Tire Formula?

[downhillhunter]

My wife has a K75 and were are going to purchase a shorter shock and the lower seat.

I have been thinking about also putting a low profile tire on it to reduce the seat height and to gain some more clearance for a shorter shock.

I would like some confirmation on the correctness of what I've been told:

Currentlly it has a Metzler 88 130/90 B17

Here's the formula I've been given: 130 x .90 = 117mm

New lower profile tire Metzler 880 140/80 17

140 x .80 = 112mm

The difference (117mm - 112mm=) of 5mm should be divided in half to get the reduction in seat height. 2.5mm = .0975" or only about 3/32"!

Can this be right?

Is there another low profile tire that I should be looking at?

Thanks,

Downhill Hunter

[Kayseventyfive]

The difference in cross section height should not be divided by two.

With the tires you mention, the rear axle will be 5mm lower with the lower profile tire. This is by the numbers. Sometimes, the real world is a little different.

[kbasa]

Dunlop used to make a bias ply 591 that had a 70 aspect ratio. I think that tire got replaced by the 501 series a few years ago, though.

Don't forget that installing a shorter rear shock will require you to lower the triple clamps an equivalent amount on the fork tubes.

The biggest issue, IMHO, with the K75 isn't as much the height as the width. It's very broad through the center, which splays a rider's legs apart, making them functionally even shorter.

[downhillhunter]

Thanks, Kayseventy five:

This ratio then, is for the distance from the rim on one side?

Any recommendations for something that will give her more that 5mm and still have the possiblity of working okay?

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Thanks, KBsa:

I'll do a search for the new Dunlop.

It looks like we have about 3/4" before we run into the handlebars, so if we go lower than that, we'll have to get some 'bar backs', which is okay for her arm length any way.

And you're absolutely right, the K75 frame at the seat front is pretty wide and with a short inseam, the spread eats up a lot of reach.
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Thanks for the advice!

Downhill Hunter

[manicmechanic]

You might investigate a custom seat that would narrow the front, thereby reducing the amount the rider must spread their legs.

[dlearl476]

You might investigate a custom seat that would narrow the front, thereby reducing the amount the rider must spread their legs.


I hear ya Dave. I'm not inseam challenged by any stretch of the imagination, but I can only flat foot on foot at a time on my K. But I have the monster touring seat, customized by Bill Mayer. IIRC, the "sport" seat is much narrower (K75S version) and there's also a "Low Seat" that requires removal of the side panels.

[PHMarvin]

Hi, Downhillhunter,
Harriet used Bridgestone Battleax BT45's in sizes 100/80V18 and 130/80V17 on her K75. The tires lasted between 6k and 7k miles, and made it more difficult for her to put it onto the centerstand. She went back to Michelin Macadam 50/50E in 100/90v18 and 130/90V17 and does not have problems with them.

[Kayseventyfive]

there's also a "Low Seat" that requires removal of the side panels.

The low seat is a mixed blessing. The low seat has sides that cover some of the area covered by side panels on other bikes. More of that area is covered by the black rubber knee pad thingee that drops down to form a side panel of its own. It has the drawback of forcing the rider's knees farther apart than (already wide) other models. Even though the low seat is low, it could benefit from some upper edge trimming of the foam toward the front to allow the rider's thighs to reach the ground more easily.

One other consideration is the rider's tailbone. Many tailbones don't like much pressure, and the raised passenger portion can put too much pressure on the tailbone of many riders. Reviews of the period mentioned this. The upright riding position afforded by the "wheelbarrow" bars supplied on some low rider bikes further exacerbates this situation. S or C bars can help some riders. So can some judicious cutting away of foam in the center of the step area. The hard foam removed can be replaced with softer foam to keep the appearance normal when unoccupied.

Another drawback of the low seat is that it forces you to sit in just one spot, which can be a drawback in the twisties.

As far as difficulty with the centerstand, mine is too high. I have a trifle of difficulty pulling the bike up on the centerstand while still astride, so I may shorten it. A centerstand needs to be only high enough to raise one or the other tire 1/16" off the ground, anyway.

[downhillhunter]

manicmechanic:
Thanks, We have a used "BMW lowered seat" on order, we'll see if it works 'as is' or if modification is needed.
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dlearl476:
Thanks, the bike came with a recovered Corbin. It is a type of bicycle saddle
design with a raised passenger rear. It's 4"+ thick, but the best seat that I've ever ridden! When I drove the bike home the 500+ miles in one day, it was unbelievably comfortable. Generally, about 125 miles and I start to squirm. Unfortunately, my wife's feet are about 2" from the ground when she sits on it!

We thought about having it customized, by removing some of the foam, but:

1. Hate to mess with this seat, since it's so good right now and it will make a
fantastic seat for someone else, with a long inseam.

2. Removing the foam would probably drastically change the comfort factor of the seat, anyway.

We're going to try the stock, lowered, seat first and if that's not the answer, then consider other options.
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PHMarvin:
Thanks, Why do you suppose the mileage was reduced so much on the Bridgestones? Did the shorter side wall cause that or is it a softer tire?

Did Harriet notice the reduction in height of the Bridgestones?

Was the handling significantly different with the lower profile tires?

My wife lowered her F650GS with a 1" Kouba link. I cut and rewelded the side and center stand for her and it made both stands much more useable and safe.

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Kayseventyfive:

Thanks, You're right, seat width has as much to do with reaching the ground as does seat height. And reducing the width and removing the foam can/does make the seat much less comfortable.

A bike with which you can reach the ground but can't ride more than 75 miles isn't much fun either.

You could put an Air Hawk over the seat....but then you're back off the ground again!

It would seem that a great lower seat would be one that was shallow and narrow at the front, for standing and wider and more plush as you slide back on it...assuming that you could work out the handle bar and foot peg positioning. Maybe we'll have to look into S or C bars. I'm not sure which bars are on the bike now, they seem to be okay for me, but we'll see after we get the seat situation figured out for my wife.

I have had emails into several seat makers for several days, but only Sargent has replied and then just to advise us of their refferal to Tech Support.

My best friend has an air seat on his R1100 and maybe that's the answer.

Leave the air out while stopping and inflate it for comfort while on the road long term.
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Thanks for your ideas and help, gang. We really appreciate it.


Downhill Hunter

[Kayseventyfive]

You could put an Air Hawk over the seat....but then you're back off the ground again!

My best friend has an air seat on his R1100 and maybe that's the answer.

Leave the air out while stopping and inflate it for comfort while on the road long term.
Downhill Hunter

Maybe we should both look into an air cushion with a small electric pump. The pump would have a thumb button for on, a pressure release valve to prevent over inflation, and a thumb deflation valve.

Or a Hall-Effect sensor or pulse counter on the front wheel that would trigger a deflation circuit and solenoid when rotation stopped.

.

[downhillhunter]

Kayseventyfive:

I'll find out a little more about his seat. I know that it does have it's own air pump and he can inflate/deflate at will.

What I don't know is: how much it changes the seat height.

Downhill Hunter

[downhillhunter]

Kayseventyfive:

Well, I was wrong about the seat. It does not have it's own pump,
it is self inflating. The foam in the seat is enclosed in an air tight
cover with a valve.

With no pressure on the seat, it will self inflate. When seated on the bike you push the valve and let out as much air as you want to make you comfortable.

A great product, but it won't do what we need.

Our used, lower, BMW seat should be here today, so will put that on this week end, along with the new low profile tires from Avon.

We ordered a new shock from Hagon, but that will take 3-4 weeks.

After this whole, cobbled, lowering process is completed, I'll post the results:

Successful or unsuccessful.

It would have been easier to buy a K75 engine and mount it on a skateboard!

I'm now looking for some sort of instructions for installing the new/used seat.

The K75 that I was going to use as a model is out of town.

Thanks,


Downhill Hunter

[deilenberger]

Thanks, Kayseventy five:
Any recommendations for something that will give her more that 5mm and still have the possiblity of working okay?
Downhill HunterBridgestone makes an 80 series in the SAME WIDTH as the stock K bike tires - lowers the bike about 1/2-3/4".. Used these on my FKY100RT, and they worked well. Look at the BT45 series of Bridgestone. Not a mileage tire - but good ride quality and good grip in dry or wet conditions.

[downhillhunter]

Thanks Don,

Do you know of an 'on line' site that I might use as a guide for installing the low seat of a K75?

Thanks,


Downhill Hunter

[deilenberger]

Thanks Don,

Do you know of an 'on line' site that I might use as a guide for installing the low seat of a K75?

Thanks,

Downhill Hunter
Sorry - no, but if you get the parts needed it's pretty obvious. There is a rubber pad that goes around the tank, the document/computer box gets replaced, and some bracketry that holds the seat down - and the lock assembly. The trick is getting ALL the parts. I've seen the seats alone on some for-sale listings, but they usually don't have all the parts.

Your best bet is to find someone with the low seat who wants the normal seat - then swap everything needed.

[downhillhunter]

Thanks Don,

I bought a seat and it arrived yesterday, but I didn't have time to open the box. Supposedly, all the parts are enclosed, seat, box, brackets, etc.

I had lined up a friend's K75 with a low seat on it, to use as a reference, but the bikes out of town, and I'd like to get it on this week end.

I have a general idea of how it works and if it is somewhat self-explanatory I probably can figure it out.

My two favorite phrases:

"How hard can it be?"
and
"What's the worst that could happen?"


With Murphy's help, what could go wrong?!!


Thanks,


Downhill Hunter

[PHMarvin]

Hi, Downhill Hunter,
Sorry to be so long in answering your questions, but I just returned from a 6k mile trip on my K1100LT.

Neither Harriet nor I had any handling problems with the B'stone BT45 80 series tires. Neither of us tries to scrape the pegs, but we also don't exclusively ride slabs, either. As far as we were concerned, it was a non-issue. The lower tire mileage is because the BT45 is a softer tire. I had the same issue with my K1100LT once when I used a set of (correct size) BT45 on it. I usually get between 10k and 14k; I got about 6k with the BT45's. Don Eilenberger also said something similar.

Harriet did notice the bike was a bit lower with the lower profile BT45's. After wearing out a couple of sets, she told me to put the standard size Michelin Mac 50/50E on the bike, as she was tired of fighting to get it onto the center stand. She doesn't have any problem, now, with the standard size tires.

If you want some guidance on installing the low seat kit, ping me off the list. I have installed the low seat kit on her (now sold) '95 K75RT. If you have questions, let me know.

[downhillhunter]

Hi PHMARVIN,

Hope you had a great trip!

We put the low profile Avons on. It lowered the bike almost an inch.

But the weather turned cold here in MN, so we've only put about 150 miles
on the bike with the new tires.

Everything seems okay, but too few miles to state something definitive, yet.

Seems to handle fine: corners, tracks, stops, well. Speedo is even further off, but will try to correct that after ensuring that these are okay tires.

Side stand works better! But, more work for the center stand. She couldn't get that to work with the standard tires. So we didn't lose anything there.

Got the new lower shock yesterday, so will put that on and see how it works.

An awful lot of changes to the bike's original geometry, so we keep watching for gremlins, but she doesn't come close to pushing the bike's limits so we're hopeful everything will be ok.

We have limited choices: can't lengthen her legs, so have to lower the seat height in every possible way. So we either try to make this "low rider" work
or she doesn't ride. The stress of not being able to easily reach the ground takes the fun out of riding. The good news - she's having fun on her 'new' K75! Even without the low shock!

I did put the low seat on...seemed to go okay, just sort of applied logic to all the parts. Am I an expert, now?

Had two short nylon straps left over, could't find a place for them. Previous seat owner said he couldn't see what purpose they served on his bike, so I guess I'll just be checking some other low seats at rallies and such and see if I can figure them out.

Thanks for your help.

Downhill Hunter

[john1691]

How about airshocks and compressor on the bike, like cars "in the 'Hood" that raise and lower, or even hop up and down?! Imagine the tricks she could do. She'd be a hit at next years National.

john1691
2000K1200RS

[downhillhunter]

I think you have a 'million dollar' idea, John!

I think Gold Wings have something like that, you just need to adapt it
to BMWs and refine it and you couldn't make them fast enough!

Imagine rolling to a stop and lowering the bike 3" so that both feet are easily flat on the ground and then as you start up, raising the bike.

Someone, somewhere had to have had this idea before. Other than cost, there must be a down side that we don't see.

Put us down as your first customer.

And we will see you in Wisc. next summer. Look for the K75 that looks like a skate board with fairings.

Downhill Hunter