Author Topic: Distorted frame?  (Read 15593 times)

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Offline KiwiJohn

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Distorted frame?
« on: 24 Feb 2006 at 02:58 »
My 80 Plus has been legal on the road for a few weeks now  but it seems to have a fondness for turning to the right.  I have repeated the straight edge alignment of the wheels but to no avail and I can only conclude that the frame and/or forks are distorted.

At some time in its life the front forks have been wacked right over to dent the petrol tank which also damaged the top cone piece of one fork tube.   There are signs of an attempt to repair this which does not look perfect but the fork legs do appear to be parallel according to every form of measure I can devise.  The top plate has been replaced with one that is slightly asymetrical to allow for the bent top cone.  I got a replacement plate from the LDMCC which is way out and it appears the Plus's may have had a different fork rake?

I guess I must bite the bullet and take it all to an expert to tbe pulled, twisted and generally tortured into alignment.

So the questions are:

1.  Do the Plus's have a more laid back rake?  What should it be?

2. What is the correct steering head inclination?

3. The rear wheel rim is wider than standard,  is this something to be concerned about?

Thanks.

P.S. I really must get this thing perfect in the next few months so that I can confidently show my face at the 100 year event in Australia.

Offline MrWright

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Re: Distorted frame?
« Reply #1 on: 01 Mar 2006 at 01:49 »
With out more info, you may want to examine the leading-links spring force at the front axial.  Your bike may measure correctly without a load (no person on the bike) and on the center stand, yet be skew on the road.  The simplest way to check the spring load on each arm is by removing the front wheel, placing a nut-bolt set in each hole and use a torque wrench at an angle.

Offline KiwiJohn

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Re: Distorted frame?
« Reply #2 on: 02 Mar 2006 at 21:43 »
Good suggestion MrWright, I will check that although I dont see how there could be much movement there with everything clamped between the leading links?

Offline Doug

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Re: Distorted frame?
« Reply #3 on: 02 Mar 2006 at 23:15 »
You will find the Douglas Radiadraulic leading link forks a lot stiffer than they look. The frame on the on the other hand is more limber than it looks!  Though for such a strong fork, I always wondered why they used such a cheap bit of sheet stock for the upper yolk. The fork was a favorite of the grass track boys, who broke up many a Mark model just to get the front forks. They had a lot of travel for the day (pre-telescopic forks), and held up well to the rough stuff. The main drawback was the lack of effective damping. 

Unless your front spindle is loose, both sides go up an down in unison without much worry of the wheel tipping side to side. If it is bent at the top, the wheel may permanently be tipped over to one side, yet pass the string test. Set the bike up vertical, and stand well off to the front and rear. Often you can sight if the wheels look 'out of plumb', or if something looks a bit odd. 

I've not seen figures for the rake or trail of the Plus models. The frame is different in other aspects to make them definitely Plus or Mark. And the forks are different too, in that the brake anchorage point is a inch further up the tubes than a Mark, the removable headlamp brackets, and stiffer springs (not taper ground.) But I will hazard a guess that the headstock geometry is the same, till proven otherwise!

-Doug
« Last Edit: 04 Mar 2006 at 04:04 by Doug »

Offline KiwiJohn

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Re: Distorted frame?
« Reply #4 on: 03 Mar 2006 at 05:56 »
Thanks Doug.

As far as I can tell comparing mine with the replacement top plate I got from the LDMCC the forks must have a lot more rake than the 'Marks', either that or my triple bracket thing is really bent which does not appear to be the case.  What is clearly bent is one of the sheet metal cones above the triple bracket, it has been damaged and partially repaired while a non-standard top plate has been made to compensate.  Bit of a dogs breakfast really and if I am going to get this bit right I need to know the angle between the axis of the central steering tube and the two fork tubes.

Considering the knock the forks took I will be getting someone to check the frame and it would be good to know what the steering head inclination should be, which on the Plus looks to be less than the rake of the fork tubes.

You mentioned damping, or lack of,  these forks have dampers in the bottom of the fork tubes, are these considered to be ineffective?  Nothing to damp the rear though.

Offline trevorp

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Re: Distorted frame?
« Reply #5 on: 03 Mar 2006 at 22:07 »
Just a thought the last time i rode mine i was suprised by the flywheel inertia had such an effect on handling very similiar to bmw im wondering if the effect is more than that


Offline Doug

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Re: Distorted frame?
« Reply #6 on: 04 Mar 2006 at 04:03 »
There are some primitive dampers in the front, but they do not seem to be terribly effective. Perhaps better than nothing, but they do tend to get a bit bouncy at the front. It does not seem to effect the handling too much, or one gets use to it. The stiffer Plus springs will require less damping by nature compared to the Mark series. 

No, the rear does not have any damping at all, but I find it so stiff running solo that it really does not need any. The Plus is supposed to have even stiffer torsion bars than the Marks series, so I do not think you will have much trouble at the back end. 

I have only ever noticed the torque reaction blipping the throttle in neutral, and it is not very pronounced. You can not discern it once underway; you can throw the bike into left or right handers will equal ease. It is amazing that they handle so well with such a limber frame, but it just goes to show that a low center of gravity can make all the difference.   

The steering stem not parallel to the fork tubes does not sound quite right. I am going to ask another Plus owner that I know if he can check his.

-Doug
« Last Edit: 04 Mar 2006 at 04:05 by Doug »

Offline Doug

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Re: Distorted frame?
« Reply #7 on: 09 Mar 2006 at 01:41 »
I checked the top plate on my Mark 3, just using metal rules and eyeballing the centers, and I measure the steering stem as about 1-9/32" back from the fork tubes. A friend did the same for his 90 Plus, and he got 1-3/8". Then I checked the top plate from a dismantled set of Mark 5 forks, again with scales but the access was better since I could easily remove it, and I got 1-5/16".

So I would say the Mark and Plus share the same distance from the fork tubes to the steering stem. The 1-5/16" I feel is a fairly accurate measure, and the 1-9/32 to 1-3/8" dimensions reasonable range either side for measuring in-situ on the Mark 3 and the 90 Plus. So if you are seeing a significant difference between the special plate on the bike and the replacement from the club, see which one is closer to 1-5/16" and this is likely the correct one to use.

The distance I measured between the fork tubes was 7-3/8".

-Doug

Offline KiwiJohn

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Re: Distorted frame?
« Reply #8 on: 09 Mar 2006 at 02:23 »
Thanks Doug,  I will take some more measurements.  It does seem odd though because if the forks were moved to make the fork tubes parallel with the stem this would move the front wheel back quite a bit towards the engine.

Do you know, were there different sized wheels fitted to different versions?

Offline Doug

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Re: Distorted frame?
« Reply #9 on: 10 Mar 2006 at 03:12 »
Quote
"...if the forks were moved to make the fork tubes parallel with the stem this would move the front wheel back quite a bit towards the engine."

True, but it is a leading link fork, so you still can have the sufficient trail even if the rake seems a bit steep.

Quote
Do you know, were there different sized wheels fitted to different versions?

An early Plus twelve page specifications data booklet the factory printed gives the following:
Front:
WM 1 x 21" rim
21" x 3" Dunlop straight ribbed
Rear:
WM 2 x 19" rim
19" x 3.25" Firestone, 90 Plus
19" x 3.25" Standard Universal, 80 Plus

Though later in the same data source it contradicts itself by quoting the 90 Plus as having the "Standard Universal" and the 80 Plus as having a "Standard"!  Later publicity material gives less detail, but still mentions the same sizes. 

The only options re- wheels I have seen mentioned are alloy rims and racing tyres, still of the same size mentioned above. 

-Doug

Offline KiwiJohn

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Re: Distorted frame?
« Reply #10 on: 10 Mar 2006 at 05:03 »
Thanks Doug,  I have 21" on the front.

I have a photocopied parts list which I believe is a "Bruce Main-Smith Retail Ltd" product of 1983.  Front forks are shown on Plate 24 and I have just thoroughly confused myself by placing a straight edge on the diagram and it shows the stem not parallel to the lower fork tubes!! 

I guess I can make the top plate however I like if I can just get some knowledgable advice on the effect of more or less rake on the fork tubes.  Would I be right in suspecting that more rake (i.e. leaning back further) would make for docile handling and vice-versa?

Offline Doug

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Re: Distorted frame?
« Reply #11 on: 11 Mar 2006 at 01:09 »
Quote
I have a photocopied parts list which I believe is a "Bruce Main-Smith Retail Ltd" product of 1983.  Front forks are shown on Plate 24 and I have just thoroughly confused myself by placing a straight edge on the diagram and it shows the stem not parallel to the lower fork tubes!! 

Artistic licence! The illustrated plate you mention is from the factory spares book. I have an original, and if I do the same, I get the stem and the lower fork tubes as parallel. Must be that cheap, knock-off copy you have! :) But really, I would not trust a spares illustration to be the last word on way or the other for that kind of thing.

Quote
Would I be right in suspecting that more rake (i.e. leaning back further) would make for docile handling and vice-versa?
Yes, leaning further back, at the top, will increase the trail and make the steering heavier and less inclined to change direction.
-Doug

Offline KiwiJohn

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Re: Distorted frame?
« Reply #12 on: 11 Mar 2006 at 05:53 »
 :D  Dont worry Doug,  I wont be using the spares illustrations for any accurate measurements!  However it is an interesting quirk that the diagram appears distorted in the same way my forks are! :lol:

Offline eddie

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Re: Distorted frame?
« Reply #13 on: 16 Mar 2006 at 19:31 »
:D  Dont worry Doug,  I wont be using the spares illustrations for any accurate measurements!  However it is an interesting quirk that the diagram appears distorted in the same way my forks are! :lol:
  John, I have access to some original factory literature that lists the parts that are different on a 'Plus' compared to the Mark series, and there is no mention of the top bridge plate being changed. Dougs comment that raking the forks at a shallower angle increases the trail is only true if the frame is modified to get the shallower angle. If you try to do it by raking just the legs of the forks (using a bridge plate with less lead), the result will be less trail because just the wheel will be moved forward. If your frame has been bent, it is not easy to check for twist when it is upright, but if you can lay the frame on its side and check accurately that the swinging arm pin is vertical - then the steering head should then lie in the horizontal plane( irrespective of the angle of rake).
                  Hope this is some help,    Eddie.

Offline KiwiJohn

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Re: Distorted frame?
« Reply #14 on: 16 Mar 2006 at 20:22 »
Thanks Eddie,  how accurate do I need to be for such checks?  Is a spirit level and plumb bob good enough?

Offline ChrisS

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Re: Distorted frame?
« Reply #15 on: 16 Mar 2006 at 22:16 »
reference 90 front forks. 
The plus series have a different bridge plate to the Marks and club spares keep both.

I have a factory GA of the front forks and the geometry is shown though not delineated as parallel

The pluses had a springs part number 38255 whilst the Marks had  spring part number 38246.

I have a number of Plus and Mark forks and they all have the same geometry, however there are differences as noted previously also the brake anchor bracket has a reinforcing strip under it.

Checking the alignment of the head stock can be done by passing a suitable sized bar through both the swinging arm pivot points and then checking that the steering head is perpendicular to this bar viewed from the front, again a suitable sized bar passed through the head stock with a couple of collars on it to locate in the bearings housings in the head stock help  The geometry can be checked with one of the cheap lasers available now or by using bubble levels.

If the bike has had a shunt the front down tubes may be bent steepening the rake, this is usually easily seen by eye. If they are not bent too badly and thats all thats wrong then its probably best to leave well alone. because the frame besides being brazed is pinned and its very difficult if not almost impossible to change tubes.

Having said that, a friend and fellow Douglas nut does push them back by jacking from a plate fixed across the frame on the lower corner lugs, against the bar  fitted through the head stock.
 
Regards
ChrisS

Offline eddie

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Re: Distorted frame?
« Reply #16 on: 17 Mar 2006 at 20:32 »
Thanks Eddie,  how accurate do I need to be for such checks?  Is a spirit level and plumb bob good enough?

John, A good quality spirit level is O.K., but I would prefer to use it in conjunction with an engineer's square rather than use a plumb bob to check that the swinging arm pin is vertical. It is also a good idea to check the accuracy of the spirit level. This can be done by putting it on 'something level', then rotate it 1/2 turn and check that you get the same reading.
                  Regards, Eddie.

Offline Doug

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Re: Distorted frame?
« Reply #17 on: 26 Apr 2006 at 01:34 »
Re- different bridge plates. Just heard back from someone that has seen a lot of these bits and pieces (and has owned a dozen Plus models over the years), and he said the Plus and Mark series share the same bridge plate. However there are early and late style plates. The early style have a hole the same size as the steering stem. The later style have a larger hole, and a spigot on the bearing race protrudes up through (so a different bearing race is required.) This was an improvement to stop the relatively thin bridge plate from worrying the threads on the steering stem. So this may account for the difference noted above and Club spares stocking two styles.

The only other difference was to the keyway to orient the speedo bracket, where some have no keyway. But this again does not alter the geometry, nor is it Plus/Mark specific.

-Doug
« Last Edit: 26 Apr 2006 at 01:36 by Doug »

Offline KiwiJohn

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Re: Distorted frame?
« Reply #18 on: 26 Apr 2006 at 04:27 »
Thanks Doug,  a new bridge plate has arrived from the LDMCC and even with my weak eyesight in the middle of a sub-Antarctic** winter and nothing but a spluttering candle for illumination I can see that the difference is very dramatic, about 5mm or more at the top of each fork tube.  I think this was deliberate and I am going to need an 'expert' to put it right.


**We are not really sub_Antarctic but there is really nothing between us except Southern Ocean and the first winds of our Southern Hemisphere winter does indeed send brass monkeys scuttling indoors.

Offline Doug

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Re: Distorted frame?
« Reply #19 on: 27 Apr 2006 at 01:41 »
John,

Interesting. I assume you mean the new top bridge plate from the Club spares is different from the previous replacement plate from Club spares, and not the special plate currently mounted on the machine.  Also that the 5mm dimension pertains to the fore-aft offset between fork stem and fork tubes. I have emailed my expert on Plus matters to see what he has to say re- this development, but it is odd as I have confidence he knows what he is talking about. Will update soon as I hear.

Meanwhile, did I mention how nice and warm it is up here?  8)

-Doug

Offline KiwiJohn

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Re: Distorted frame?
« Reply #20 on: 27 Apr 2006 at 08:11 »
It is a long story Doug,  when I got the machine it had a bent (i.e. wonky) top to one fork leg and an asymetrical plate.

I got a new plate and straighted the wonkyness as far as practical with what I have on hand.  When the new plate arrived it still didnt fit apparently because of a different fore-aft offset which I assumed to be a Plus characteristic.  So I altered the plate to suite by cutting and welding each side.  However the bike handles very badly, wants to go around corners unless I keep a bit of pressure and would scare the living daylights out of me if I had to do a heavy brake manouvre.  So this is where I am now except I have another new plate on hand.

As far as I know both club plates are the same (or would be had I not bodgered one).

The owner back in 1965 said a prior owner had 'choppered' the bike, (or at least tried to)  and I am forming a strong suspicion that part of this process included reducing the trail by altering the angle of the fork tubes through the triple bracket.

I need someone to finish off fixing the wonky conical piece at the top of one tube and now that I am convinced that the fork tubes should be parallel with the stem piece I need someone with enough heating gear to reset the angle of the tubes through the triple bracket.  Or if I can figure out how to do it I might try to fix this myself,  I just need to figure out exactly how to do it.

Nice day today,  warm breeze from the north and beautiful autumn day, pity I had to spend it at the office.