My take on it.
Biker Cop makes a valid point that just because you think something is OK it doesn't mean it will pass legal scrutiny. This could apply to more than tires.
If one was to accept that a motorcycle can not run a car tire one would have to accept that trikes and sidecars have to run motorcycle tires because they are licenced as motorcycles. Two possible exceptions are the Spyder and HD trikes as they are factory built and and the manuals may have some credibility with regards to using a specified car tire.
I'm not talking common sense, just the legal angle. Has any government sanctioned body approved car tires for trikes and hacks?
Has any car tire manufacturer stated that their tires are approved for trikes and hacks?
While I have great respect for the knowledge and skills that some members have regarding rim designs and vehicle setups I'm not familiar with the licensing procedures of modified motorcycles.
I am unfamiliar with what a person or company has to do to become a certified manufacturer of licensed motorcycles. With the exception of the HD and Canams most trikes are based on a heavy bike front end and power train attached to, typically a narrowed Mustang axle.
With the possible exception the Spyder, trikes and hacks present stability issues for purposes other than parking. Three wheel ATVs were banned over the issue but since the Spyder is a reverse trike with stability controls it is probably OK. Hacks are asymmetrical so how does one predict how a tire gets loaded. Kiting the car brings in some side loading that neither a car or motorcycle rim / tire combo will ever see in conventional use.
The greater concern is the survival of the activity of motorcycling. We are outnumbered 50:1 by cagers. We don't need to go at each other throats over full face or beanie, leather or textile, cruiser or R series.
Anecdotal evidence doesn't hold much water in the legal system but if enough anecdotal evidence is collected it may encourage certified testing which may shed new light on subjects. If the anecdotal evidence is collected at no cost to anyone but the individual involved it seems foolish to complain.
My apologies for not replying lately, I've had a couple busy shifts... but I've read up on the latest posts. I do have some contacts that are P.Eng's for the MTO, and I mentioned to them about the tire issue and they both looked at me like I had two heads. They were very surprised that anyone would even consider darksiding. Maybe it's more of a niche thing than I thought. I will keep my eyes open for more examples at Dover in June.
I do have another question that seems to be coming up in this thread, at what point does a motorcycle cease to be the vehicle that it was designed to be? Changing forks, shocks, handlebars, lengthening swingarms, really in some cases the only constant is the engine. I don't have an answer for this one... but is there a line in the sand where it becomes a home-built vehicle?
I've written this in response to both of you - I've touched on some of the below, in previous posts - from a sidecar pilot's perspective, and 20 year experience.
It's the way MTO rules are written. As long as the registration is still valid for a vehicle class, is attached to the vehicle, if said vehicle passes a mechanical safety, or an engineering assessment, then it's all good. It's still a 'factory' motorcycle even if the only thing remaining on it factory, is the portion of the frame holding the VIN. People have taken sidecar outfit modifications to that point of change, on both sides of the pond.
Europe/UK standards are a bit different, and a bit more rigorous. All associated UNECE/TLV/MOT standards that are in place, are documented and enforced - including vehicle modification engineering standards. They do have people on staff in their Standards/Inspections offices that can do this, apparently.
At it's core - a sidecar is considered an accessory for a vehicle class - motorcycles. They are treated as such. Attached to a motorcycle, it should be assessed by a knowledgeable mechanic for vehicle mechanical fitness and for any potential instability issues. If it is being registered for a safety, the whole assembly together needs to be assessed for road safety and braking/lighting standards. A knowledgeable mechanic would know that this includes alignment, geometry, braking, suspension, and any other mods that are potentially needed to make it a safe vehicle for road use. An ignorant mechanic will eyeball it, make sure it's lights work, give his shoulders a shrug, and sign the safety. Unfortunately, people do die because of this.
Fundamentally though, A motorcycle, with a sidecar, is still considered a motorcycle with a very large accessory, by MTO and Federal vehicle standards. As far as vehicle standards go, it's nothing more than an over-sized saddlebag, with a bogie wheel. Very few juristictions even have set standards for lighting placement, braking requirements (ie. yes or no, and if so equipped, is it actually required to be hooked up?) etc etc.. There are certainly no set crash-safety, crush-zone or silly air-bag requirements for a sidecar.
At the the base level of sidecars - you are looking at your average every-day full cradle-framed motorcycle, mated to a generic sidecar. Minimum mods have been made to the vehicle, separately or in whole to make it work together as a unit. Perhaps stiffened shock-absorbers and stiffened springs/collars up front in the stock fork tubes, a steering damper, perhaps beefed up rim sizes running larger sized motorcycle tires, or running a mild car-tire modification. Matching 'weight-class' of motorcycle and sidecar to each other appropriately, as well as assuring ensuring correct geometry of the overall rig conforms to acceptable practice in terms of sidecar wheel lead, toe-in and lean-out, is part of the total setup process, and makes the difference between a good handling rig, and one that'll scare the piss out of you or worse, try to kill you, the first corner that you try to make.
In this class, are the conventional commercial sidecar outfit offerings - Ural, Dnieper, CZ/Jawa, HD fabricated kits are some of the more recent commercial offerings available.
The next class, are the medium performance rigs. Usually involving substantial wheel-set, suspension, braking, steering, and performance design changes, but usually along 'traditional' sidecar fabrication lines. This is where major structural changes start occurring in the sidecar design and setup.
Case in point - mine. My bike shares the swing-arm, frame, engine, fuel tank and triple trees of the original. Added on top of that ... 600lb spring shock-units up front, aftermarket 900lb spring shock in the rear, custom designed trailing arm swing-arm and hub/brake/wheel assembly on the sidecar - over-designed to survive all but a land-mine hit (I broke the original factory suspension/frame, very spectacularly!), new front-end leading-link design, custom full-girdle sub-frame fabricated to mount to original motorcycle frame (bolt-on) in order to attach the sidecar at 4 points without bending the hell out of or breaking the motorcycle frame the first time that i put a loading stress into it, custom/car-adapted rim and hub assemblies on all three wheels, sway-bar, remote hydraulic sidecar braking system.. and much more than that, that i've since forgotten about. I've added on ~700 lbs of weight, in the process of bolting on the sidecar and all associated modified and changed bits. I would estimate that about 100 lbs of that might well be considered 'extra' weight coming out of building things in the "when in doubt build it stout" school of industrial-engineering.
As mentioned before, mis-matching of a sidecar to a motorcycle in terms of both comparative size and weight, will also pose significant challenges in making a rig, 'safe'. Significant changes will be required for a mis-match - my current sidecar rig was a three year project in this sort of mis-match adaptation - my sidecar was built for a much larger motorcycle, in a touring configuration. I wanted it on a physically smaller motorcycle, in a semi-sport configuration. Major changes obviously occurred to make that happen. Consideration of weaknesses exposed, and issues needing to be overcome, resulted in changes being made in all aspects of that bike/sidecar combination in that 3 year project in order to make the whole mess work together, well. Rather much like teaching a hippo to dance like a ballerina.
I also had it looked at, over, inspected, and commented upon many times since - including by individuals inside of the sidecar industry. No voiced critique or concern of any of the mechanical or engineering changes that were made. I involved qualified welders and machinists in any parts that i could not handle.
It handles and performs much like a short wheelbase 3-wheeled Miata would, i imagine.. it's considered a medium performance sidecar rig, mainly because of the dated (but updated) 1940's Leading Link front-end design that i chose to fabricate. It's of a traditional design, but modified/updated to use stock braking components in full floating capacity, and a very wide car tire (obviously). It still has limitations in certain handling circumstances in higher performance situations, whereupon the next class of sidecar rig generally does not - performance is usually dictated by how much nerve the sidecar pilot possesses, in the 'high-performance class'.
'High performance sidecar rigs' - this is the fewest of the sidecar few. More popular in the UK/Europe, but still has a small following on this side of the pond. Usually owned/fabricated by the very technically oriented. There are only a few commercial entities that are trickling out hand-fabricated factory sidecar rigs of this type. They tend to borrow from 'race-tech' technology, and are fabricated with advanced hub-centre steering variation front-ends, integral sidecar/motor frames/bodies, etc etc... these are the comparative Ferrari's of the sidecar world. They are still registered and classed as a motorcycle in most jurisdictions, using the donor motorcycle registration.
With the exception of the comparative few factory built conversions in the sidecar class/low production runs of high performance sidecar designs, it's a very individualistic thing. The rule of thumb, no two sidecar rigs are the same - in setup, configuration, or even owner riding style. They very much need to be mechanically setup, and inspected for their own merits and/or short-comings or limitations. Certain bike designs lend themselves better to sidecar installation, than others. Certain bikes are easier to adapt, than others. No current factory manufacturer of motorcycles (with the exception of Ural) recommends installation of a sidecar to their motorcycles - it's an instant voided warranty. A valid reason - no motorcycle manufactured today is going to work with a sidecar, without modifications being done to it by necessity - to address safety related, or suspension, frame, and weight distribution short-comings that may compromise the vehicle combination on the roadway.
I can't really speak for other motorcycle adaptations and changes for 3 wheeled variations of trikes or reverse trikes, or doing substantial solo-track motorcycle structural changes. Do keep in mind that there are at least two manufacturers in the US and Canada (Hannigan, Lehman) that build straight from OEM factory conversions to trikes (1800 Goldwings are most common) as well as limited OEM factory offerings (Hannigan builds/built HD's factory trike special), and at least a couple of other manufacturers that build kits.
Only recently has Ontario legislation changed to recognize 3 wheeled configuration motorcycles and the recent popularity of the Bombardier Can-am Spyder - in respect to licencing only. If you test your M1/M2/M on a 3-wheeled configuration motorcycle class, your licence reflects that class limitation to 3 wheels only. If you test for an M1/M2/M on a solo motorcycle - you can ride both 2 and 3 wheeled motorcycle class vehicles. Sidecar outfits get thrown into the same licencing restriction as tadpole trikes like the Can-am, and traditional trike conversions like the modern Hannigans or Lehman goldwing/cruiser conversions.
The vast majority of these - sidecar, tadpole and traditional trikes (with the exception of the base class of sidecar rigs), use automotive purpose radial tires in the conversion process. Those that are involved with and sell these as conversions as DOT/MOT certified manufacturers, sell them with automotive rim conversion package setups only, for obvious safety and liability reasons. A true "dark-sider" might creep into the sidecar class on occasion on the homebrew side.. i haven't really seen any previously - but then i haven't paid attention the past few years either.