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Makotosun

Front suspension fork upgrade?

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Replied by RT325 on topic Front suspension fork upgrade?

Well there ya go above. MX250B forks 1975. Had one of those bikes new in "75 & great forks!!. Oh, & as said, if ya put long ones on the front you need to balance out the rear. Maybe add a coupe of inches to the arm & longer shocks so it looks right. Not like mine though as is overdone at the back with an RM125 arm. I used to have a PE 250 arm which was a bit shorter & looked right. Then sold the bike 30 years ago & oneday out of the blue the guy gave me the main frame back so started again with what i had left. It's what we do for fun---umm--i think. Also, my frame had has a Triumph twin in it before i got it. #ta or tiger90 & was used on the road. Looked like it nearly dropped straight in with only the front tubes pushed out a bit but no cutting or welding. The shortened subframe was my butchery after i got it.--suspension wars had started lol. Just ran the steel original arm back then but needed a couple of inches extra. Had long gas girlings on which didn't like the angle & wore out. Ones on now are also gas girlings arc welded into PE bodies to get the length. Only short travel shocks as was all i had but still plenty of travel being forward mounted & heavy damped. Don't think i'd be game to weld them again with 200psi of nitrogen oil mix on these trying to escape haha. Welded them where they're thicker around the base. Made two pairs, one medium damped & one hard damping. Shop was girling importer for the country 'nz'. But just didn't hack the pace when people were modifying everything in those days but were good in the vertical position. Sorry, getting of course again.
Last edit: 07 Oct 2020 15:36 by RT325.
07 Oct 2020 15:25 #21

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Replied by bluntz465 on topic Front suspension fork upgrade?

On his Honda,he says his Honda oh the shame!
1957 Cushman Eagle 1969 Honda 90 ATC 1969 Honda CB250
1973 Yamaha AT3 1974 Kawasaki KH440 1974 Honda XR75
1975 Suzuki TS75 1975 Kawasaki GreenStreak 90 1975 Suzuki GT550
1975 Suzuki TS250 1976 Suzuki GT550 1976 Indian Chief
1976 Yamaha YZ80 1978 Yamaha IT175 1980 Suzuki GS550
1982 Yamaha IT465J
Last edit: 07 Oct 2020 17:53 by bluntz465.
07 Oct 2020 17:50 #22

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Replied by YZBill on topic Front suspension fork upgrade?

Attached is a list of fork tube diameters broken down by size and brand.
kiwavmotors.com/en/faq/about-fitment/mot...fork-tube-size-chart
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07 Oct 2020 17:56 #23

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Replied by bluntz465 on topic Front suspension fork upgrade?

So as you can see, every one of these guys has swapped out their forks.
Dont be scared of trying leading axel modern units if your not racing it. You can just let whatever feels best pass through the top of the clamp and maintain geometry while gaining all the extra travel that extends below the axel.
If you a taller rider youll get by far better.
If your a heavy guy you may need to change springs. But you can buy 4 sets of forks to play
With for the cost of those emulators.
Unless of course that extra step pushing your bike around in the driveway means everything!
One other small point is the tiny change in geometry of the axel moving forward will be more high speed stability.
Good luck
PS. One small confession
I have a set of emulators for my IT
Please keep IT too yourself....
1957 Cushman Eagle 1969 Honda 90 ATC 1969 Honda CB250
1973 Yamaha AT3 1974 Kawasaki KH440 1974 Honda XR75
1975 Suzuki TS75 1975 Kawasaki GreenStreak 90 1975 Suzuki GT550
1975 Suzuki TS250 1976 Suzuki GT550 1976 Indian Chief
1976 Yamaha YZ80 1978 Yamaha IT175 1980 Suzuki GS550
1982 Yamaha IT465J
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Last edit: 07 Oct 2020 18:36 by bluntz465.
07 Oct 2020 18:29 #24

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Replied by mdscott on topic Front suspension fork upgrade?


Thanks Bill for posting, cool site
Thanks Mark
Roseville, Ca.

'03 ST1300
'75 Norton
'73 CB500K
'81 Honda CT110
'87 PW50
'64 McClane Edger
'02 Honda XR70R
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07 Oct 2020 18:41 #25

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Replied by YZBill on topic Front suspension fork upgrade?

Here’s some of the bikes I’ve modified the forks on.
‘83 Honda XR100 with ‘83 Honda CR80R forks. I used the triple clamps off a ‘77 Honda MR175, which bolted right up. To balance it out I used 360mm long shocks, also off the MR175, and a chain tensioner.


‘77 Honda CR125M with forks off an ‘82 Honda XR200R. These bolted right up to the stock triples. I did shorten them 3.5 inches, but they now provide a full 9” of travel which is the limit for the Historic class.


‘74 Honda CR125M, which is a vintage class bike. The forks are off an MR175 and have been modified with R-T springs and emulators. I rode this bike once before modifying the forks and handling was downright spooky.


‘78 Bultaco Pursang 250 (Mk11 M206 if you’re a purist). I also modified the forks with R-T springs and emulators. A few posts back MarkT mentioned about sticking with straight-rate springs. I made the mistake of putting progressive springs in the forks on this bike. Compression damping seemed a bit slow, so I changed from 20wt to 5wt fork oil and the front end had a tendency to pogo on flat landings. When I came up short on a tabletop it launched me off the top and I landed in the soft soil on the edge of the track. The bike stuck and launched me onto the ground and into the guy behind me. Broke four ribs in two places. Sneezing brought a whole new meaning to the word discomfort. Don’t be a cheap bastard and buy progressive springs.
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07 Oct 2020 21:00 #26

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Replied by RT3Sam on topic Front suspension fork upgrade?

Nice Bikes YZBill and Thanks for the advice. So I'm thinking I'm for sure going with R-T Springs. So do you recommend Emulators or not? I want to get the bike dialed in right, but I still want it to be close to stock in appearance. I also don't want to spend more money on the bike than its worth.
08 Oct 2020 04:58 #27

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Replied by YZBill on topic Front suspension fork upgrade?


Personally I would start with getting the correct springs for your weight/ability. You should also get a fork oil setting tool (see pic below). Set the preload on the springs and start with the fork oil weight and level recommended by R-T. For fine-tuning you can start with varying the oil level in +/-10mm increments. Next would be using a higher or lower viscosity fork oil and finally you can change the amount of spring pre-load by +/-5mm increments using washers. The best is to find a test area you can ride that has the obstacles you would encounter on the type of riding you’ll be doing. Using a zip tie on the fork tube you want to make sure you’re using all of the fork travel, but not completely blowing through your compression damping (bottoming out the forks) on every root, rock or whoop section. Rebound damping is how the front feels when there are repeated obstacles such as a section of roots, rock garden or a long whoop section. If it feels like you’re using a jack hammer, your rebound damping is too slow. If it feels like you’re riding a pogo stick, your rebound damping is to fast.

The reason I have emulators installed in three of my bikes is because I could not get the compression damping tuned for motocross. The tracks we race at aren’t “big air” SX tracks, but they do have jumps that will push the limits of the suspension.

Also keep in mind you don’t have to buy the parts direct from R-T. You can get the part numbers off their site then do a web search. Often times you can find them cheaper through a re-seller on eBay, Amazon or even places like Rocky Mountain ATV.
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08 Oct 2020 09:36 #28

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Replied by PMK on topic Front suspension fork upgrade?

Emulators are kind of a love hate setup. Some folks swear by them others swear at them.

The goal of the emulator was to mimic the feel of a more modern cartridge fork on older damper rod style forks.

Emulators are technically a speed sensitive type valving, where damper rods are position sensitive valving. This in simple terms means that the firmness of the damping changes either with how quickly the fork is compressed OR where the fork is in the stroke.

Damper rod forks can be altered to work very well. Not uncommon to read how guys would braze the oem holes in the damper rods, then drill new holes in a better location or a better diameter.

Correctly getting the spring rate and oil quantity correct is absolutely needed in all forks. A heavier rider on soft springs would love emulators, but certainly is a less than optimum setup. Correctly holding the forks up with proper springs will allow better use of the forks available travel.

Insufficient oil quantity will let the forks ride low in the stroke and lack progressiveness to avoid bottoming. Add to this, riding low in the stroke with damper rods will detrimentally alter the forks ability to work well. The damper rod is position sensitive, and riding low in the stroke places the damping in a less than optimized setting.

I may have overlooked it if already posted. It was common back then but less common now with long travel. You can modify your fork caps by installing inexpensive Shrader valves. I do not recommend running the fork 100% on air pressure, but running the lighter spring with a small amount of air pressure, typically 10 psi or less can have great results. Doing this can help support the bike better and when correctly setup with proper oil quantity will give a good feel at extension, have better mid stroke support and most times better bottoming control. Oil quantity and viscosity, as already mentioned by others will dial it in also.

Not sure if any damping rod mod specs are out there on the internet. Myself, I would research for that, modify my fork caps, and start out with the oil viscosity recommended for the modified damper rods.
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19 Oct 2020 03:18 #29

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Replied by PMK on topic Front suspension fork upgrade?

Wanted to add, I would prefer 6 inches of good quality fork travel over longer travel that was poor quality and not properly set up.
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19 Oct 2020 03:21 #30

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