Hey guys,

I've been looking at substituting the stock front end on my LT for the GSXR forks and front wheel. As I feel this would upgrade the suspension and braking of my K bike significantly. However the key element to make this work would be a custom triple tree. Look at past peoples attempts I've seen that the CognitoMoto triple tree (https://cognitomoto.com/collections/triple-clamps_handlebars_controls/products/gsx-r-fixed-offset-triple-tree-conversion-30mm-45mm-50mm-55mm-60mm?variant=26788597441) would fit on my K bike. 

However I am an extreme noob and have no idea how to configure the triple tree for example I don't know what 'offset' is. Along with this how does this fit onto the bike which part turns lol as I know that it needs bearings. 

Basically I just need it all explained. I have read other peoples attempts but none go in enough detail for me. 

Thanks,
DBRMN

The offset is how far forward the centre line of the forks is in relation to the centre line of the steering stem and the Suzuki forks are about 40mm too far back so that the fork leg hits the frame and tank ans severely limits the turning of the steering.
To rectify this you need to make new triple clamps with the correct forward offset. Sounds easy so far but it's not at all easy. To get an engineering place to make them will be expensive, very expensive or you will need a quality milling machine and the expertise to use it plus heat treatment to strengthen the alloy.
I would strongly advise to use a complete K1100 front end and replace the bushes and then use a K1200RS wheel and brakes adapted to the forks. It's been done before and have a search around as TWB has done it and there is a write up on it here somewhere.

Hello chaps, have a look at this link. I have also thought about fork upgrades and found this a little while ago. Cheers, Dave.
https://cognitomoto.com/products/gsx-r-fork-on-bmw-r50-r60-r75-r90-frame-conversion-stem

Oops, sorry DBRMN, just seen that your link is the same, apologies. Dave.

RicK G wrote:The offset is how far forward the centre line of the forks is in relation to the centre line of the steering stem and the Suzuki forks are about 40mm too far back so that the fork leg hits the frame and tank ans severely limits the turning of the steering.
To rectify this you need to make new triple clamps with the correct forward offset. Sounds easy so far but it's not at all easy. To get an engineering place to make them will be expensive, very expensive or you will need a quality milling machine and the expertise to use it plus heat treatment to strengthen the alloy.
I would strongly advise to use a complete K1100 front end and replace the bushes and then use a K1200RS wheel and brakes adapted to the forks. It's been done before and have a search around as TWB has done it and there is a write up on it here somewhere.

So on the link I provided would I select 45mm offset to eliminate the steering problem that you mention? I know it would be far easier but i really want the look of chunky forks which the K1100 doesn't have if you know what I mean? I'm probably being stupid but if I purchase the triple tree from CognitoMoto it should work I just need to work out what offset i need. Please check the link though and tell me what you think. Thanks, DBRMN

bigdavergs wrote:Oops, sorry DBRMN, just seen that your link is the same, apologies. Dave.

No it is a different link I hadn't seen this so thanks. My only worry is like RicK G said the offset will be 40mm to short and I don't think the conversion stem allows for this. What do you think?

Hi again, I was just going to buy the item and see how it fitted. There's currently a K100 with R1 forks attached which looks to be ok. I think it's on Oz Gumtree. You could always contact the seller and ask him how that set up works. Sorry, can't be much help otherwise. Regards, Dave.

bigdavergs wrote:Hi again, I was just going to buy the item and see how it fitted. There's currently a K100 with R1 forks attached which looks to be ok. I think it's on Oz Gumtree. You could always contact the seller and ask him how that set up works. Sorry, can't be much help otherwise. Regards, Dave.

Would you mind sending me the link to that? Also let me know how it goes if you do try the conversion I'd love to know!

Thanks
DBRMN

I  have been down this road trying to fit upside down forks (KYB from a Suzuki RM 250) to a K75. From memory the offset had to be at least 40mm but I would have gone to 50mm to get extra lock for off road work. These guys have done the hard yards so I would be asking them about it. They make a stem for a K100 so they would know the right offset.
You have stirred my interest again so do get back with the results please.

Honestly - just drop a pair of progressive springs in there. I can guarantee that it will feel like a new bike, but you'll need the RamShok (or equivalent) at the rear too. I spent £400 upgrading the suspension on LFB and it was like someone had given me a new bike with a perfect replica of the old scratches on it. I'm in agreement with something Rick said about the brakes quite a while ago: the Brembos are adequate and have the advantage of not overwhelming the front tyre with braking force.

RicK G wrote:I  have been down this road trying to fit upside down forks (KYB from a Suzuki RM 250) to a K75. From memory the offset had to be at least 40mm but I would have gone to 50mm to get extra lock for off road work. These guys have done the hard yards so I would be asking them about it. They make a stem for a K100 so they would know the right offset.
You have stirred my interest again so do get back with the results please.

Ok thank you for your help because I think provided this Triple Tree fits it should be a very easy conversion. I'll keep this thread updated but it will most likely be after new years.

Dai wrote:Honestly - just drop a pair of progressive springs in there. I can guarantee that it will feel like a new bike, but you'll need the RamShok (or equivalent) at the rear too. I spent £400 upgrading the suspension on LFB and it was like someone had given me a new bike with a perfect replica of the old scratches on it. I'm in agreement with something Rick said about the brakes quite a while ago: the Brembos are adequate and have the advantage of not overwhelming the front tyre with braking force.

Yeah that was my first thought however my view is more 80% cosmetics 20% performance. For example any K bike cafe racer fan will have seen this thread by Mike Flores, more specifically his mock up with the GSXR front end and I just fell in love. The performance element is just to persuade myself its worth the money and effort . http://www.motobrick.com/index.php/topic,8807.0.html

You can also read about an USD fork conversion at this link: http://k75projeckt.wordpress.com/

robmack wrote:You can also read about an USD fork conversion at this link:  http://k75projeckt.wordpress.com/

Yeah I've seen this guys project before he has done an amazing job but seems to use a lot of modifications and isn't very specific on the dimensions and details of the modifications. Maybe to someone with more experience these mods are obvious but not to a noob like me lol.

https://www.gumtree.com.au/s-ad/norman-park/motorcycles/1987-bmw-k100rs-cafe-racer-custom-/1154561402

Hi again, this is the link to the K for sale with R1 forks. Dave.

Quick question does anyone know the stock offset of a 1991 K100LT ABS triple tree?

Thanks
DBRMN

I have a set of triple clamps in my garage, damn it, I know I do, but bugger if I can find them!
Here's what I have from various interwebtron sources. Perhaps what you seek is in there somewhere.

Pivot Stem:
Length from top of lower triple clamp to top of threads: 176mm
Length from top of lower triple clamp to where tapering begins: 128mm
Diameter (to fit inside bearings): 28mm
Steering head length: 165mm

Rake & Trail:
27.5°/3.98 in (10.1 cm)

~26 degree head angle, and ~2.9" fork offset

Two Wheels Better wrote:I have a set of triple clamps in my garage, damn it, I know I do, but bugger if I can find them!
Here's what I have from various interwebtron sources. Perhaps what you seek is in there somewhere.

Pivot Stem:
Length from top of lower triple clamp to top of threads: 176mm
Length from top of lower triple clamp to where tapering begins: 128mm
Diameter (to fit inside bearings): 28mm
Steering head length: 165mm

Rake & Trail:
27.5°/3.98 in (10.1 cm)

~26 degree head angle, and ~2.9" fork offset

You see I'm not entirely sure I know what I'm looking for but 2.9" fork offset is equal to 73.66mm which to me sounds like too much but them again I have no idea what it should be. I trust you but I think I just need someone to confirm it for me to really believe it lol 


This i the measurement I'm looking for if anyone was wondering.

DBRMN wrote:

Two Wheels Better wrote:I have a set of triple clamps in my garage, damn it, I know I do, but bugger if I can find them!
Here's what I have from various interwebtron sources. Perhaps what you seek is in there somewhere.

Pivot Stem:
Length from top of lower triple clamp to top of threads: 176mm
Length from top of lower triple clamp to where tapering begins: 128mm
Diameter (to fit inside bearings): 28mm
Steering head length: 165mm

Rake & Trail:
27.5°/3.98 in (10.1 cm)

~26 degree head angle, and ~2.9" fork offset

You see I'm not entirely sure I know what I'm looking for but 2.9" fork offset is equal to 73.66mm which to me sounds like too much but them again I have no idea what it should be. I trust you but I think I just need someone to confirm it for me to really believe it lol 


This i the measurement I'm looking for if anyone was wondering.

So I found this chat from past forum threads:

https://servimg.com/view/17702881/63

However i don't understand the chart nor which measurement is the one I need.

DBRMN wrote:

DBRMN wrote:You see I'm not entirely sure I know what I'm looking for . . . 
. . . which to me sounds like too much but them again I have no idea what it should be.. . .
I trust you. . .  lol  

. . . However i don't understand the chart nor which measurement is the one I need.

What I'm interested in is how you determine a measurement "sounds like too much" when you also assert that you have no idea about the subject. Are you active in politics or are you remote viewing? You've also used the word trust Reaganesquely—a nostalgic touch. 

The standoff for your bike in that chart is 2.3 inches/56.0mm. I used Google Translate. Careful! That chart might not be trustworthy either. For one thing, its color scheme really isn't all that pleasing.
I'm probably not trustworthy either. It's a jungle out here.

Laitch wrote:

DBRMN wrote:

DBRMN wrote:You see I'm not entirely sure I know what I'm looking for . . . 
. . . which to me sounds like too much but them again I have no idea what it should be.. . .
I trust you. . .  lol  

. . . However i don't understand the chart nor which measurement is the one I need.

What I'm interested in is how you determine a measurement "sounds like too much" when you also assert that you have no idea about the subject. Are you active in politics or are you remote viewing? You've also used the word trust Reaganesquely—a nostalgic touch. 

The standoff for your bike in that chart is 2.3 inches/56.0mm. I used Google Translate. Careful! That chart might not be trustworthy either. For one thing, its color scheme really isn't all that pleasing.
I'm probably not trustworthy either. It's a jungle out here.

Honestly I just don't know how this is not somewhere on the internet that I can find and now I'm getting different numbers. I'm sorry if I sound too much like a noob but I'm 16 so years of experience are pretty low   .

DBRMN wrote:
Honestly I just don't know how this is not somewhere on the internet that I can find and now I'm getting different numbers. I'm sorry if I sound too much like a noob but I'm 16 so years of experience are pretty low   .

You've been at this bike since October so so you're not exactly still wrapped in swaddling clothes.

When you want to take advantage of being an inexperienced 16-year old, it is good strategy to not express contrary opinions about information given to you when you don't know what you're talking about, otherwise, you start sounding like an adult.

RickG gave you a lot of information at the beginning of the month. Now, you've got the prettily colored chart with info on it that has been translated for you. I'd say you're on well your way to somewhere.

Laitch wrote:

DBRMN wrote:
Honestly I just don't know how this is not somewhere on the internet that I can find and now I'm getting different numbers. I'm sorry if I sound too much like a noob but I'm 16 so years of experience are pretty low   .

You've been at this bike since October so so you're not exactly still wrapped in swaddling clothes.

When you want to take advantage of being an inexperienced 16-year old, it is good strategy to not express contrary opinions about information given to you when you don't know what you're talking about, otherwise, you start sounding like an adult.

RickG gave you a lot of information at the beginning of the month. Now, you've got the prettily colored chart with info on it that has been translated for you. I'd say you're on well your way to somewhere.

Well my principle is cautious is always better to avoid making a death trap even if that means questioning everything. And being inexperienced does not make me incapable of evaluating information. Thanks for your replies I'll keep this thread updated.

bigdavergs wrote:https://www.gumtree.com.au/s-ad/norman-park/motorcycles/1987-bmw-k100rs-cafe-racer-custom-/1154561402

Hi again, this is the link to the K for sale with R1 forks. Dave.

Thank you, I sent him a message so I'll update this thread if he replies.

Bon voyage!

As you say, the problem about these cafe racer modifications is that most are cosmetic, but dramatically reduce performance.
You need to consider the chassis geometry before replacing a fork. A GSXR one is more rigid*, but also much more shorter than the original one; the offset is different, the stanchion diamaeter also so you won't be able to turn your bar more than a few degrees before it strikes the tank.
As it's shorter, if your front wheel does not hit the radiator, you'll have to face a very unstable behavior of the front end, and to cope with the reduced angling capacity of your bike.

*So what? Who cares about it, if the rest of the bike is not rigid and if you're not driving only at hi speed in curves?

An easy and efficient improvement is to fit a K11 fork, with new springs (not stiffer, not progressive as the original ones are progressive, just springs designed for the new weight of the bike, so probably softer!).
If you want an excellent and adjustable front end, use M1R tubes , stanchions and hydraulic on the K11 clamps. The K11 fork is derivated from this fork, and it will be coherent with your bike ( same era), where a USD fork looks weird on a 80's motorcycle.

jbt wrote:As you say, the problem about these cafe racer modifications is that most are cosmetic, but dramatically reduce performance.
You need to consider the chassis geometry before replacing a fork. A GSXR one is more rigid*, but also much more shorter than the original one; the offset is different, the stanchion diamaeter also so you won't be able to turn your bar more than a few degrees before it strikes the tank.
As it's shorter, if your front wheel does not hit the radiator, you'll have to face a very unstable behavior of the front end, and to cope with the reduced angling capacity of your bike.

*So what? Who cares about it, if the rest of the bike is not rigid and if you're not driving only at hi speed in curves?

An easy and efficient improvement is to fit a K11 fork, with new springs (not stiffer, not progressive as the original ones are progressive, just springs designed for the new weight of the bike, so probably softer!).
If you want an excellent and adjustable front end, use M1R tubes , stanchions and hydraulic on the K11 clamps. The K11 fork is derivated from this fork, and it will be coherent with your bike ( same era), where a USD fork looks weird on a 80's motorcycle.

Hopefully though with the triple tree I want to buy has an increased offset which would eliminate the limited turning capacity. However you make very good points about the chassis geometry that was my main concern and especially if it makes the front end unstable. 

Sorry quick question what brand of bike is a K11 is it Suzuki? and what are M1R tubes?

DBRMN wrote:

Sorry quick question what brand of bike is a K11 is it Suzuki? and what are M1R tubes?

K11 is for BMW K1100LT and K1100RS bikes. All the 16 valves engine and Paralever K (so including the K100RS1 and the K1) use the same fork, made by Marzocchi, using the basis of the Marzocchi M1R fork, a famous race fork in the mid-80's.
The early K100RS1 and K1 were using a fork with traditionnal damper inside, close to the ones used in the M1R, but with no external settings available on spring preload and hydraulics.  But tubes and stanchions are the same.
Then, the dampers were changed for a cartridge system (around 08/1990 in suppose), than does not allows to modify the compression or rebound settings as easily as it was with the traditionnal system.
On these forks, one tube is in charge of the compression hydraulic dampening, the other is for rebound. The springs are the same, and particularly weak on heavy models (k1100LT). They are progressive by design, so changing them for progressive springs with the same characteristics won't change anything.
Consider that if you remove parts for your racer, the weight factor is modified also and it's nonsense to mount stiffer springs whereas you need softer.

Another issue is the modification of the offset. It determines the trail  which is the value that gives stability to your bike. The more offset you'll have, the less trail you'll get, the less stable it will be. If you add a shorter fork lenght, which reduces also the angle of the steering column (the rake), you'll get a very unstable motorcycle: bar shaking is waiting for you.

DBRMN wrote:

You see I'm not entirely sure I know what I'm looking for but 2.9" fork offset is equal to 73.66mm which to me sounds like too much but them again I have no idea what it should be. I trust you but I think I just need someone to confirm it for me to really believe it lol 


This is the measurement I'm looking for if anyone was wondering.

I knew I had a set of triple clamps! Here's piccie with a tape measure whacked up against it as in your illustration.


It is approximately 57mm or 2-1/4" from centre of fork tube to centre of steering stem. The K1100RS Marzocchi triple clamps are less "relaxed" and therefore provide a  slightly quicker steering feel when used with K75/K100-8V Monolever final drive/swingarm, otherwise about the same when used with K100-16V/K1100RS Paralever final drive/swingarms.

Keep in mind that if you're looking for "quicker" steering or just something that might fit and look 'cool' (like a GSXR1000 or 1100) the further forward the fork tubes sit from the steering pivot then the more relaxed or slow steering the feel will be. The illustration you provided us potentially shows a quite quick steering arrangement, all other dimensions like steering head angle, fork length, etc., being equal.

Two Wheels Better wrote:

DBRMN wrote:

You see I'm not entirely sure I know what I'm looking for but 2.9" fork offset is equal to 73.66mm which to me sounds like too much but them again I have no idea what it should be. I trust you but I think I just need someone to confirm it for me to really believe it lol 


This is the measurement I'm looking for if anyone was wondering.

I knew I had a set of triple clamps! Here's piccie with a tape measure whacked up against it as in your illustration.


It is approximately 57mm or 2-1/4" from centre of fork tube to centre of steering stem. The K1100RS Marzocchi triple clamps are less "relaxed" and therefore provide a  slightly quicker steering feel when used with K75/K100-8V Monolever final drive/swingarm, otherwise about the same when used with K100-16V/K1100RS Paralever final drive/swingarms.

Keep in mind that if you're looking for "quicker" steering or just something that might fit and look 'cool' (like a GSXR1000 or 1100) the further forward the fork tubes sit from the steering pivot then the more relaxed or slow steering the feel will be. The illustration you provided us potentially shows a quite quick steering arrangement, all other dimensions like steering head angle, fork length, etc., being equal.

Perfect this was exactly what I was looking for thank you so much. I think I'm going to try and keep the offset as stock as possible but I know I will have to increase it a bit because the forks are so big, which hopefully doesn't make the steering too slow. Thanks again for you reply much needed.
DBRMN

Don't forget that the wheel axle is not always in line with the fork tubes. There's often an additionnal offset at this place! Big offset at the triple tree + offset at the wheel axle may result in a trail = zero.

jbt wrote:Don't forget that the wheel axle is not always in line with the fork tubes. There's often an additionnal offset at this place! Big offset at the triple tree + offset at the wheel axle may result in a trail = zero.

Oh yeah thats a good point I forgot about axle offset, I believe though that K100's are in line and the same with the GSXR front end but I will have to research this thank you for reminding me.

In some cases, I've seen inverted forks, placing the wheel axle behind the tubes. This may compensate a huge offset at the triple clamp.

Two Wheels Better wrote:I knew I had a set of triple clamps! Here's piccie with a tape measure whacked up against it as in your illustration.


It is approximately 57mm or 2-1/4" from centre of fork tube to centre of steering stem.

Keep in mind that if you're looking for "quicker" steering or just something that might fit and look 'cool' (like a GSXR1000 or 1100) the further forward the fork tubes sit from the steering pivot then the more relaxed or slow steering the feel will be. The illustration you provided us potentially shows a quite quick steering arrangement, all other dimensions like steering head angle, fork length, etc., being equal.

I came across this while researching what had been on USD conversions on K75/100 - I've done conversions on multiple Triumph models using different donor forks, as well on my TL1000S and my Mrs SV650

That graphic illustration depicting what defines Offset measurement is actually mine! I created that exact graphic LONG time ago!  

But (most respectfully - this is only my first post after intro) the post statement quoted above is actually completely converse to what happens as you increase or decrease offset: 
As you INCREASE OFFSET (forks move further away from the steering stem) that DECREASES the TRAIL and that makes the steering Quicker (i.e. requires less input to turn in) 
And if you SHORTEN OFFSET (forks closer to the stem) that INCREASES the TRAIL and that makes the steering SLOWER (requires more input to make it turn in). 

I wrote this article to help explain geometry on a Triumph Forum - http://www.triumphrat.net/sprint-forum/102102-the-definitive-usd-fork-conversion-for-sprints-thread.html#post1195267
(You'll find my little graphic from above there too!  ) 

So if the OEM offset is 57mm, that graphic is 32mm (I know this because I know what its from because I created it) - therefor a swap to that triple clamp would make the steering a little slower. 

In real World however there are other factors - on my Triumph for example stock offset was 45mm and my new triples are 30mm - yet it is NOT difficult to turn in and actually responds extremely well to corrections even mid-corner. So Trail is not necessarily the complete story. 

Hope that is helpful

D'Ecosse wrote:In real World however there are other factors - on my Triumph for example stock offset was 45mm and my new triples are 30mm - yet it is NOT difficult to turn in and actually responds extremely well to corrections even mid-corner. So Trail is not necessarily the complete story.

That would seem to shorten the wheelbase length which might have some slight quickening affect on steering response.

Here's an uncomplicated illustration that helped me to understand some of these concepts. If it's incorrect, please chime in.

Laitch wrote:That would seem to shorten the wheelbase length which might have some slight quickening affect on steering response.

Yes there would be some effect from that too. 
These factors cannot be taken in isolation - there are obviously lots of factors in play. So you can't just say 'the shorter wheelbase' is what makes it quicker (but I don't think that's what you are saying) - but it helps counter the increase in trail. But actually even the trail would not have (in my case) been a significant net increase - the forks being shorter made the rake steeper and that contributes a negative (quantitatively) effect on the trail number. And of the course the rake by itself also makes for a shorter wheelbase, in addition to that of the difference from the offset itself. 
Note that for what seems like ever, it is common knowledge that raising the forks up through the triples (i.e. making them effectively shorter) makes the bike have quicker steering. Primarily it makes the rake steeper, (shortening the trail) but also changes the weight distribution. 
My main point was just to offer that the effect of offset alone (all other things being unchanged - which is almost impossible to do) is opposite to what was stated.

D'Ecosse wrote:

Two Wheels Better wrote:I knew I had a set of triple clamps! Here's piccie with a tape measure whacked up against it as in your illustration.


It is approximately 57mm or 2-1/4" from centre of fork tube to centre of steering stem.

Keep in mind that if you're looking for "quicker" steering or just something that might fit and look 'cool' (like a GSXR1000 or 1100) the further forward the fork tubes sit from the steering pivot then the more relaxed or slow steering the feel will be. The illustration you provided us potentially shows a quite quick steering arrangement, all other dimensions like steering head angle, fork length, etc., being equal.

I came across this while researching what had been on USD conversions on K75/100 - I've done conversions on multiple Triumph models using different donor forks, as well on my TL1000S and my Mrs SV650

That graphic illustration depicting what defines Offset measurement is actually mine! I created that exact graphic LONG time ago!  

But (most respectfully - this is only my first post after intro) the post statement quoted above is actually completely converse to what happens as you increase or decrease offset: 
As you INCREASE OFFSET (forks move further away from the steering stem) that DECREASES the TRAIL and that makes the steering Quicker (i.e. requires less input to turn in) 
And if you SHORTEN OFFSET (forks closer to the stem) that INCREASES the TRAIL and that makes the steering SLOWER (requires more input to make it turn in). 

I wrote this article to help explain geometry on a Triumph Forum - http://www.triumphrat.net/sprint-forum/102102-the-definitive-usd-fork-conversion-for-sprints-thread.html#post1195267
(You'll find my little graphic from above there too!  ) 

So if the OEM offset is 57mm, that graphic is 32mm (I know this because I know what its from because I created it) - therefor a swap to that triple clamp would make the steering a little slower. 

In real World however there are other factors - on my Triumph for example stock offset was 45mm and my new triples are 30mm - yet it is NOT difficult to turn in and actually responds extremely well to corrections even mid-corner. So Trail is not necessarily the complete story. 

Hope that is helpful

This actually makes a lot of sense from a physics point of view if you regard turning moments, increasing distance from pivot (increase offset) reduces force needed (to turn).

I would be interested on your opinion on a GSXR fork swap on a K100, there are quite a few examples out there my favourite being the swap made by Mike Flores a famous K100 Cafe racer.

Thanks,
DBRMN

Laitch wrote:

D'Ecosse wrote:In real World however there are other factors - on my Triumph for example stock offset was 45mm and my new triples are 30mm - yet it is NOT difficult to turn in and actually responds extremely well to corrections even mid-corner. So Trail is not necessarily the complete story.

That would seem to shorten the wheelbase length which might have some slight quickening affect on steering response.

Here's an uncomplicated illustration that helped me to understand some of these concepts. If it's incorrect, please chime in.

Surely offset like D'Ecosse said would have no impact due to the parallel shift it makes? But I definitely agree with you about the angle change due to shorter forks. My assumption would be if you decreased the length of the forks but increased the offset enough there would be no change in wheelbase however a change in rake would always be inevitable

Not sure what you mean by 'parallel shift' - but offset certainly DOES make a difference. 

DBRMN wrote:I would be interested on your opinion on a GSXR fork swap on a K100, there are quite a few examples out there my favourite being the swap made by Mike Flores a famous K100 Cafe racer.

Well the two immediate concerns are fork length and the offset, which are going to affect things. 
A lot depends on what type of riding/bike classification you're looking to do. 
With street/sport the height may not be issue, indeed possibly advantage to get it lower at the front; if you want dual-purpose then you probably want more height (which then also aggravates the offset issue, as opposed to countering it) 
I did look at the bike project you mentioned last night - thought the bottom bushing in the steering head was extremely creative! 
And I have only just started looking into the BMW and haven't done ANY measurments but caught some posts suggesting the narrower fork spacing (and shorter offset) made for limited lock to lock arc? 
You would have to get honest opinion from him on how it handles. 
A lot of these mods end up being for aesthetic reasons than necessarily performance improvement. That's OK - you just need to understand what you're getting into & what trade-offs (if any) you might end up with. 

Since you're in UK there is an outfit called topyokes.uk that I believe would do a custom service should you wish to change the offset - so create somewhat of a hybrid between the BMW and Suzuki (or whatever other option you choose)

I just recognized another important factor:
OEM front wheel is 18", yes? 
So if you go to a 17" that goes a long way to recovering the effect of a shorter offset
Shorter offset would make trail longer; smaller diameter tire would have net shorter effect on trail. 
See the post I linked above for the math on how the respective parameters affect the end result 

It's important to take all the factors into consideration when determining what the net result is going to be. 
If having custom triples made it's probably not necessary to go to the OEM Steering Offset if you also plan on using a smaller wheel. 
And decide whether you really need the existing ground clearance. 
If a cafe-racer you might want to go as low as you can without scraping; for a trail/scrambler, you probably want to maintain at least standard. 

But take all the factors into consideration:
Fork length - the length itself is not a factor in the math - however the length of the forks (or their position in the triples) will have an effect on the rake angle (and weight bias). 
Having shorter forks (or positioned accordingly in the triples) will make the rake steeper, the trail shorter and the steering quicker.  
Offset - as already discussed - shorter offset will give net longer trail
Wheel diameter - smaller wheel will give net shorter trail. 
Note that a smaller diameter tire will also have a secondary influence in additionally changing the rake angle - but if you selected a fork length/position that yielded the same rake angle, just changing the wheel size will already bring back the trail. 

Shorter forks (more acute rake) -> shorter trail 
Shorter offset -> longer trail 
Smaller wheel -> shorter trail 

One other advantage re fork swaps - you are probably going to get a much better braking system. 
On my Mrs SV650 in order of reasons for doing, the performance of the suspension was actually only THIRD in line - preceded by 1) the brakes and 2) aesthetics. 
If fork PERFORMANCE is your primary criteria there is good argument for just upgrading springs and cartridges.