted: October 25, 2017, 2:26 pm 
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Yes, It will be for street use.

If you are asking about track width, the rear will remain 950mm, and the Front will be 1380mm.

The wheelbase will be about 2300mm.


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PostPosted: November 11, 2017, 2:54 pm 
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Before choosing the wheels and adapters I reverse engineered the Raptor 700 front suspension geometry. I took as precise measurements as possible and plugged it into the Vsusp.com modeling tool.
To see the Raptor 700 front suspension geometry model -> Click here

One of the challenges is to use as much of the donor suspension as possible to reduce costs. However, the geometry of the donor is tuned to off-road use and has poor handling characteristics on the street.

For the front suspension,
  • You should always start the re-engineering at where the rubber hits the road. I swapped from the low pressure tires on 10" rims and went to a 185/70-14 DOT auto tire.
  • The Raptor has a scrub radius of 11mm. I didn't want to make this any larger so I chose a wheel/adapter combination that reduces the scrub radius to 1.5mm. This scrub radius is so small, that it may induce some wheel wobble on high speed straight-aways. If so I can add a thin wheel shim to increase the scrub angle.
  • The Raptor has 285mm of level wheel travel and a Roll-Center of 154mm. In addition, there is almost no outside wheel camber inclination to improve the tire contact patch on hard turns. These characteristics are all tuned for an off-road vehicle with narrow tires that have a rounded profile (vs flat tread contact patch). The chassis I'm building will have more width between the A-arms, so I can cut the Raptor's A-arms down to achieve better manners on the street. The result will be lower ground clearance (125mm), lower Roll-Center (63mm), and 2 degrees positive camber on the outside wheel during hard turns (@ 4.5 degree lean).
  • There is almost no anti-dive in the Raptor geometry. This causes the soft long travel front shocks to nearly bottom-out on hard stops. I will add some forward angle to the A-arms to reduce dive.

For the rear suspension, I will keep the donor's solid axle.
  • I swapped the low pressure tires on 9" rims for 195/70-14 on the rear. These tires have a flat tread profile and are narrower, but are about 30mm taller.
  • I swapped axle hubs for a set that add 95mm to the track width.
  • The result is that the outside edge of the rear tires are about the same width as the original donor wheel/tire set. Since the tire is narrower, the center of the tire tread is about 50mm wider. Since it is a solid axle with no differential, I kept the track width shorter than the front to reduce scrubbing and ensure the front tires have adequate leverage in turns.

As an initial test of the tire/adapter combination, I did a midnight run to terrorize my neighborhood. I did several high speed runs and there was no wheel wobble. I then ran each front wheel over the curb at speed to ensure the scrub radius didn't cause the steering to jerk out of my hands. The Rapture handled better, steered with less effort (even with flat profile street tires), the larger diameter rear tires put my engine at a better RPM for highway cruising. Overall, the test was a success!

Swapping the low pressure knobby ATV tires for DOT Auto tires significantly reduces rolling resistance. On the high speed runs, the engine doesn't work nearly as hard. In fact, I was surprised by my "Run-Away Raptor" later that night. I normally takes quite a bit of force to push the Raptor around my garage. But on DOT auto tires, my Raptor tends to roll away on its own due to a very slight slope in my garage floor. I now have to set the e-brake on my Raptor to keep it from rolling to the other side of the garage on its own!

To see the new TG700 front suspension geometry model -> Click here


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Vsusp models.jpg
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PostPosted: November 11, 2017, 4:47 pm 
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What's the wear rating for those 14" tires? As light as it's going to be, the car needs very soft rubber else the tires will never come up to temperature and will have poor traction.

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PostPosted: November 11, 2017, 7:03 pm 
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The wear rating is "400", which is a harder compound :( . However they also have a "AA" traction rating, which is the highest traction rating :lol: .

Typical auto DOT tires are not the best on any light weight car. Even the lightest DOT tires are made to carry 900lbs per tire, so they always ride hard on a light weight car. I have run lower air pressure (around 25lbs) to soften the ride on past light cars I've built.

I haven't had a DOT tire traction problem in the past on a light weight car. But its a good point to watch as I get it on the street and start testing its handling...Thanks


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PostPosted: November 15, 2017, 12:26 pm 
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After the front suspension design geometry was refined in the Vsusp online tool, the next step was to uses those measurements to design a 3D model of it. The front subframe is made mostly of 1"x1"x0.083" wall square tubing and carries the front suspension, coil-overs, rack & pinion, and the driver's pedal assembly. The front subframe will be mounted to a separate main frame via rubber mounts.

The front subframe will use many or the parts from the donor Raptor 700. Reused parts include the coil-over shocks, upper A-arms, lower A-arms, spindles, brakes, etc. The upper A-arms and lower A-arms will be shortened on the side that attaches to the frame. The design incorporates cantilever lower A-arms that hide the coil-overs and keep the exterior body design very clean. The lower A-arms will also be strengthened to handle the cantilever forces.

To see an animation of the front subframe -> Click Here

To see an video of the body design clay model -> Click Here


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Front 34 e.jpg
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Front subframe.jpg
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PostPosted: November 23, 2017, 1:51 pm 
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I now have the chassis design in place. This will be a "Mule" chassis, whos purpose is to ring out chassis and geometry problems. The Mule chassis is disposable and will be ultimately replaced by a final more refined chassis.

You will notice that the central chassis is shock mounted to the front subframe and the rear subframe. This helps to isolate the driver from noise, vibration, and harshness (NVH). It makes it easier to replace the central chassis down the road.

The different colors highlight the various tube sizes I will use.
  • Green - 1"x2"x0.083' rectangle tube. These tubes are used mostly on the sides to provide side impact protection
  • Blue - 1"x0.083" round or square tube. Square tube will be used in the front subframe to make fabrication easier. Round tube will be used elsewhere to reduce weight.
  • Purple = 0.5"x0.083" round tube. These tube are used for short braces. Pre-cut 90 degree 0.083" gussets will also be used for bracing in tight areas.

I started the design by building a virtual 3D template in SketchUp. This provided the side view and top view envelope in which the chassis needed to occupy.

Maple the supermodel had another engagement, so I inlisted the help of "Thump" the crash dummy. Thump helped me layout the dimensions of the cockpit. Please be aware that the term "crash dummy" is considered politically incorrect. Thump prefers to be called an "Impact Specialist".

The rear subframe contains the engine/transmission and rear suspension from the Raptor. The front of the Raptor chassis will be cutoff and leaned downward. This new downward angle will lower the center of gravity, provide more appropiate anti-squat geometry for the street, and shorten the overall wheelbase.

To see an animation of the chassis -> Click Here


Attachments:
Steel chassis design f34.JPG
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Steel chassis design r34.JPG
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PostPosted: November 24, 2017, 12:28 am 
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What do you mean by "shock mounted"?

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PostPosted: November 24, 2017, 8:54 am 
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Buy shock mounting I mean that the front and rear subframes are isolated from central chassis where the driver sits. Chassis isolation is done by removing metal-to-metal contact by using rubber or urethane Donuts or spacers.

All production cars use shock mounting to make the cabin quieter and give the car a quality feel. The trick is to do it in a lightweight Manor that does not reduce chassis rigidity or Road feel. The size and the material used in the isolators can make all the difference in how the car feels. If the isolators are too soft, then the car will feel sloppy.


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PostPosted: November 24, 2017, 1:19 pm 
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Sorry. I got a chuckle out of

Quote:
You will notice that the central chassis is shock mounted to the front subframe and the rear subframe. This helps to isolate the driver from noise, vibration, and harshness (NVH).

and

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being used in the same post.... Nothing personal intended. NVH will not be discernable from all the engine, wind and tire noise IMO. Besides that, I'm still following this thread. :cheers:

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My RX7 powered Locost is now for sale http://www.locostusa.com/forums/viewtopic.php?f=40&t=18460

or visit my Cushman Truckster resurrection log: over HERE


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PostPosted: November 25, 2017, 9:07 am 
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Point well taken. It won't be Lexus quite for sure. But there is much that can be done to reduce NHV. For example, my last build is a mid-engine V8 car and the engine is only a few inches from my head. I was able reduce cabin noise to the point where the engine is just a low rumble when cruising on the freeway.

I was able to do that by working noise reduction into every phase of the build, instead of trying to address it at the end. Basically the the 4 tools of noise reductions are:
  • Reduce the sources of noise - Use a relaxed motor, quiet exhaust, choose quieter tires, etc
  • Isolate noises from the cabin structure - Chassis isolators
  • Block noises from entering the cabin - air tight firewall, weather-stripping, multi-density acoustic foam barrier, etc
  • Dampen noises in the cabin - Dyna-Mat, carpet, mat, upholstery, headliner, door insulation, etc

Much of the above cannot be done on an open top car, but every little bit helps.


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PostPosted: November 25, 2017, 10:00 am 
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When you consider that tire and wind noise at 45 MPH alone were measured @ ~ 95dBA on my Locost.........I can see isolating the engine vibrations for sure. Beyond that, being conscious about sprung vs unsprung weight is about the best you can hope for. Like you say, every little bit helps. But some efforts may not pay back with measurable results.

Keep on keeping on. And update us as you progress. :cheers:

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“Any suspension will work if you don’t let it.” - Colin Chapman

My RX7 powered Locost is now for sale http://www.locostusa.com/forums/viewtopic.php?f=40&t=18460

or visit my Cushman Truckster resurrection log: over HERE


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PostPosted: November 25, 2017, 12:45 pm 
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Yup, I agree with you 100%


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