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theoretical 4 wheel conversion

33K views 40 replies 25 participants last post by  chad warren 
#1 ·
What would you guys think of if some one made a 4 wheel conversion to turn it into a poor mans ktm x bow?

The car is already incredibly wide and with a long wheel base for its oal and weight. Adding that fourth wheel would probably add the needed traction and control that would boost this things performance to play with cars by the likes of lotus and boxsters for a fraction of the price point.

Would any of you consider it as an option for your slingshot?
 
#2 ·
You would be re-engineering the ENTIRE vehicle. Not to mention you turn it into a car, it will not longer be a motorcycle. At that point I would assume it needs to be registered as a reconstructed vehicle and suffer through that entire process. I say just enjoy it for what it is..... They will develop upgrades for this design. It probably is already a great handler and is more performance capable than 99% of the peoples driving ability to use such capabilities.
 
#3 ·
I do not believe it would make the transition to car by adding a wheel legally speaking but perhaps some one more informed can answer that question as I do believe their was one 4 wheel motorcycle that was made in the us.

I personally just want more rear wheel traction, spinning the wheel through the first three gears tells me I need more rubber in the back.
 
#4 ·
I do not believe it would make the transition to car by adding a wheel legally speaking but perhaps some one more informed can answer that question as I do believe their was one 4 wheel motorcycle that was made in the us.

I personally just want more rear wheel traction, spinning the wheel through the first three gears tells me I need more rubber in the back.
Well, to be exact about it at least in Maryland (this will vary state to state of course with specifics refining basic Federal Guidelines) according to Transportation Code of Maryland Article 15 Section 136, subsection a;
(a) “Motorcycle” means a motor vehicle that:
(1) Has motive power;
(2) Has a seat or saddle for the use of the rider;
(3) Is designed to travel:
(i) On not more than three wheels in contact with the ground; and
(ii) At speeds exceeding 35 miles per hour; and
(4) Is of a type required to comply with all motor vehicle safety standards applicable to motorcycles under federal law.
(b) A detachable sidecar is an accessory to and not a part of a motorcycle.
 
#5 ·
I have to agree. There are already options out there if you want to go four wheels and an open top. Granted the price of entry may seem initally higher but by the time you make all of the necessary changes to this vehicle you would most likely be better served with an Atom, Vette, Mustang, modified miata, etc.....
 
#6 ·
I can't speak to the legality aspects of it, but adding a real rear end setup to this system appears do-able, although somewhat advanced fabrication. Remove the right angle drive/swingarm/belt setup. Create a subframe that attaches to the chassis that is triangulated and stiff. Install a standard style automotive driveshaft running to an IRS style rear center section, such as the aluminum 8.8 out of the T-bird or explorer (light, strong, easily obtainable and large aftermarket support for gears and diff). Create rear A-arm or trailing arm suspension, or possibly utilize existing suspension components, to complete the package. Lots of work, but totally doable with the proper tools and know how.
 
#9 ·
I'm sure there will be other options out there, possibly the miata diff, but the 8.8 is the one I am most familiar with, and isn't really that big. It's nothing compared to a Dana 60, Ford 9", etc, which would be way overkill for something like this. That rear end family is also extremely popular in general so pretty much any gear ratio, various types of locking/torque biasing differentials are available, as well as stub shafts for 930CV's. They are also stacked up like cordwood at every one of the U-pull-It yards around here. I am all ears for any other alternatives if you know of some.
 
#14 ·
Physically, there are a bunch of Harley trike conversion kits that also say they can custom fit to your bike with full rear IRS if you want. if you provide them your rear cog, they can build it to your frame. There's one on ebay for $7k, and comes with brakes!

It doesn't address how rigid it will be, but I'm sure a good crafts worker can work in a rear stabilizer and an extra cross member. Heck to simplify things, you can even even get rid of the slingshot propshaft to belt-drive assembly and replace it with a rear diff.

Can anyone comment on registration issues? I would assume when you purchase it, and the dealer licenses it, the DMV registration will say Motorcycle (at least in CA), assuming all these modifications are made after you purchase the "motorcycle" or 3-wheel cycle. What happens if the highway patrol pulls you over, your regsitration says motorcycle, but you obviously have 4 wheels?

If you want to register it as a 4-wheeler, do you have to meet current crash safety standards like a regular car, like bumpers, airbags, windshield, SMOG, etc?
 
#15 ·
Perhaps if you can register it as a kit car you can get away from airbags, windshield wipers, etc. Kit car laws also vary state to state though.

I agree with others that there are kits out there already that fulfill that fantasy and can be relatively cheap (Exocet).
 
#18 ·
Sorry, but why would you buy a three wheeled motorcycle and then convert it to 4 wheels? Sort of defeats the whole idea/purpose. As someone else mentioned, buy a sports car. Hell, you can get a used Boxster, Z4, or Miata for a lot less.
I bought the SS because I couldn't buy an X-Bow. I only wish US laws allowed a 4 wheel version of the SS. My Z3 just isn't the same thing. I feel every pound of every machine I own. Lighter is better.
 
#19 ·
Not worth all of the effort it would take.

There's no chassis structure there to support rear suspension; it would be less effort to start from scratch.
Not that I have an interest in doing it but my thought was more like a racing ATV. They don't have much out the back in the way of framing either but shoving the axle through the swingarm seems "plausible" along with updating the suspension to take on the extra weight etc.

Again, no interest myself but I've kookier things done. Just search for VW trikes. Makes me wish for the days of the Spanish Inquisition and hot pokers to the eyes again. Thems things ugly.
 
#20 ·
If anything what would be most plausible would be to offset the existing rear drive and mount a second rear wheel on the opposite side. It would have a gap between tires only wide enough for the belt drive. Would look like the SRT Viper bike rearend but with wide tires. Still the roll or
torsional strain would kill it.
 
#21 ·
I was thinking about something along the lines of this. It would take some obvious testing to figure out if the benefits were worth it. Not as good as a typical solid axle, but there might be a little room for improvement.

To act like no one would consider some kind of modification to increase the overall grip is absurd. Weight, price, ability to finance. All of these factors will push people to consider this as an alternative to some other vehicles (used Elise for instance).

Being limited to which tracks I could place drive this on might be the only thing keeping me from picking one up. No, I'm not talking about drag strips.
 
#22 ·
I would assume people will create 4 wheel conversion kits. I think the uniqueness of a 3 wheel vehicle is part of the coolness of the slingshot. I wouldn't think there would be any issue DMV wise if you converted it. I use AAA when i register vehicles and they've NEVER come out to make sure the vehicle i was registering was actually what was in the parking lot.
 
#23 ·
A 4 wheel conversion interests me a great deal. I would love to get a KTM X-Bow, however, they are around $80k. The Slingshot with 2wd would have very similar look and specs (weight/power).
Insurance and registration isn't a problem where I come from. It would just be considered a u-built, which is how all kit cars and custom bikes are set up here. The conversion would likely void the warranty, but I would still do it regardless myself.
I have thought about how I would go about converting it after looking one over in person, and studying pictures of a bare frame.
1. Purchase a premade IRS rear suspension. My personal choice would be the one offered from Dave Held at team321.com
2. Remove rear swingarm and belt drive differential.
3. Build a simple tube frame rear section to support the IRS suspension that ties into the existing frame.
4. Build a longer driveshaft to connect to the new differential.
5. Re-plumb and re-bias for extra rear caliper.

This of course is just the basics, but looking at the simplicity of the slingshots design, makes this look very feasible to me. Here is a quick photoshop showing the team321 rear suspension and a simple tube frame rear section on a Slingshot.
 
#24 ·
The other hurdles I am pondering are the vehicle dynamic aids.
ABS
Traction control
Stability control
The simple solution is to turn them all off. However, they would be nice to have, especially the ABS. The programming for them now is based on three wheels, adding the fourth would probably require some ecu tunning, and additional sensors on the new wheel.
Any thoughts on this?
 
#27 ·
Orangeman,
Your killing me. I know the Miata makes perfectly logical sense in every way. But I don't want one, and it is not the same at all to what a 4 wheeled SS would be, in my mind. There are many people who think the same, and they buy cars like this:

They are not sensible cars at all. They are very different then a Miata (yes I know it is called an MX-5, but to me it will always be a Miata)

The Miata is a street car that is fun at the track. The cars above are track cars that are fun on the street. Totally different experience in my mind. I know the Miata is a fabulous car, but it's not what I want.

For me the problem with the Ariel, KTM & Radical, is that they are still out of my budget. From my very rough idea of adding a 4th wheel to the SS, I think it could be done for less then $30k. Then add a turbo, and be on par for performance, with the above mentioned cars for around $35k. Plus you would have a car that probably nobody else has.

Rant over.
 
#28 ·
I working on ideas of how to attach a tubular sub frame to the existing SS frame. I was wondering if a owner could verify a couple things?
1. There is some differences in the drawings of the frame as to how the swing arm attaches. Some show it with brackets, some show it with a thru axle. Can it be verified which way it is, or maybe this changed during production?

2. Is the steel "girdle" that supports the differential welded to the tube frame? Or is it removable?
 
#30 ·
Is anyone seriously working on a 4 wheel conversion. It seems like I heard from one of the supporting vendors there was going to be something very different unveiled at the SSITS16 ? Anybody with some insider info on what that might be ... ?
It's too cold to ride... might as well daydream ! :(
 
#31 ·
I am sure you will see 1 or 2 people with this in 2016!

With all the mods people are coming out with, this has to be something you will see; even if it is only for personally use and not for sale to the general public.

Just a blind guess....
 
#34 ·
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