91 K100LT Tail Trunk On 95 R100RT

I would like a larger trunk, so I can remove the saddlebags most of the time. The current 22 liter trunk is hardly big enough, so I need the saddlebags to supplement. With all prior Airheads, I used a large tail trunk, usually Luftmeister, and only mounted the bags when needed. In looking at candidates, the 91 K100LT trunk is specific to that year, and looks very similar, in styling, to the 95 RT. Not sure about capacity, but would estimate maybe 35-40 liters. The mounts look to be transferable from the K to the R, but may need a bit of tweaking. I have located a couple of 91 KLT trunks, but have not yet found the mounts.

I like to keep Airheads near stock but, when pressed, choose function over form. Anyway, either trunk is bolt off & bolt on, so no permanent mods involved.

Has anyone where ever installed a K bike trunk, or any other larger trunk, on a mono shock Airhead? If so, any words of wisdom?

Thanks In Advance,
Joe H

How do the two trunks attach?

20774 said:

How do the two trunks attach?

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The same basic rear mount that sits on top of the fender, other mount similarities are such that, I believe, if I can obtain 1991 K LT mounts, they'll be transferable to the RT, perhaps with a spacer here or there, lengthening, shortening, etc..

I just bought a 1991 K100LT trunk off eBay. I've been studying K100LT & K1100LT trunks awhile, and most K100LTs came with the same 22L trunk that's already on my Airhead. I dislike the 22L because the lid opens from the wrong end, and the capacity is too small for me. Also, it only attaches with a single locking pin, so not sturdy enough to support the backrest (if actually used) without warping and misaligning the lid locking mechanism. In researching the K100LTs' optional 35L trunks, I discovered they are very hard to find. On a positive note, it appears the forward mount already on my Airhead can be easily adapted; the rear mount would be a simple bolt on, but they are even harder to find than the 35L trunk itself.

Though both are 35L, and the K1100 trunks are much easier to find, I much prefer the K100LT version. It sits low, very similar to the Airhead's 22L. But the K1100LT trunk is perched up high, and looks ridiculous. Even though it has a quick detachment and the lid looks easier to open/close, I'd never put one on the Airhead. I attached a pic of a 91 K100LT, to give the basic idea of what I am striving for on the Airhead. Will put the 22L on the shelf, in case I ever decide to sell the Airhead.

Have you given any thought to a Givi trunk. The base plate attaches to most racks and the trunk can be popped off with only the base plate left behind. They are very versitile, if you don't mind it not BMW branded.

GIVI 2 Thumbs Up

Chunk,
I had a GIVI tail trunk on a Suzuki Strom, and loved it. I seriously thought about transferring it to the R100RT before selling the Strom to my son. It was a spacious 33L and, as you say, easily detachable and all around convenient. Though not a purist, I wanted to stay with something that looked, "correct" on the RT, sat low, an as far forward as possible while still allowing clearance for opening the seat. In my opinion, the 35L trunk shoulda also been optional for mono shock Airheads, instead of just for some years of K100LTs.

Two thumbs up for the GIVI though, nothing bad to say about it!

Joe H

jhall said:

In my opinion, the 35L trunk shoulda also been optional for mono shock Airheads, instead of just for some years of K100LTs.

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We need to keep in mind that in the latter '80s and early '90s BMW had two, and then three design and engineering teams that barely talked to each other. The early Classic K Bike team was heavy with guys from the car side. The few on the Airhead team just plugged along. And then with the Oilheads emerging in '93 but the K Bikes continuing to '95 there was spillover but not much.

Weight Difference

The 1991 K100LT trunk arrived a few days ago, and NO disappointments. It's in very good condition, and the painted lid even matches the dark charcoal paint on the Rt's fairing.

As predicted, it's gonna be an easy mount.

Only drawback is weight difference. The original, 22L trunk weighs 6 pounds, whereas the 35L weighs 21 pounds. But it is beefed up in all the right places, i.e. the areas that support the passenger backrest. I consider it a winner!

jhall said:

I dislike the 22L because the lid opens from the wrong end,

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That's the optional air brake system

Lee said:

That's the optional air brake system

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LOL, and I just know one of these days I am gonna test it out. I try to remember to reach back and insure the lid is closed and locked sometimes. But one of these days I am gonna reach back there and think, OOPS! With the 35L that is not an issue. I also talked to a guy who removed his 22L because he carried a passenger, who tried to use it as a backrest, and severely misaligned the lock mechanism. It seems the 22L was never meant to be more than a place to stow jacket and gloves, or something similar.

The 35L is more akin to the large Luftmeister trunk that came on the R100/7 SPECIAL I once had. I used that trunk a lot, and just kept the saddlebags stored 99 percent of the time. With subsequent Airheads, installing a Luftmeister trunk was one of the first orders of business, if not already present. I am sure this trunk will see plenty of use too.

jhall said:

LOL, and I just know one of these days I am gonna test it out. I try to remember to reach back and insure the lid is closed and locked sometimes. But one of these days I am gonna reach back there and think, OOPS! With the 35L that is not an issue. I also talked to a guy who removed his 22L because he carried a passenger, who tried to use it as a backrest, and severely misaligned the lock mechanism. It seems the 22L was never meant to be more than a place to stow jacket and gloves, or something similar.

.

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We had ours open a few times, even when locked after hitting a big bump. At first we did not use a liner and things like gloves would get sucked out.
I think after the first year we bought the BMW trunk liner and that worked to prevent things flying out.
To make sure the lid never opened by accident my wife made a nylon strap with a quick release Fastex buckle to hold the lid shut.
Since we both had our own bikes I liked the small size trunk.

A word to the wise. The plastic rear portion that the trunk frame mounts to on a K100 is not strong enough to support much more than the stock 22 l trunk. I have tried larger ones and the tail piece will crack.

cycleman2 said:

A word to the wise. The plastic rear portion that the trunk frame mounts to on a K100 is not strong enough to support much more than the stock 22 l trunk. I have tried larger ones and the tail piece will crack.

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A large (1 inch or more in diameter) fender washer under each leg of the rack spreads the load and can minimize the risk of cracking.

35L Trunk Mounting

It appears the1991 K100LT's optional 35L trunk sat directly on the fender tail, with only a thick foam pad in between. I say this because the fender shape and roundel is imprinted on the bottom of the foam pad of my recently acquired trunk. It was also supported by out-riggged braces, and anchors on the forward sides. Sitting it down on the fender is how they achieved the lower positioning. Not sure I wanna do that on the RT, as it would definitely ruin the fender paint. Instead, I plan to mount it on the RT's original tail rack for the 22L, which is about 2" above where it sat on the 91 KLT. I recently sat the trunk in place and mocked everything up; it appears easy enough, but will be tedious in order to avoid looking ratty when done.

Will get back to it when colder weather is here, and more time to tinker with little stuff.

35L Trunk Mounts

cycleman2 said:

A word to the wise. The plastic rear portion that the trunk frame mounts to on a K100 is not strong enough to support much more than the stock 22 l trunk. I have tried larger ones and the tail piece will crack.

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Having inspected my bike and looked at pics of R and K bike rear sub-frames, I now understand what you are describing. Comparing R and K bike subframes, the K is squared and beefier, with mount tabs welded on. The forward mounts, which anchor to the front side of the 35L trunk, attach to a metal brace, which is attached to the two welded frame tabs. The R frame has no such metal brace, nor tabs; it's forward mounts are simply anchored to the fiberglass tail piece. I think that's what you have seen crack? Not surprised, and I will not anchor the larger 35L trunk to that fiberglass tail piece. I am likely gonna mimmic the metal brace and welded tabs on the K frame. However, the rearward mounts are very similar on both bikes, and will be easy to fabricate in mounting the 35L.

Seeing how flimsy the rear sub-frame is, and how the tail trunk is attached to all R frames, including twin shock versions, I now know why the large Luftmeister trunks vibrated to much. They are just perched on the rear corner of the flimsy sub frame. Still, I never had a problem with a large Luftmeister trunk on a twin shock airhead, but they always shook a lot.

I also read the (optional) 35L trunk mounts on the K100s were prone to breaking. So I am sure, with the flimsier R sub frame, it may be even more of a problem. Then too, the bike handles so well now, I am hesitant about hanging a heavier trunk behind the rear axle. Still, I am gonna give it my best shot, and set is low and far forward as possible, as it originally was on the K100LT.
Thanks,
Joe H

The stock 22 litre trunk was the same on both the R100RT & K100RT. I can't tell you how many hats and gloves I lost out of the trunk because of the lid flying open going down the highway. Not a good design. I tried to keep it locked but you know how that goes.

I did try a slightly bigger aftermarket trunk using a Givi metal mounting bracket that was designed for the K100RT. The Givi bracket mounted in the same holes as the stock trunk, but all that is holding that in place is the fiberglass tail piece and I never could keep it tightened up with the bolts that threaded into the Givi mount, I tired Locktite and nothing would stop the bolts from backing out of the mount. The bigger trunk was setting up a vibration which in very short order would loosed the bolts. Because of this the areas around the holes where starting to show some slight cracking so I went and reinforced the underside of the fiberglass , where the bolts go through. I had tried the larger fender washer between the bracket and the fiberglass but it didn't help spread the load and it still wanted to crack.

So what I did was to drill out the Givi metal mount so that I could put a bolt and a locking nut through it and the fiberglass underneath. In the end it just provides a big flat surface that I can mount a waterproof bag or tent if I need it. I have the two saddlebags if I need some space for longer trips. The tank bag provides space for the day ride type of stuff. I might look into a soft tail bag, but it is tough to find one that is waterproof and worth the money.

This winter I still may try again to mount a different trunk, as I have two older aftermarket trunks that I might adapt.

Tail Piece Cracked

Thanks again for the feedback cycleman2. As you, I have already had bolt problems where the trunk mount bolts to the tail piece. One bolt was missing, and one was very loose. I replaced the missing bolt, applied blue lock-tite to both bolts, and snugged them down. Then, maybe 1000 miles later, discovered the tail piece had cracked all the way through on left side, and half through on right side. I also read somewhere that tightening those bolts may cause the tail piece to crack, true indeed. I have looked at the entire rear section far more closely than I did on 2 previous twin shock RTs. I am pretty disgusted with how flimsy the whole setup is. I no longer believe it possible to install a larger, heavier tail trunk without major restructuring. Not sure I wanna even bother, as is, the setup is merely capable of supporting an empty 22L trunk.

The tail piece cracks are forward and adjacent to the leading edge of the saddlebag mounts.

QUESTTION: Does anyone know if the early K-LT tail piece will fit on a mono shock RT? I can see the forward section is different, and attaches differently, but OK with modification, as long as it does not show on the outside, is reversible, and will beef up the overall structure to support a larger trunk, or even just properly support the POS smaller 22L trunk.

Thanks Again