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FF: Forks Modified Steering Stem



timetraveller

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VOC Member
Just heard from Mike Hawthorne who was racing in Belgium over last weekend. At his first race last year after fitting one of the new steering heads he broke the track record for his class at one track in Belgium. Last weekend he did the same at another track and apparently knocked four seconds off the previous best time. Congratulations.
 

macvette

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VOC Member
I've done a number rides now on the same route, a couple of miles on an A road, around 20 or so on N. Yorks B roads, humps, bumps and recessed manhole covers, but no severe potholes. It does have the sleeping policemen I mentioned before. Speeds from 20 to 70ish. I measured the suspension travel as tabulated below. The bike is a SeriesD with the steering stem mod plus lower link sealed bearings and Chinese steering damper although the damper pivots on it's centre line unlike Norman's set up. This makes no difference to the suspension travel but I do have it set at its mid point. I have not shortened my spring boxes and have 44 lbs/ins springs one of which has a couple coils removed. I weigh 210 lbs kitted up for riding.
With me on the bike, the front of the lower link is 10mm above the rear measured through the centrelines of the spindles. The guy who measured it doesnt do inches. 20180903_134504.jpg 20180903_134504.jpg
 

timetraveller

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VOC Member
Thanks for that Mac. Do you find the bike comfortable or is the front still too stiff for you? Sorry if I am being dense but do I understand that you are only getting two and five sixteenths minus one and a quarter inch movement while riding? i.e. less than one inch of movement. If so then that is not enough and either you need softer springs or a lighter setting on the damper. Please correct me if I have misunderstood the figures.
 

macvette

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VOC Member
I didnt measure the suspension travel before I did the mod so I have no objective data for that but the front end was quite compliant then. I did post, with Big Eds help, a video at the time. At that time the bike had standard D single springs in each spring box.
These measurements are compression only using 3 tie wraps, one on the damper and one on each spring box so I cant measure rebound from static sag with me riding the bike.
The bike is comfortable to ride and the links rise at the rear appreciably under acceleration. Prior to the mod, it was not possible to compress the front end by pushing down on the bars with the brake applied but it is easy to do that now. You can see that there is more damper movement with the longer eyebolts and it is easy to pull the bars up when astride the bike whereas with the short eyes this was impossible.
 

timetraveller

Well Known and Active Website User
VOC Member
If you would like to try a pair of 36 lbs/inch springs then I can send you a pair. If you prefer them then they will be £30 plus P&P. If you don't then send them back with no charge. I would like to see more movement.
 

chrislaun

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VOC Member
Perhaps you have set it up with too much static sag, I only have 3/8" with the bike alone and 3/4" when aboard. then a further 2-1/8" travel used over speed humps, that's on the spring boxes.
 

bmetcalf

Well Known and Active Website User
VOC Member
I put a ty-wrap on a springbox while the bike was on the Hills stand. During a subsequent ride, I hit a bump that was about as bad as I want to try and the ty-wrap ended up 3.125" down. I have an AVO, shortened springboxes, and davidd's short springs. During riding on smooth roads, it seems to hardly move at all.
 

timetraveller

Well Known and Active Website User
VOC Member
I think that one problem with this is that you used the position of the ty-wrap when the bike was on a Dave Hills stand as a zero point. The stand acts as a pivot with the rear of the bike partially levering the front of the bike upwards. I think that if you had used the lower end of the outer box as the zero point when the bike was on its wheels then you would have found much less movement. The short springs mostly stop the steering problems but they greatly reduce the total movement available. If you get the chance, try one of the bikes with a JE steering head I think you will find a major improvement in comfort.
 

chrislaun

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VOC Member
I have no experience with davidd's short springs but surely they were made to alleviate a problem you no longer have and possibly what is happening is on the centre stand the forks are going to full extension but settling very low on to the short springs when off the stand leaving limited available travel on stiff springs.
Chris.
 

greg brillus

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VOC Member
When I first got my bike back on the road after I restored it back in 2015/16 I had no other springs to use other than a selection of David's springs. I tied a pair of the red 75 Lb springs which did actually work ok but I could tell the front end was still quite stiff. That was when I had some made up in different lengths and spring rates, but I found they were actually way too long, and after chopping them down they became quite usable. My friend who's Comet I installed the stem kit to some time ago, has been playing around with a coilover only on the front the same as I did on the racer. We need to make up some suitable springs, but it seems that the shocker is actually too strong in it's damping. We know this because if the original spring boxes are refitted, the fork action is too stiff............This confirms my findings about the singles needing a much softer shock absorber to soften the ride. I know some of you might not believe me, but we have been trying out many changes, and this seems to be the case. On the same bike we have also been trying the same coilover on the rear with the seat set up as fully sprung.........This is in an effort to make the bike much more comfortable to ride front and rear. I want to carry out these exact mods to my next project.
 

timetraveller

Well Known and Active Website User
VOC Member
So you have the JE mod and Davidd's short springs; that is not a combination I would have thought would be good. If you recall people found that Davidd's springs intended for Comets were much too soft on road equipped bikes and over here we ended up using one sidecar strength spring and one twin solo strength spring to get a reasonable amount of movement on twins and even some singles. These springs were 14" length, if I remember correctly, and the 36 lbs/inch and weaker springs that I supply are intended to be fitted with a length of 13.5" and 3" of pre-load. There is now a known problem of different spring boxes having different length ends but I will ignore that for now. This means that you only have about half an inch of pre-load. What is the angle of your lower link when the bike is on it wheels or when you are sitting on it? It should be horizontal or even very slightly down at the front. It is good that you are getting over 3" of movement so perhaps your combination works as well. Certainly the experience of Greg and his chums is that stronger springs and weaker damping works for them while over here stiffer damping and weaker springs seems to be the preferred option. Maybe you have found yet another combination. HELP!!o_O
However, feedback is what is useful so thank you.
 

davidd

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VOC Member
My short springs were designed to minimize the problem caused by the poorly designed stock steering stem. They will do that when used with a stock steering stem.

The red springs, which were 75 lbs/in., worked on my racer very well and are still in use with the present owner. On all my springs I used the original stock figures for bottoming out in an effort to prevent it from happening. To get to that rather stiff figure to prevent bottoming out, the spring had to be reasonably stiff. But, because the spring could start at 0 lbs/in. it mitigated the stiffness quite a bit. It goes up quite fast to prevent the bottoming out problem. Finally, it made some sense to have a "softer" spring to use with other heavier springs to cover all the weight variations.

Once the stem is changed, you should not need to worry about the geometry problem. You can pull the brake lever and the fork will still be able to go up and down. If you have a new stem you can use any combination of springs you wish. The limitations are comfort and performance and should not involve the safety issues that have caused issues in the past.

The DD stem is not quite the same as the JE stem. They both solve the geometry problem, but the JE stem has a good amount of telescopic motion, which is welcomed by most modern riders. The DD stem does not have telescopic motion, it is more like a girder fork where the wheelbase changes little. The design differences exist because I did not know John Emanuel and he did not know me. We both chose slightly different designs and we both worked alone, at least I did and I suspect John did, also.

I suspect that the differences in spring and damper choice is due to the need to counteract the attitude changes that come with the JE design. Some have chosen stiffer springs and some have chosen stiffer dampers. I would tend to choose soft damping with stiffer springing, but this is normally something the manufacturer works out preprodution. Unfortunately, the messy work of testing is being conducted by all of the participants. The good news is that the quest is now for comfort and no longer involves safety.

David
 

bmetcalf

Well Known and Active Website User
VOC Member
TT, I just put in the springs you sent me with the stem and the bottom link definitely points down now when the bike is on its tyres. It is supposed to dry out on Wednesday, so I'll go for a spin to see how they fare.
 

timetraveller

Well Known and Active Website User
VOC Member
OK Thanks Bruce. I looked through my records but you must have had your springs quite some time as you are not in the latest spread sheet. If the springs were shiny then they are stainless steel and are the 36 lbs/inch ones. If dark steel then probably 45s which some people are finding too stiff. Let us have the feedback when you have had chance to test it out.
 


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