Tubeless versus Tubes.
IN DEPTH discussion: tubeless
on Snowflake tube type rims??
© Copyright, 2014, R. Fleischer
section54, subsection 6
This article will discuss
in depth: Rim shapes (including BMW rims); using
tubes (or not)
on tube-type rims; tubeless operation; repairs; arguments for &
against using tubes with
rims designed for tubes. Other articles on this
website have further information on these
subjects, and much more, such as:
Make a pot of coffee...you are going to need the caffeine to read through the following.........after which you will be reading the rest of MY article....stopping, pondering, thinking....
Here is a link to an article that has enough math (but enough photos too), from a Goldwing'er
(Honda Wings have somewhat unique rims) to spin your head, for the nerdy amongst us. Fine if you want to scan the article, but do see the second part of the article....and then
REPEAT that look-see.
That article has extensive vector & other diagrams, which may bore you....but, also has some conclusions about contact size & pressures/forces, that may well astonish you. As you read keep in mind what REALLY happens when you are cornering; & think about the effects if you change a tire size...from, example, a 90/90 to a 100/90; or, 3.25-90 x 19 to 3.50-90 x 19; or, from 4.00" to 120 metric, etc. Think about what REALLY happens when you go to larger size tires (larger width and/or profile change). You may be very interested.
The below article will present data, discussion, & arguments, for tube/tubeless tires &
Some of the concepts/discussion may require reading one paragraph, & then stopping
to think. It is UNlikely that readers will be able to just read this article straightaway
through, & gain the information I would like you to absorb.
There are folks (including some that I highly respect) that say you must "never" run any sort of tire without a tube on wheel rims that were not designed to be run as tubeless. I am personally not 'that much' against the practice. What I believe is not the pertinent thing...what IS pertinent, and why you are reading this article, is that YOU gain a good understanding of all aspects & be able to make your own decisions.
I will try to inform you herein with UNbiased information. There ARE a lot of wrong thinking, misconceptions, failure to be open-minded enough to see the whole picture.
There are real reasons to do what the manufacturer of tires, tubes, rims, & motorcycles too!...say to do...which, generally, are to not use rims designed to be used with tubes, as tubeless.
There are things not at all widely known. Keep in mind that manufacturer's are not at all interested in long-winded information, like in this article you are reading. Such verbose information would tend to cause them a lot of labor in further explaining things, as there are always people who 'need to contact the manufacturer' to make specific
points, commentary, or have the makers provide additional information, etc. Thus, they make simple easy to understand statements...do this, or, don't do that. Their company lawyers would agree with that attitude.
Motorcyclists & sidecar/trike drivers do all sorts of non-standard things. Car tires, wide rims, change of rim diameters, LOTS OF "ETC.". If it was REALLY so vastly unsafe to run tubeless tires withOUT tubes on tube-type motorcycle wheels, how come sidecar rigs, which produce SIDE FORCES that can be EXTREME (2 wheelers do NOT have anything even close to those forces), do NOT have lots of accidents with blown tires, tires coming off rims, etc....? Sidecarists, sometimes, depending on situation & equipment, install tires in the 'wrong direction'. NOWADAYS many tires are marked on the sidewall as to what usage is (rotational direction), but SOME tires are marked for one direction OR THE OTHER, depending on if used as a front, or a rear? SOME have BOTH directions, depending on usage! THINK!!! These are only two of the many variable uses. Did you know that sidecar rigs have an EXCEPTIONALLY safe record? I'll try to answer questions in this article.
However...just because many folks use tires & rims in all sorts of non-standard ways hardly means that 'anything goes'!
There are levels of safety. In many things in life, the lower levels of improvements are large jumps. With improvements, every slight bit more improvement takes more and more effort, & one then might well question why one should go to huge lengths for a tiny improvement. The car manufacturer's, and the motorcycle manufacturers all well know this.
Ultimately, the safest tire/rim usage is to NOT RIDE AT ALL! I am not being facetious here. Just what is the chance that your particular decision about modifying your motorcycle rim or tire/tube usage will result in an accident, versus not having done the modification? What are the REAL facts?
Did you know that some have raced Airheads on paved tracks, at high speeds, using BMW Snowflake wheels that were specified for tubes only? Did you know that arguments that tires greatly expand in diameter at high speeds & thus will be likely to have the tire tread move to the center of NON-safety rims, & loose all air very suddenly, is FALSE? In fact, tires mostly expand in width at speed/temperature, & that is typically not any factor in a tire moving to the center of the rim.
Tube vs Tubeless, from several viewpoints:
The primary method by which a tire 'becomes unglued from the rim' is from a massive bending of the rim from the tire hitting a large object in the road. THAT can happen with tubeless....or tube type... tires/rims/etc. The 'safety' bump in a 'safety wheel/rim', as used with tubeless rims, does not prevent air loss from a bent rim. It does 'help' prevent a very fast air loss that would occur if the tire became un-beaded, & conditions & the rim were such that the tire bead would otherwise have moved to the center. Then, if you think about it, what about safety bumps or ridges in automotive wheels??.....don't those tires get exposed to SIDE FORCES that 'should' try to force the bead off or inwards, compared to 2-wheeler's that hardly have much side-forces, comparatively? What about radial tires on motorcycles, where the tread can move sideways even more?
Below photo is of a BMW Snowflake wheel which is designed to be used with a tube. A tube was installed, the rider ran over a brick, which bent the rim. The tube held air; the rider returned home safely. If the rim had been converted to tubeless use, would the tire have lost pressure? How fast? No loss? It is entirely possible that if the tire/rim in the photo did NOT have a tube, it may or may not loose air, but certainly would have if the bent area was deep enough. There is no question in MY mind that IF the bent area was severe-enough, the air would have disappeared from the tire VERY rapidly in a no-tube situation. If the rim was already of the tubeless type with no tube, a severe bend could also cause the tire to loose air very quickly. A valid argument can be made for situations where there IS a tube, & a puncture or otherwise a 'flat' causes the tube to be squeezed outwards some, and then sliced open by the rim itself. That will result in a fast exit of air. Loss of air, how & how fast, is the PRIME argument for/against using a tube-type rim as tube-less (AND EVEN WITH TUBES!!). Yes there are general arguments about which is better, tube or tubeless, even on rims being used as designed.
(1) Easier on-road tire repair. I think THAT is the PRIME REASON. Other reasons include tubeless tires MAY loose air from simple punctures more slowly. With tubes, a puncture MAY be similar, but they usually leak faster from a puncture; thus HAVE TO BE FIXED by a patch, which typically means wheel & tire removal.
(2) Possibly reduced weight, which means slightly better handling & mileage. Maybe; maybe not.
(3) No need to carry a spare inner-tube(s); repair kits can be simpler, take up less room. That can be true. However, there are other factors, not the least of which is if the tubeless tire deflates and moves inwards some....you may have trouble getting it back to full bead contact so you can inflate it.
(4) Possibly deeper tread on some tires, as tubeless tires can run cooler, so tread might be made deeper. Without a tube, the tire temperature is less, & the tire tread may last longer; or, a better rubber compound could be used. Note that these arguments in (4) are not necessarily true; all tires are designed to be run between certain temperature limits, otherwise the rubber does not perform as desired.
(5) A tubeless tire can be ruined by some types of punctures, that means $$$ replacement; while a tube type might just need a tube patching...or tube replacing. Seldom do folks say that they know that tubeless tires with a seriously bad puncture must be REPLACED, NOT repaired (or, repaired only very temporarily, & ridden at low speeds, until they can obtain a NEW tire). OFTEN they know, & ignore these things.
(6) A tube should be considered, as far as carrying a replacement, and not just, or in place of, patching equipment. Tubes take up a fair amount of room.
(7) In the boonies, tubes allow you to reduce the tire pressure greatly, when needed. What type of tires are used on BMW's bikes that are sold for on/off road use, such as the G/S and GS? What is different about the rims?....and I don't mean just the later GS rims that have the spokes outside of the normal 'in rim' area. THINK!!!
(8) YOUR argument, here...:___________________.
I think it reasonable for me to state that I have mixed feelings about many of the things mentioned so far in this article. While many have ridden on tubeless tires without tubes on tube type rims quite successfully & safely for many decades & hundreds of thousands of miles, MY CONCLUSION IS THAT IT IS LESS SAFE. That hardly means it is totally unsafe or even largely, or?..... what IS the percentage of "UNsafe"...?? It is my belief that 'less safe' is far safer, over-all, than many will say or believe. It's all going to be your choice anyway.
It is only reasonable to point out that motorcycling, in itself, is a more dangerous thing to do, than to use many other forms of transportation. Humans like to do certain things that are fun, yet dangerous. Perhaps the better statement might be what is YOUR level of safety? Do you wear All The Gear All The Time (ATGATT)?? ALWAYS...100.000%? Wear easy to abrade to your skin common jeans? Always wear proper boots? Armor padding? Leather is safer than nylon products like Cordura...what about YOU? What is more dangerous, not wearing ATGATT, or running tubeless on snowflake wheels? OR, is it really fair to submit that question at all? Would this argument/question be better if I put it in other terms?
When I started riding there were no such things as tubeless tires for motorcycles.
That means that the inner walls of the tires were not officially sealed to prevent air
loss. The truth probably is SOME tires were sealed OK on the
inside & might have been useable as tubeless......but we never thought of it...or, if we did think about it, no one I knew had tried it; and one of the reasons was that flat tires were repaired by installing patches on the TUBES. All riders knew how to repair their own tubes, and did so. BUT....tubeless tires are NOT a recent development, at least not for CARS. Although invented
& patented decades earlier, it was ~1955 that tubeless tires became mostly standard on
automobiles. Before that time, tires required a separate inner tube
(besides punctures) now & then because of incorrect tire fit, friction
between the tire
wall and inner tube generating excess heat, rusted rims, etc.
Tubeless tire technology does away with the need for an inner tube & it is, REALLY!... accepted by the various manufacturer's, that tubeless tires INCREASE safety (at least on rims designed for them, as the manufacturer's don't want to discuss this too much further). A reason is that a simple puncture typically releases air more slowly than with a tube, mostly due to the valve fitment (adding sealing rubber at the tube type valve stem is an entirely special argument and discussion). Another reason is, of course, that it could be CHEAPER, with no tube required. That also means less labor to install the tire. Another reason is longer tire life would be possible (less heat build-up), so one could design for longer life.....or higher performance with some additional life. To a MUCH LESSER extent was Public Demand. The reason it was so little desired by the public, hype & advertising aside, is that cars carry spare tires & FEW drivers would actually repair a tire themselves with a punctured tube.
If a tubeless tire (no tube installed) gets a small puncture, air escapes only through the puncture hole, leading to a slow & typically gentle deflation. The actual point of DEflation where YOU feel something may be wrong, will vary. This is due to the various types of tire construction, sidewall stiffness, your speed & riding technique at the time, etc. Neither tubeless nor tubed type of tires often burst like balloons, which means rapid deflation, and possible loss of control. Tires almost never came off the rims on motorcycles, less rarely on cars. The argument was that if the tire did come off the rim, or partially, on a motorcycle, it was probably going to be due to a large rim bending allowing the inner tube to protrude & be gashed/slit by the rim, badly, for a fast DEflation. It was thought more likely that a full deflation, or nearly, would allow the tire to rotate on the rim, and because of friction, carry the tube along with the rotation, and snap the valve stem off the tube, allowing fast deflation. Valve stems were not to be solidly tied to the rim by a nut, so as to allow the tube to slightly rotate, without tearing out the valve. A burst-balloon effect could not happen if the tube remained inside the tire & was not bulged out from under the tire bead. With CARS, drivers tended to be oblivious to a slow to moderately fast deflation, until the car handled terribly bad...by which time the tire might, or might not, have been ruined by running so flat. Motorcyclists are much more sensitive to changes in handling, and are extremely unlikely to continue to ride if the pressure in the tire/tube goes down by even half.
Tubeless rim & tube type rims usually have different ANGLES of bead area metal....but not necessarily what you might think. I will get into that a fair amount in this article, with photos/sketches.
The 'safety bead' or safety-bumps in a tubeless rim are supposed to prevent the tire bead from moving into the rim's center. Go back & see the photo at the beginning of this article. The tire hit an object in the road. Which way did the rim bend?...did the tire go into the rim? Remember, this wheel was designed to be used WITH tubes, and would have no safety ridges or bumps. Suppose this was a tubeless tire, and on a tubeless or tube type rim. Have you EVER seen anything written about how impacts may cause tire movements outwards, or inwards, in any real detail, or do they 'just' say that safety ridges/bumps are to prevent the tire bead from moving inwards. It is obvious that the air pressure inside the tire (or tube) is what keeps the tire bead in contact with the wheel rim bead area...but what happens if you just get a flat tire, no rim bending impact is involved. What does the tire do?...does it move inwards? Does it stay in contact with the rim? Can you think this through? I'm going to get into things in a lot of depth in this article.
Fast exiting of the air pressure in a tubeless tire more or less can depend on the rim design & rim damage. Manufacturer's have sought ways of keeping the tire bead in contact with the rim area, even for rather extreme instances of the rim being seriously bent from such as a large pothole, or the tire hitting an object in the road. While tire beads & rim shapes have changed, with improvements for bent rim problems, there is no common method in use, nor has one ever been widely sold nor promoted, to keep any type of tire 100% in contact with the sidewall area of the rim, no matter what problem occurs.
Cars: Rim is VERY seriously bent or cracked/damaged. The rim damage, & escape of air from the tire, is not likely to be any problem for loss of control....the accident has already caused that loss of control, more or less.
Motorcycles: Rims are lighter, think of them as less strong, easier to bend from
road debris, rarely crack badly, but possible. Compared to cars, loss of control in
an actual collision accident may be instant from such as impact with another
vehicle, striking a deer, etc.. Hitting even modest sized debris in the road can cause loss of control or
momentary loss of control, or none such; but the bending of the rim might be
enough to let all the air out quickly from a TUBELESS OR TUBE TYPE usage, since
in a tube situation the rim could slice the tube; and tubeless, the rim can be bent
enough to cause air loss.
MOST motorcycle 'flat tires' arise from a modest to slow loss of air from a simple puncture. Loss of control, would, in my estimation, be rarer, as almost always there is an indication of mushiness & a weaving feeling that considerably precedes any complete loss of air. It is rare for a rider to lose control and have an accident because of a pressure leak.
Manufacturers have tried rims with various angles to the bead area. Some also put bumps in the rim insides so that the tire would not likely be able to move into the center area of the rim. That does not help matters if the rim bends towards the hub enough to let air escape between tire bead and rim bead. It is not the same thing for the edge of the rim to be moved/bent, as in the earlier photo, and an angular or side impact, which bends areas inwards. That is a very rare, indeed, type of bent rim. A considerable amount of bending is needed to cause the tire to come off the rim; and much more common is what you see in the photo, above. If the bending in the photo was enough, the air in a tubeless tire releases VERY quickly. If a tube was inside, it might get pushed out by the air pressure, as sort of a bubble, and then the continuing rotation of the wheel might cut the tube, and air would release fast.
Answers to questions like those, & maybe some other questions, MIGHT give some
to what any safety change REALLY means.
"We all know", & " it has been proven over & over"; "more lights added to a motorcycle, & use of reflective clothing & certain bright colors on a helmet, ETC.......all reduce accident rates." UNfortunately, 'common knowledge' is NOT NECESSARILY NOR ALWAYS TRUE. Fixation effects have been studied; as has blending effects. Taller and wider illumination works well, ...more lights in a small area can be worse. I know, you don't believe me. The initial reports, done by the military/government, are here: http://bmwmotorcycletech.info/recog-safety.htm
But, my point:....is there a limit or some point on some sort of plotted accidents
curve, that shows how much of anything gives how much better protection? Would using
tubeless tires on tube-type rims, without the use of tubes.....be as less safe
as, perhaps, not having the brightest
attention-getting reflective color on your helmet? Not having ...and how
many....extra rear lights? What about
a headlight modulator?
WE DO NOT KNOW... OR, DO WE?
I have presented some ideas for thought. I am not promoting the use of no tubes in tubeless or tube tires as tubeless on tube-type rims, but I would like a real discussion, or at least some sort of thinking, about the depth of safety.
Enough photos and ideas to make your head swim....
Below is a 19" front Snowflake wheel, basically the same as the above wheel in design.
Flat bottom type of rim, similar to above wheels,
except for the flat bottom area, & there is
knurling for the tire bead! What do YOU think those
knurled areas really are for or really do? Keep the
tire from rotating as the inner tube looses enough air, & forces are trying to rotate the tire, and maybe
rip the tire valve off the inner tube, releasing air
faster? What is YOUR idea about the knurling???
Note the lack of any knurling. Tubes are not used,
so no need for knurling to avoid tire movement on
the rim, which would 'take along' the tube, and rip
the tube valve out of the tube.
If you are thinking, really thinking, you probably just asked yourself a question....quite a lot of literature says it is OK to install a tube...but this rim does NOT have knurling to help prevent the tire from rotating on the rim, pulling a tube along with it and ripping out the tube stem. Take a close look at
the rim bead area ANGLE & compare to the rims,
above, with no ridges.
BMW was more
concerned about the tire bead
than outwards. You are often told that you can add a tube to a tubeless tire; think about what that means with this rim. While thinking about that, think also about how the tube is being stretched to fill the tire, and the rim shape, NO MATTER WHAT TYPE OF RIM.
This next group is of safety bead/ridges styles:
Following are self-explanatory; and, yes, this is BMW's famous spoked wheel where the spokes are OUTSIDE the tire area.
Flat bottom. Same basic rim design as the R100GS, see photos above.
These 3 above photos are Morris Mag's, rear wheel, for Airheads. See any safety ridges? Nope, I don't either.
In the TL-H2 sketch above, the BUMP in the rim is generically shown. The actual bump or ridge might be just barely inwards of the tire bead when the tire is mounted & sealed to the rim; or; that bump could be as shown in the sketch, where the purpose is to prevent the tire, which might have disassociated with the rim, from moving inwards to the center-well area. A compromise is usually made, & some rims even have had both bumps & ridges. If a tire moves to the center of the rim, the loss of air will always be very rapid....on any type tire not containing a tube. Note also, how 'easy' it would be for a tire using the TL (not TL-H2) to move into the center area.
Above is another sketch, showing WM, a version of the MT/TL, & a version of the CP. Especially note how the CP type rim will help keep the tire association with the rim. If you think about all these sketches, & think about what REALLY might happen under several assorted circumstances of using & not using tubes, you just might get the idea that a CP rim WITH bumps or ridges, might be near-ideal. How about a TL-H2 with BOTH bumps and ridges?
For the NERDY: Metzeler says that if you use a TL-H2 rim, with a tube, it must be a Metzeler tube. They did not specify why. I think it due to the higher safety of a natural rubber tube; the particular design slightly up from the valve stem on the 'sidewall' of the TUBE; & the Metzeler tube design at the core. Probably a good suggestion by Metzeler....yet totally forgotten by probably everyone.
Has ANYONE ever said anything to you; or, have you EVER read anything, about the bump or ridge REALLY being there to help hold the tire to the rim so you CAN MORE EASILY inflate a tubeless tire?
The MTH2 rim is often used on OFF-ROAD motorcycles, where the rider may, under some circumstances, use a vastly lower than normal tire pressure.
Some few of you may be using car rims with motorcycle tires, or motorcycle rims with small car tires. This is done by some sidecarists, and sometimes with 16" wheels on Harley Davidsons, and I know of other instances. There is a difference between all these things that is not commonly known. Motorcycle tires have a THINNER bead area, where the tire fits between the edge of the side (where it meets the horizontal, before the safety ridge). If you think about this you will understand that if you use a car tire on a motorcycle rim, the safety ridge is fairly INeffective, as the tire bead will ride over it. If the motorcycle tire is used on the tire rim, the motorcycle tire's bead can move back and forth a bit.
OK...now that you have plowed through & read all that (and likely gotten confused)......below is the easier (?) to understand information...on what you need to know.
The REST of the information:
Some of these things are MY personal interpretations/ideas; read & do what you want, at YOUR RISK, NOT MINE:
2. NEVER fail to use
tire talc when installing a tube; rub it all over the tube & inside
tire with your hands. This allows the tube to move about a bit, avoiding excessive
friction against the tire; & also helps the tube, upon installation, avoid any folding.
3. Almost all tires marked tubeless, are OK for use WITH tubes at a reduced maximum speed; per the manufacturer's. Exceptions MAY exist in trying to use SOME RADIAL tires. As a general rule, I suggest you do not use old-style radial tires on your Airhead. You generally will not find old-style radials that fit your Airhead anyway. If you do try an old-style radial tire, approach handling with slowly increasing steps of aggressiveness & speed, until you know what the tire and motorcycle will do.
I suggest you adjust the steering head bearings carefully, for just a trace of weaving at maybe 25 mph, before doing radial tire testing. This is a good adjustment for any tire on an Airhead.
There are now 'radial' tires specifically made to work with old bikes like the Airheads. You might be interested in trying them, preliminary reports are GOOD, but tire pressures seem to be a bit more critical, IN MY....preliminary....
opinion. I may be wrong.
4. Tires marked tubeless are supposed to be specially made to ensure they hold air for long periods of time. This is typically done by a special coating on the inside of the tire. Talk of special bead areas, etc., are WAY exaggerated. Tires not marked tubeless MAY SLOWLY loose air if not used with tubes, as they could be porous in tread or sidewall, or bead. I have seen tubeless AND tube-type tires hold air for a long time on CAST tube-type rims. Occasionally I have seen tube type tires that would very slowly leak air, when used on tube-type rims, such as BMW tube-type Snowflake wheels, withOUT tubes. I am talking about just a FEW psi per week. In every instance, it was due to lack of a "tubeless coating" on the inside of the tire, or, poor cleaning of the rim bead area before fitting the tire. In most circumstances these were OLD STYLE FLAT TREAD SIDECAR-RATED TIRES. Mostly what I have seen is very SLOW pressure loss. Weekly topping-up is adequate; pressure loss not usually being much more than normal found if using 'natural rubber' tubes, which quite normally leak faster than other types of tubes. Some types of tubes, whether used with tubeless rated tires or tube-type rated tires, WILL lose pressure slowly, compared to the 'plastic' type tubes...but the tubes said to be 'natural rubber' are probably much less likely to be ripped from a puncture.
Tire pressures are supposed to be checked before every day's ride, right?
SOME folks have used the snowflake cast wheels with tubeless tires, & installed a tire sealant; & had good success in slowing any air loss & have even had the sealant work well for them with punctures. MANY have had hardly any air loss over time.
Some very few, that is, RARE, snowflake wheels may be SLIGHTLY porous. You can clean and seal the insides with a coating.
Purposeful off-road tires are, in the majority, meant for use with tubes, many of these tires do not have any interior sealing coating, SOME DO! These tires may be used at times as low as 5 or 8 pounds of pressure. Avid off-road riders typically have other methods in keeping tires on the rim, besides special rim bumps/clamps, when using such very low pressures.
5. Tube Mounting NUTS:
When installing a tube (tubeless or tube type rims), normally do NOT leave the mounting nut against the rim. There reasons NOT to screw the tube valve stem nut to the OUTSIDE of the rim. THAT is done, although somewhat loosely, DURING INSTALLATION OF THE TUBE, TO MAKE THINGS EASIER. So, why should you not leave the outside nut against the rim?
(a). The tube could fail to release air from between the tube & inside of the tire; via the valve stem clearance to the rim...which it is SUPPOSED TO DO.
(b). Leaving the nut against the rim defeats the safety of allowing the tube to be able to move slightly, upon a flat occurring. Run the nut up against the cap, or throw it away, or put it in your tool tray. MANY tubes are sold with that nut, & the purpose is TO HELP DURING INSTALLATION OF THE TUBE INTO THE TIRE. It is better/easier, to use the special tool for inserting the valve into the rim hole. http://bmwmotorcycletech.info/TireRepair.htm
Using the nut against the rim prevents tube movement if one has a flat; then the tire CAN rotate on the rim some,
and rip the valve completely out of the tube, causing a FAST air loss! Remember the photos of the KNURLED tire rims, much earlier here? BMW has Service Information bulletins out on NOT tightening the nut to the rim...see (d).
(c). If the nut is tightened to the rim and the tube seals to the rim too well, it can trap air from tube to rim, & allow tube chafing. The tube could chaff & fail early. This is particularly so if you did not rub the tube & tire insides with tire talc.
(d). I have tire manufacturer's manuals & a BMW bulletin on this: BMW has had Service Bulletins (SI's) on the valve nuts. One SI gave an additional reason to have it up against the cap: improper inflation could cause the tube to be weakened at the valve stem & if the nut was at the rim, the stem could suddenly & disastrously tear out. What BMW did not say, & I will, was that this can also come from very low inflation AND over-inflation during seating of the tire.
(e). TUBELESS TIRES are, as you have seen in this article, sometimes used on TUBE TYPE RIMS. The stock tube-type rim hole is 8 mm. If you do not wish to enlarge the rim hole for a standard pull-in type of tubeless stem, there is a special valve stem that is available...even from BMW!... that seals at the inside of the rim by means of an O-ring built into a recess of the stem unit. The stem area of the rim, inside, must be machined flat for this to work well; although I have seen folks seal with a bit of rubber or thin O-ring added. Some clever (?) folks have used the stem from an old tube, cut from the tube, and maybe a piece of tube rubber, and one of the nuts, INSIDE, to make a valve stem that seals when used with an UNmodified rim hole...that is, no need to enlargen the diameter, and no need to have any flat area for the metal special stem to seal (has a rubber O-ring). I have seen this done where it works WELL. There are those, often sidecarists, who seal the tube-type stem with rubber doughnuts or by other means. Sidecarists do lots of things that some may consider wrong (or, unwise, or at least strange). Carefully done, I have no problem with you using a tubeless tire on a tube type cast rim (snowflake, for example) and this type of a valve stem arrangement.
13. Snowbum has, on two of his Alaskan adventures, installed tubes WITH rubber washers
14. The snowflake tube-type wheels will handle major rim denting & still hold air, if tubes are installed as they were designed to be. HOWEVER, there have been numerous reports on these rims of very serious rim bending, with NO problems with tubeless tires without tubes!
Thus, considering everything I have mentioned so far in this article, there is PROBABLY NO clear-cut 100% answer, in MY opinion, on whether or not it is safe, or safe enough, to remove the tubes from tube-type snowflake wheels.
ADDENDUM & CLARIFICATIONS:
SOME manufacturer's tubeless tires & tube-tires have slightly different beads, but I have been TOTALLY unable to get DEFINITIVE information, & my personal inspection has not shown enough difference to make it more than a minor mention. The flat portion of the tubeLESS rim just inside the bead is usually a bit wider (depending on the particular tubeless rim design), and I have previously noted the angle differences. I do NOT think this any major thing. When any tire containing a tube gets a bump big enough to damage the rim severely, you may well retain air with a tube, & less likely to without a tube.
I usually advise against any tubeless tire withOUT tubes on tube-type snowflake wheels. I think that such use is SLIGHTLY LESS SAFE, OVERALL, all things considered, than WITH tubes. How much less safe? Well, maybe very little, overall, in practical use. You are totally on your own in this regard.
NOTE!...this is an OFFICIAL DISCLAIMER!:::
.....I am NOT advising that it is, in ANY way, safe...or not... that you run tubeless tires withOUT tubes....on your tube-type BMW Snowflake wheels! You are on your own in doing modifications...I am NOT responsible, & I am not advocating changes to the wheels, nor using tubeless tires without tubes on wheels BMW did not intend that for. I am simply trying to inform you of how things work as best I understand them, what others have done, and the caveats...as I know them. This is a legal disclaimer!!!
©Copyright, 2014, R. Fleischer
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