Author Topic: MK5 front fork springs  (Read 3501 times)

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Offline hoejmark

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MK5 front fork springs
« on: 23 Oct 2015 at 14:48 »
Hi All
I am starting to rebuild a MK5, and have a question about the springs that was fittet into the front forks.
The cross section of the wire gauge are round , but I can read in The Spannerama Book, that the cross section of the wire should be square.
As far as I remember, the wire type of the springs in my MK3Sport are square.
The dimensions of the springs are:
Length 265 mm
Inside diam. 26,0 mm
Outside diam. 42 mm
Wire diam. 8,4 mm
Number of windings are 24

Does somebody out there know about these springs, are they of Douglas manufacture for Mark models, or do you any idea about how they will function?

Hoejmark
B29
T6
MK3Sport
Mk5

Offline Doug

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Re: MK5 front fork springs
« Reply #1 on: 25 Dec 2015 at 04:25 »
Hello Soeren,

Looks like the response is "no"!    :)

The sources that I have show they had the square wire springs from at least July 1947 onward (T35/Mark 1), so it is quite likely they only ever used the square section springs. While it is possible they used round wire on early prototypes, more likely these are a replacement set of springs that serendipitous fit in the fork tubes. Another possibility is they are aftermarket pattern springs made for the Douglas, but given that factory spares were available from Graeme Brown through the 1980s, I cannot see it being economical for someone else to produce them. Perhaps those using and abusing their Dougies through the sixties, seventies, and eighties could comment if aftermarket springs (with round wire) were available.

Without knowing the spring rate of the originals or yours, it would be hard to say how well they would work. There are formulas on the internet for calculating spring rates of coils springs with round wire, but not sure if you will find one for square. You would have to make a guess as to the spring material, or just use the same material in both cases for a relative comparison rather than an absolute value. Die springs use square wire, so someone ought to have a formula.

The LDMCC Spares have (had?) new springs for the Marks series in stock.

Regards,
Doug

Offline hoejmark

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Re: MK5 front fork springs
« Reply #2 on: 26 Dec 2015 at 15:19 »
Hello Doug.
Thanks for your reply.
Before dismantling, the front fork suspension was too stiff -I think - compared to my MK3 Sports.
Later I have realised that there were severe rust on the pivot bolts in the front end of the reaction links, that stiffened up the system, so maybe the round wire springs will turn out be not that bad at all.
The total distance between the windings indicates the total displacement of the springs to be app. 60 mm. Assuming the length ratio of the inner link to the leading link to be 1:3, the outcome should be a total up and down displacement of the front wheel to be abt. 180 mm. I have no expierence on this, but doesn't it sounds reasonable?
The next to do, I believe, is to reassemble the front fork, to test out the stiffness of the suspension. My MK3 Sports has the tapered springs and is perhaps a bit soft in the front wheel suspension.
Regards
Soeren

Offline Doug

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Re: MK5 front fork springs
« Reply #3 on: 27 Dec 2015 at 03:44 »
It has been so long since I have had the springs out of my Mark 3 Sports, that I have no recollection what the total spring travel might have been. The only set I have are in the fork. Douglas claimed six inches of wheel travel, but like their sales demonstration of riding on and off a four inch curb, it is not something I have ever put to the test. But I do agree that the front suspension on mine is also a bit soft in my opinion. It could also use more effective dampening, but having said that it handles quite well.

-Doug

Offline eddie

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Re: MK5 front fork springs
« Reply #4 on: 27 Dec 2015 at 13:12 »
Doug,
          Re: Front fork damping.  I got the same impression with the front forks on my Comp. Fitting a light springs to the washers that pass for non return valves to keep them against the inner pistons transformed the action of the forks - both with regard to the amount of damping and also the fact that the damping was evident right from the start of the rebound. This was most noticeable on bumpy uphill climbs where previously the forks would start to extend without damping, then the damping would come in, wanting to lift the front wheel, often resulting in a loss of steerage!
  Regards,
               Eddie.

Offline hoejmark

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Re: MK5 front fork springs
« Reply #5 on: 30 Dec 2015 at 14:13 »
Eddie
I use SAE 50 oil in the MK3sports forks, to get reasonable damping. If I place a weak spring below the damper disc do I then have to use a lover SAE grade of oil for damping?
Soeren

Offline eddie

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Re: MK5 front fork springs
« Reply #6 on: 30 Dec 2015 at 17:13 »
Soeren,
            When I modified my forks, it was pure experimentation, so I started off with 20/50 engine oil. With the springs fitted, the damping was so much improved that I didn't try any other grades of oil. The thinking behind using multigrade was that the viscosity would remain more constant over a greater temperature range - this has proved to be the case, but in mid winter 10/40 grade would probably give a softer ride. From the days when I rode motocross, I soon found that better dampers with thin oil gave better handling than using thicker oil to obtain more damping. The thicker grade oil tends to froth rendering the damping ineffective.

    Regards,
                 Eddie.