I have several forks apart on my bench right now, and it brings up some questions.

2 pair for a Paso 750, one pair from a 1991 907, and the third is supposed to be a 907 fork, but has a few minor differences (e-bay purchase) (both 907 forks are for a 300mm rotor and 4 piston caliper)

The 750 forks have 470mm long springs that are 5.00mm diameter wire. These are progressive wound. No spacer. 140mm travel.
The 907 forks have 276 mm long springs that are 4.75mm diameter wire. These are straight wound. A plastic spacer 245mm long on top. 125mm travel.

Of course, the 907 fork tubes are longer, and the innards appear to be the same (damper rods and such). Travel on the 907 is shorter by 15mm.

Springs by nature are progressive, but if they are long compared to the travel, the progressive nature is reduced.
750 - 470mm spring length / 140mm travel = 3.357 : 1 compression ratio
907 - 276mm spring length / 125mm travel = 2.2 : 1 compression ratio

So, with the much longer compression ratio, the 750 would benefit from a progressive wound spring more so than the 907's shorter spring.

So what does this all mean? I don't know. That's why I am asking you. Anyone up on suspension/spring theory? I just always assumed that longer springs are better, but maybe not?

So, a couple of questions to ponder:
- Would longer springs be more compliant than a shorter spring?
- Has anyone thrown a set of 750 springs into a 907 fork, or visa versa? (Adjusting the length with a top spacer of course)
- What length are the Hyper Pro progressive springs for a 907? Are they different from the 750 kit? (again, adjust for length with a spacer) They are different part numbers for 750,906 and 907. Could be different spring rates.

Just for reference, the ST2 springs I happen to have in the garage are 365mm. (right in between the two)

Of course, assuming the same amount of preload.

Cheers, Phil

Complicated subject with no one single correct answer. This is a good place to start

Ok, after a few days of pondering, and some reading supplied by G, I have come to this conclusion.

The 907 configuration of the M1R fork is a better setup than the 750 version.

- Straight wound coiled springs are indeed linear in their compression, (not progressive as I had previously stated) A leaf spring is more progressive, but a coiled spring doesn't get anywhere near the deflection to create that scenario. (my bad)
- You can have the same spring rate with a coil that has half the number of coils. Metallurgy has a lot to do with it, but spring design is also a big factor. (hence the 907 spring and 750 are similar spring rates, but the 907 spring is much smaller)
- A smaller spring has less weight, so your wheel and fork will react faster (better)
- A smaller spring has less volume in the fork. That volume is needed for oil and more important, air.
- Air is a very progressive spring, and is factored into the overall spring rate. The air volume is set by your oil level. If you run a straight wound spring, the air is your second spring making it progressive in combination. If you take up too much of your fork volume with a spring, the progressive nature may be too progressive, and may lock out on full compression. Anyone remember the air reservoirs that were put not he top of forks of the MX bikes in the early 80's?

So, a smaller spring (of correct spring rate and travel) is a better setup.
I would think that the 750 fork would benefit from a 907 spring (with appropriate preload spacer) The 750 fork may be too progressive with its smaller air/oil volume, and a progressive wound spring. The oil level for the 750 fork and the 907 should certainly be different. (160mm for the 907 fork, 180mm? for the 750?)

Anyone have pictures of a HyperPro spring set for a 750/906 or 907? They are different part numbers in their catalog, and I'm interested in what the differences are. Do HyperPro go with the less volume theory, and supply a new preload spacer tube?

Cheers, Phil

I found the instructions that came with my Hyperpro springs (P750). They say oil SAE 2.5, oil level 130mm (air gap), static sag 24mm (+-5).
I had written down; original spring 465mm, Hyperpro 500mm.
The Hyperpro kit does not come with a spacer. After checking the instructions again it is mentioned that original spacers should be reinstalled (and that the cap should be screwed on carefully as the spring preload can be approx 35-50mm).

Bock900R (Rudiger) had got himself a set of Hyperpro springs for his 907ie a year ago. He may still have the instructions to them.

I`ve checked the TUV documents of the Hyperpro springs and these list spring type and spring ident. number.
What`s weird is that this document lists a 906 88-90 and a 91. Both with different springs because for one it says M1R: 775mm (like the P750) and the other M1R: 795mm.
However the spring type for the 906 88-90 (and early 851) is the same as the one of the P750. The spring ident. number is not. I`m assuming that this means these models share the same springs. The different ident. number is the number of the complete kit and this consists of the springs and a bottle of fork oil. The P750 and 906 kit probably just come with a different weight oil (just like the WP kits for the 750S & 900SS see below)
The numbers of the 907ie springs are totally different.

That two length M1Rs are listed might not mean anything as many parts lists i.e. for brake pads or things like that list bikes like a Paso 350 or others that were never built.

I`ve been going through countless (TUV) documents to find some infos but the only thing I found is this from a WP homologation document.
It`s not really helping as it only lists the 907ie and for a proper comparison all springs should be from the same manufacturer. You can only compare the Sport & 900SS.
However, Da is outer diameter, Lo length, d wire thickness, n number of windings (fork oil and air gap)

Boy, you can sure tell it's winter.

Addendum, ad nauseam.

Of course simply dropping 907 springs into a 750 fork isn't straight forward. The spring was designed for 15mm less travel, so it will probably not work well at full travel in the 750 fork, and may take a set over time as it would be compressed passed the design limits, and its a short spring already. Adding some length on top may help, if you know the spring rate, and can get a cut section of spring to add to the top, but a longer spring with the same wire and wind, is a softer spring.

With the tapered bottom of the spring, getting a spring from the common sources isn't so easy. Best course of action is to simply buy the HyperPro kit, which indeed does have less coils than stock from the pictures I have seen so far. I know, its a long way to go to say, buy the only alternative that we have.

Anyone have a source for Hyperpro springs for less than $150 USD?

Cheers, Phil

Options from Hyperpro (Dutch)

From the US Rep for Hyperpro in asking about the differences between the 4 sets that they offer for the M1R fork.
Hi Phil,
Thank you for your interest in our products.
We are the importer and distributor for Hyperpro in the US. (He answered me on Christmas morning. Go figure. )

It is a tall order you are asking for I will try my best:

750 PASO - `88-`90 - SP-DU07-SSA 005 - 775 mm long fork - #0173 - oil SAE
2.5 - oil level 130 mm - OD 30.80 mm - L 505 mm - rate 4.7-12.6 N/mm

906 PASO - `88-`91 - SP-DU09-SSA 007 - 775 mm long fork - the same as 750
PASO but oil level only 120 mm

906 PASO - `88-`91 - SP-DU09-SSA 006 - 795 mm long fork - #0409 - oil SAE 10
- oil level 150 mm - OD 35.5 mm - L 520 mm - rate 6.7-17.9 N/mm

907 PASO - `92 - SP-DU09-SSA 008 - #0121 - oil SAE 7.5 - oil level 120 mm -
OD 35.3 mm - L 280 mm - rate 6.5-12.3 N/mm

Price for either spring set including the oil = $ 159.00 plus shipping of $
18.00

Please call the office in NJ at 732-786-9777 with your questions and/or
order.

Regards and Merry Christmas

Klaus Huenecke
EPM Performance Imports
Email: info@epmperf.com
Phone/ Fax: 732-786-9777
http://www.epmperf.com


I'm guessing that because Hyperpro sells into TUV countries, they have followed the stock spring lengths for easier qualification and approval.
Next, another supplier
Cheers, Phil

And from RaceTech
http://racetech.com/index.aspx
Straight rate springs. Recommend 15mm of preload.
Racetech has chosen a middle ground length of 340mm, similar to my ST2 springs, and offered a range of spring options. You could use these springs in any of the M1R forks with a proper spacer.

FRONT FORK SPRINGS for the 750
Stock Fork Spring Rate: 0.59 kg/mm(stock)
No recommended alternatives.

FRONT FORK SPRINGS for the 851 w/ M1R
Recommended Fork Spring Rate: 0.958 kg/mm (for a 230 lb. rider(me))
Stock Fork Spring Rate: 0.78 kg/mm(stock)

Available FRSP S3534 Series Springs are:

RT FRK SPR 34.7x340mm .80kg FRSP S3534080 $114.99
RT FRK SPR 34.7x340mm .85kg FRSP S3534085 $114.99
RT FRK SPR 34.7x340mm .90kg FRSP S3534090 $114.99
RT FRK SPR 34.7x340mm .95kg FRSP S3534095 $114.99
RT FRK SPR 34.7x340mm 1.0kg FRSP S3534100

FRONT FORK SPRINGS for the 907
Recommended Fork Spring Rate: 0.988 kg/mm (230 lb. rider)
Stock Fork Spring Rate: Call kg/mm(stock)

Available FRSP S3534 Series Springs are:

RT FRK SPR 34.7x340mm .80kg FRSP S3534080 $114.99
RT FRK SPR 34.7x340mm .85kg FRSP S3534085 $114.99
RT FRK SPR 34.7x340mm .90kg FRSP S3534090 $114.99
RT FRK SPR 34.7x340mm .95kg FRSP S3534095 $114.99
RT FRK SPR 34.7x340mm 1.0kg FRSP S3534100


So, for me, this is the direction I will be going, and cutting my own spacers.
Any other suppliers you may know of?

Cheers, Phil

Wirth, Wilbers, WhitePower, Promoto (which I think is the economic series from Wilbers)

Hyperpro listed above.
Progressive does not list any spring options for an M1R
Wirth does not list any springs on their site for the M1R.
Wilbers has a spring listed for the 907, and 851, but no details on the differences. Nothing for the Paso's http://www.wilbers-bmw-racing.de/pdf_fi ... 220213.pdf
White Power doesn't seem to offer just fork springs, but frankly, after 15 minutes trying to negotiate their site, I gave up. I never found any fork springs on their site.

Feel free to add any info if you find it. If you have a contact at one of these places, they may tell you differently, as Hyperpro did.

Cheers, Phil

I know Rick Tannenbaum at Cogent Dynamics(motocd.com) has done some work on 907 forks, he may be able to help with the 750.