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Old 10-13-2010, 08:54 PM   #1
edray OP
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Airhead steering/suspension geometry

Hello,
I have a 78 R100S, w/ 1" shorter Ohlins rear shocks. I bought them thinking that it'd be better for my short inseam, but of course not realizing at that time that it would slow down my steering...Do you guys think that puting an 18" front wheel on would return my bike to stock geometry??
Also puting wire wheels on, w/offset spoke rear, and wondering what the optimum tire sizes would be..as well as availability for modern rubber..Woody says 150 max for rear.. 110, 120/18 for front??? (these seem to be the best available tire matches(so far..)
Thoughts??
Thanks in advance,
Ed
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Old 10-14-2010, 02:08 AM   #2
Rucksta
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about halfway

The 18" wheel drops the ride hight 1/2" over a 19" wheel.
The smaller diameter will also pick up a bit of steering speed over the basic geometry issues.

You could try a bit less preload on the front to even things out a bit more.

Fat tyres are just going to jack the bike back up again and introduce a bunch of offline forces the frame isn't really set up to deal with.

120/80 18 rear + 100/80 or 90 18 front front seems to be a sweet spot.

110 on the front feels heavy to me and increases bump steer to an unacceptable level.

Bridgestone have good size ranges available in paterns & compounds that suit

http://mc.bridgestone.co.jp/en/produ...tlax/bt45.html

Some say the fronts scallop easily but that is not my experience.

Metzeler ME33s are a popular choice but 110/80 18" is about the narowest available.
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Old 10-14-2010, 01:41 PM   #3
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Sorry edray but I think you've painted yourself into a corner.

Dropping the rear of the bike an inch is a drastic change to your bikes geometry and really screws up the handling, but at this point, reducing the size of the front wheel will only get you part way back to normal, with the side effect of much reduced clearance in turns.

Ruckstas right... Usually, when guys talk about "Modern rubber" they mean wider/fatter tires and thats another thing that'll slow down your steering and make the bike feel heavier and more difficult to turn. Generally, narrower "V" profile tires are the way to go to lighten and speed up an Airheads handling.

If you're an experienced rider (And even if you aren't!) getting both toes on the ground is usually good enough and it works for me but thats personal. YMMV. I would probably have left the suspension stock (For starters.) and cut down the seat. Then if that didn't work, I'd have tried an 18" front wheel, which would quicken the steering somewhat and lower the front 1/2". Another good Airhead option would be an R65. Smaller wheels and tires make the bike a better bet for the shorter people among us.

I'm wondering what your goals for your bike are. Racer-like handling? A lower, more comfortable ride? A lighter handling chassis to everyday use?
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Old 10-14-2010, 05:07 PM   #4
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A R65 might work for ya. They have A smaller 18" front wheel. The tires are the same size as the bigger bikes except the front one being 18". Sag being equal between a R65 and a bigger model, the R65 is lower because they have quite a bit less suspension travel front and rear than the bigger bikes unless you are talking about the rear of a SWB /5. I think they are about the same. The problem is that if both bikes have sacked out suspension, the R65 is only going to be that 1/2 in lower in the front for the smaller 18" wheel.

Otherwise, you are going to have to get a "real" top triple tree and raise the fork tubes. If you do that, be careful of dragging stuff. Some caster wheels on the cylinders might help.

Good luck!
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Old 10-14-2010, 06:12 PM   #5
edray OP
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Thanks everyone,
Evidentally I didn't have much luck reinventing the wheel (sorry..), so now I'm going to go back to square one, get rid off the shorter version and purchase the correct Ohlins i should have gotten in the first place.
Then really research rubber..(Thanks for the link,Rucksta)
Appreciate the thoughts,great info.
Ed
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Old 10-15-2010, 12:32 AM   #6
pommie john
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Just to throw a spanner in the works...I reckon that if you have 1" shorter shocks and an 18" wheel on the front, you'd be so close to standard geometry, very few people could tell the difference.


I've tried longer shocks on various bikes to quicken the steering and although it makes a bit of a difference, it's not that great.

If you have to swap from cast wheels to spokes, then yeah, maybe it will be more expensive than buying another pair of correct length shocks. But if you really want to lower the seat height, my opinion is that you have the right idea.
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Old 10-15-2010, 06:48 AM   #7
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John

Before giving up on BMW forks I ran a stepped billet top clamp that allowed for adjustable ride hight.

I found that
+/- 3mm was splitting hairs.
+/- 6mm was a noticble change.
+12mm the bike pushes through corners.
-12mm and the front end tucks and washes out.

If you'd never riden a well set up Airhead and rode the +12mm I reckon you'd come away thinking 'suitable as an old man's touring bike'.

Once you'd riden the well set up Airhead you would likely no longer be counted among the many who wouldn't notice.
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Old 10-15-2010, 10:56 AM   #8
edray OP
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OH OH, Mathematics!

Like a lot of people,,well, less analytical math-types, my head gets spinning w/ mathematics..and I originally thought I could sort this out with....math.
John,thanks; that was my reasoning,about the 1" shorter rear,and the 18" front.
Rucksta,thanks; I do want a well sorted out airhead.
Mindsok; can I have all three choices?? kidding, i know they'll be compromises...
I've owned my S for 28 years now,about 170K miles,(riding it in a real sporting manner),along w/ other bikes and now realizing I have to sell my fantastic loud red Ducati after my latest ticket..commercial license at risk..
owwww, so i'll have a small budget for the airhead of my dreams.
So here's the other half of my dilemna:
Last year I upgraded my 2001 SV650 w/ a GSXR front end, very cool and worked out well..I've always wanted to do a swap like that for the airhead.
Nella, w/ his posted R100/7 build has really inspired me to do it..and I think I have the desired wire spoke hub figured out for the front..
Nella ,I believe has real success w/ his geometry; 140/80 18 rear w/ stock length shocks, and I believe a 19 on the front(stock-looking Lesters).
So I was hoping since my short shock current geometry might allow an 18" front and bringing it back to correct geometry,then the GSXR swap, I could have more tire choices, as I mentioned, and possibly dual compounds,etc..
This may be opening Pandora's box, and apples and oranges,etc..but I think
this is workable.. of course I'd like to avoid an expensive roll of the dice,like redoing front rim sizes and rear shock length after the already spendy build.
I certainly welcome your thoughts.
Thanks in advance,Ed
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Old 10-15-2010, 11:21 AM   #9
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Quote:
Originally Posted by edray
Thanks everyone,
Evidentally I didn't have much luck reinventing the wheel (sorry..), so now I'm going to go back to square one, get rid off the shorter version and purchase the correct Ohlins i should have gotten in the first place.
Then really research rubber..(Thanks for the link,Rucksta)
Appreciate the thoughts,great info.
Ed
I have a 1978 R100RS and am 5'8" on a goo dday when I stand up straight. my Corbin is pretty low though...which helps I am sure. BUT I love the way the thing handles stock. I agree you should go back to the right length shock and set the front sag and rear sag perfectly and I suggest Metzeler Lasertehcs as I love them and almost every R100RS I see has them so they must be a wise tyre choice for the Airhead RS.
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Old 10-15-2010, 09:21 PM   #10
Hawk Medicine
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Sniper-X has it pretty much right.

Sell the short shocks and replace then with either stock length (13") or either the Olins 13.3" or 13.5" in shocks. I like 13.5 inchers myself but either of the longer ones will liven up the handling. It ends up being he same thing as swapping on an 18" front wheel.

Once you've done that, you have the option of stiffening the frame (I have the stuff.) or leaving it stock and you'll at least want to rebuild the front end and check the steering head bearings.

LazerTechs are very good tires. (I'm running my first set and they're pretty nice .) but I'm not convinced that they're much better than the old Lazer/ME88 combo but they aren't cheap and they do wear faster. I'm going to try Battleaxes next time.

By the way...

I have loooked very closely at Nella's photos and I think his fork swap probably lowered the front end, thereby waking the bike up. Any comments Nella?

Lastly: You can get some pretty wide tires on an Airhead if youre willing to put up with removing the rear drive to change tires but OTOH, if you're gonna ride your bike on the road, thats something that you might want to think about. Twice!
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Old 10-16-2010, 05:59 AM   #11
nella
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I measure my Ohlins at ~13.3" eye-to-eye. I'm running a top and bottom triple clamp that only has 30mm of offset on those GSXR forks and the net effect of all that is much quicker steering than stock and the front end is lower. However, nothing I have done in very spirited riding has shown me that the front is is too low. That's not to say a better rider wouldn't be grinding the valve covers, it's just not an issue with my setup and riding.

I'm running the widened rear Lester with a 140/80 Battlax and the front Lester has a 100/90 on it.

There are no issues with straight line stability or any wobbles or other handling issues with what I have done. My last motorcycle before this one was a 900 Monster that had been modified with an Ohlins shock on the rear and was set up for track day use. After the suspension and brake mods, the /7 is just as fun for me.




Quote:
Originally Posted by mymindsok
Sniper-X has it pretty much right.

Sell the short shocks and replace then with either stock length (13") or either the Olins 13.3" or 13.5" in shocks. I like 13.5 inchers myself but either of the longer ones will liven up the handling. It ends up being he same thing as swapping on an 18" front wheel.

Once you've done that, you have the option of stiffening the frame (I have the stuff.) or leaving it stock and you'll at least want to rebuild the front end and check the steering head bearings.

LazerTechs are very good tires. (I'm running my first set and they're pretty nice .) but I'm not convinced that they're much better than the old Lazer/ME88 combo but they aren't cheap and they do wear faster. I'm going to try Battleaxes next time.

By the way...

I have loooked very closely at Nella's photos and I think his fork swap probably lowered the front end, thereby waking the bike up. Any comments Nella?

Lastly: You can get some pretty wide tires on an Airhead if youre willing to put up with removing the rear drive to change tires but OTOH, if you're gonna ride your bike on the road, thats something that you might want to think about. Twice!
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Old 10-16-2010, 02:39 PM   #12
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Quote:
Originally Posted by nella
I measure my Ohlins at ~13.3" eye-to-eye. I'm running a top and bottom triple clamp that only has 30mm of offset on those GSXR forks and the net effect of all that is much quicker steering than stock and the front end is lower. However, nothing I have done in very spirited riding has shown me that the front is is too low. That's not to say a better rider wouldn't be grinding the valve covers, it's just not an issue with my setup and riding.

I'm running the widened rear Lester with a 140/80 Battlax and the front Lester has a 100/90 on it.

There are no issues with straight line stability or any wobbles or other handling issues with what I have done. My last motorcycle before this one was a 900 Monster that had been modified with an Ohlins shock on the rear and was set up for track day use. After the suspension and brake mods, the /7 is just as fun for me.
Personally, if I were going to quicken up steering I would do it by raising the back instead of lowering the front for ground clearance. I would also not run that much bigger of a back tire. Wide rear tires really get into cornering clearance a lot. The wider the rear tire, the more the rear contact patch moves to the inside of center when leaned over. That phenomenon makes you have to lean the bike over even further to make the same line thusly reducing cornering clearance. Plus that wider rear tire probably slows down your steering. Your robbing Peter to pay Paul.

I run pretty much stock geometry. I get into enough tank slappers as it is when I ride my airheads hard. It's usually from the front wheel coming back down a bit crooked or landing from a jump a bit sideways. My airheads have always shook their heads every once in a while right when I straighten them up out of a corner. All bikes do that a bit but when I have raised the back of my beemers they start doing it a bit too much!
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