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Pinned Trail - Fitting USD Forks?

Discussion in 'Tech Tips' started by maelstrom, Dec 2, 2014.

  1. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    If you are going to fit different forks then you need to check that the offset of the steering stem centre to the fork centres is close to original. Changing this offset will change your trail, and trail is a very important element of your steering. Increasing trail means that your steering point is further ahead of your axle centre and will increase high speed stability and reduce manoeuvrability and vice versa. If you know the geometry for your standard bike then some simple trigonometry and you can calculate what the trail will be when you fit your new front forks.

    old-fox-albums-rake-trail-picture7100-rake-trail-lr.jpg

    Tony Foale seems to think that it is not so critical but rather a combination of factors http://www.tonyfoale.com/Articles/RakeEx/RakeEx.htm
    I personally think that sports bikes, that have a more forward weight bias, are more sensitive to changes in trail than the older BMW that he experimented with. When I did some experiments of my own with some of the belt drive Ducati models I found the changes to be quite obvious. These were increases in trail that resulted in slower steering and more high speed stability that I mentioned before.

    The 'jacking up the back', that is so popular with the 'track day' riders, quickens the steering response as a result of the reduced trail not the rake. I often found that the rear slicks that were being used on track bikes had such a low profile, compared to the OEM tyre, that trail had actually increased from standard. Riders thought that they had adjusted their ride height etc to quicken their steering when in reality they had only got the steering close to standard again and it was the lower profile of the tyres that was giving them the different feedback (combined with the belief in their head; never underestimate the placebo effect). Knowing these values is very important when setting up a bike for racing as you can adjust settings when trying different tyres etc and know what is actually causing the difference rather than assuming that it is just the tyres etc. The old "What you can measure, you can manage" thing.

    There is a trail calculator (spreadsheet) available from RaceTech and you can find it by doing a search for "racetech trail calculator". It also has a list of tyre specs on the second sheet. Explanations for "Real Trail", a term coined by Tony Foale, can be found here http://www.tonyfoale.com/book/Geom.PDF. RaceTech also produce a very useful guide called "Race Tech's Motorcycle Suspension Bible", and a lot of that content can be previewed online.

    You can use your smartphone to get a rake angle; just download a suitable application to use it as an inclinometer (I use http://www.plaincode.com/products/clinometer/). Then you can try adjusting your rake angle by adjusting the position of the forks or the rear ride height. If you cannot get the trail value that you want, you will have to source some replacement fork crowns.

    clinometer_logo.png


    Part 2
    Since I am fitting USD forks on my own bike, a Yamaha FZR400RR 3TJ1, I will write about the process. In my effort to find some forks and triple clamps that are suitable I discovered two things. First of all, there is not that much information available; there are a lot of owners who just bolt something on and hope for the best. Secondly, a lot of the information that is available is hearsay, very little is verified. In an effort to do something about this I have made a table of information. I started with an existing table that I found here http://www.svrider.com/forum/showthread.php?t=135430 the credit for the diagram given to jeff721. I have changed this because I know that a lot of the information will never be added and there were also some very important items missing.

    Fork Database
    http://www.litetek.co/Guide_USD_ForkDatabase.html
    At the moment there is not much there but I will add to it as I can and all contributions are welcome.
    USD_Template.jpg



    Part 3
    Once you find a front end that you think you can use, you need to consider that you want the front axle to be in the same position, relative to the frame, as before. Fork length will not do it because the triple clamps may be in a different position. So before you start tearing your bike apart, you need to measure from the axle to some fixed point on the frame, as per the attached image (forks fully extended of course). This is how I measured my Yamaha 400.

    Fork_Length.jpg

    LINKS
    Table of steering head bearing fitments:

    Part 4 - The Triple Clamps:
    The chances that your new triple clamps will fit your steering head are slim to none, so you will need to make a new steering stem.
    1. Start by pressing the stem out of the triple clamp that you have chosen to use with your USD forks.
    2. The section from the bottom of that stem where it corresponds with the bearing face of the bottom clamp is the new bottom section of your custom stem. (section 'A' in diagram)
    3. The section between your top and bottom bearings from your bike's original stem will now be measured and used for the middle section (section 'B').
    4. The top section will be copied from your original stem and will probably require a custom nut (section 'C').
    Stem.jpg

    This is my own example of a custom top nut. The thread on the top of the stem is the same as the original FZR400 Yamaha but I made a custom top nut in the style of the Ducati. I prefer the clamped nut to the double nuts which i think are a stupid design.

    [​IMG]


    Part 5
    Since I am keeping the original wheel, to make it fit into the R1 forks I bored the wheel to suit the larger R1 wheel bearings. Of course all of the spacers will need to be custom made to suit. Then I used a vertical laser mounted above the bike to centre the wheel.
    MachiningWheel Bearings_01.jpg FrontWheelSmallAlign.jpg

    The final step is to ensure that the discs align with the R1 calipers.
    Finally, here is the rolling chassis with the front end in and original wheels.
    [​IMG]

    cheers
    Blair
     
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    Last edited: Nov 10, 2019 at 9:10 PM
  2. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    So true.
     
  3. Tim_

    Tim_ resident nutcase Premium Member Dirty Wheel Club Contributing Member

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    Amazing post mate... I agree with you.. but instead of trying to get a tighter turn im searching for as close to original as i can get.. There was 2 main reasons for the conversion..
    1) To have fully adjustable front suspension..
    2) My MC22 disc and pads barely passed roadworthy.. so it need new ones.. the ZXR has brand new discs and pads....
    3) the look of USD forks is just sexy... lol
    4) having ridden it i now know it is an upgrade.. bike just holds the road better and less twitchy..
    I have been measuring a lot... and i will be re-measuring alot more.. but its come down to a MM game.. because even 5mm up or down can make very noticeable changes... also with the non drooping pegs my riding position has changed again... so ultimate I am now looking for a comfortable riding position that does not hurt the engine


    I haven't taken the bike for a proper ride yet.. but as i type this my rosso 2's are being put on the ZXR rim.. so i will have good tyres so i can go up the mountains and test and tune the steering..
     
  4. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    @Tim_
    I suggest that you measure the offset of the Kawasaki fork crowns to determine the change in trail first. Of course you should start with the fork length set to the same value as your Honda.
    [​IMG]
     
    Last edited: Dec 9, 2014
  5. Tim_

    Tim_ resident nutcase Premium Member Dirty Wheel Club Contributing Member

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    I still have the CBR crown.. So i laid it over the crown of the ZXR and the CBr is closer together in 10mm forward...
     
  6. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    So you have lost about 10mm of trail when the forks are the same length. Not good!
     
  7. Tim_

    Tim_ resident nutcase Premium Member Dirty Wheel Club Contributing Member

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    My Bike:
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    But because of the H frame. I can not go to the same height as the CBR forks. Other wise the ZXR forks will hit it. Its actually sitting 15mm higher up the forks. So I got some trail back there.

    The nose of the bike is not sitting up. And the tank is just under level. But the bike levels out when you sit on it. So I think I will stiffen up the rear. Because when I went riding the front held the road and all the movement was the rear shock.

    Also I went to my tyre guy. And the ZXR tyre that was on it was the to wide and to big for a light bike. So now the bike has Pirelli Rosso 2 on the front and back.

    I only just put the wheel back on the bike now. So I won't get to ride it until tomorrow.
     
  8. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    Good Post Blair

    I've lowered the front of my drag bike by about 30mm to move the weight forward a bit. I assume this will make the bike a little less stable (not noticeable I would think). Purpose built drag bikes run raked forks like speed trial bikes for stability (zero handling). A mate of mine was moaning about his bike's crap handling...he put a bigger rear tyre on and now he raves about it. Just raising the back that little bit made a lot of difference (according to him)
     
  9. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    When I lived in Brisbane, a friend of mine bought a CBR250 and asked me to take it for a spin. It handled like a pig because of the woeful tyres (not because of their size). The local Tyres for Bikes shop used to sponsor 250 production racers at the time and they would sell off the very slightly used tyres at a good price. Installed a set and the bike was transformed.
    Big tyres are like like big jets, they make everything slower except in the owners mind. :)
     
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  10. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    Sorry, I didn't mean bigger, I meant taller...it raised the back of the bike up a little and thus changed the angle of the front forks. Cheers
     
  11. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Yes I figured that, but it is always hard to know what makes the difference. If the old tyres were rubbish, which is what I was trying to get at in my story, then he is just comparing apples and oranges. Even if it was exactly the same type of tyre but just the different profile it does not mean that the reduced trail is the source of the improvement. It may be that the previous rear profile was too low and upsetting the handling due to mismatch between front and rear. This is the problem with people making claims about something being an improvement or being good. Without direct comparisons it is all conjecture.

    I set a Suzuki GSX-R750 back to standard trail for racing at Lakeside because the downhill entry onto the straight seriously compresses the front and the bikes become very unstable. The rider did his fastest ever lap times and got his first podium. Best to take everything with a grain of salt and trust the stopwatch. But hey, I like to bench race as much as the next guy :)
    cheers
    Blair
     
  12. Frankster

    Frankster See the World before you leave it Staff Member Premium Member Ride and Events Crew

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    What's going on here? I looked at this thing at the island and realised the trail on these front ends varies based on weigh being carried, braking and accelerating. Is that right Blair?
     

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  13. Murdo

    Murdo The Good Doctor Staff Member Premium Member Ride and Events Crew Contributing Member

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    Welcome to the (weird) world of Girder Forks.
     
  14. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Yes, the motion of the axle describes an arc. Different front suspension configurations will have their own specific geometries. Trail shortens on conventional telescopic forks too, but it is linear.
     
  15. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    I have added to the original post a new section, Part 2, which contains a link to a fork database page
    http://www.litetek.co/Guide_USD_ForkDatabase.html
    At the moment there is not much there but I will add to it as I can and all contributions are welcome.
    cheers
    Blair
     
  16. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    If any of the ZXR250 owners would like to supply any details it would be appreciated and useful for the forum members. Photos showing that verify the measurements would also be good.
    cheers
    Blair
     
  17. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    I am adding some non-USD (RWU) forks to the database as well, if anyone volunteers any information that is, the model name is in blue.
    cheers
     
    Last edited: Sep 2, 2019
  18. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Bump, bump, whinge, nag, nag, grumble.
    I have posted about this USD datasheet on 7 web forums so far and to date, zero input. Methinks it may take a while to fill in the blanks :)
     
  19. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    The ZXRs are on my list in front of me ol buddy

    At the moment it doesnt reach double figures on the thermometer during the day and its been -3 at night ... my effort during these times is to put another log on the fire ..... the workshop lies dormant :thumb_ups:
     
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  20. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    No worries Mr Grey. Stay right where you are :)
     

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