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Allan G, linker48x
Total Likes: 8
Original Post (Thread Starter)
by linker48x
I posted about my T140 forks up in the "What did you do to your bike today thread" but I thought I would start a thread here, in the Triumph section, in the hopes of getting some ideas about how to proceed.

I have a mildly modified 1979 T140D and I have the engine about where I would like it--pretty much all stock, with just a few mild changes, such as MAP 9 1/2:1 pistons, retimed cams, JRC carbs (although I am in the process of changing over to Euro jetted Amal Mk 2s), Kibblewhite valves and springs, and Boyer and Podtronic, and a low restriction muffler on the stock 2 into 1 header--and I am now turning my attention to the forks, which need help. I am concentrating here on getting them functioning better,. There is no question that installing a more modern fork will improve things, but I don't want to make a change like that on my street bikes--I want to continue with the standard period forks and get them working as well as I can.

The forks seem to suffer from the typical T140 complaint of failing to comply with smaller, sharp bumps that require the wheel to move quickly--seemingly because of either too much stiction, or too much high speed damping, or both. In other words, they "kick" or deflect off small sharp bumps, quite noticeably.
They seem to work fine, on slow roller type bumps, and in cornering and braking loads, which don't make the wheel move fast. This suggests to me the problem is either stiction and binding, or too much high speed damping, or both. I just did a mild initial tryout on a fork modification-- I just replaced the 97-4003 o-rings with a piston ring replacement I got from L. P. Williams in England (97-4003P), and I replaced my 20 weight fork oil with Honda Proline 10 weight, 125mm from the top, fully collapsed with springs out, and I put them back together carefully to avoid binding and stiction. Unfortunately, I don't think I have improved them much because with the wheel still out, and either with both no oil (other than a little for assembly) and with oil in, each fork leg is still somewhat difficult to move and doesn't feel very free--about like they were before I put the piston ring replacements in. Then, I put them back together fairly carefully, in an attempt to get the forks centered on the axle and to avoid binding from that source--the grooves in the axles and the use of clamps on the axle on both sides are an unusual feature of these forks and could intorduce "pinching" the forks together, and binding, so it makes sense I thought to pay attention to that in putting them back together. Anyway, I guess I will figure out whether I made any real improvement once the snow melts in late April in Alaska. I have a lot of time yet to wait.

So, I am wondering, has anyone been able to do anything to these forks that improve their action on the sharp and little bumps, without degrading their low speed damping function? I am all ears at this point. I am guessing whatever is out there that works either addresses the stiction, or the apparent excess of high speed damping, but what do you have to suggest? I am guessing here that fixing the forks means modifying the damping somewhat. The trouble is, changing the high speed damping usually involves some kind of popoff valve that lets the wheel move quickly in small increments. Anyway, please let me know what you might have done?

Race Tech makes an Emulator for these forks, but I am not ready to try that yet, before I try less major modifications, since the complete changeover is fairly expensive and requires permanent modification of the damping rods, by turning the tops off in a lathe--no going back. Emulators modify older design damping rod forks in a manner to "emulate" the function of modern design cartridge forks, by doing away with the existing compression damping circuit and replacing it with a single stage popoff valve that kicks open on sharp bumps, and with an additional set of fixed damping orifices different than stock; the fork's original rebound damping remains in effect. This is all in a small aluminum damping valve body (like a suspension piston) that sits on top of the damping rod, under the spring. Emulators are adjustable for spring preload on the blowoff valve, and they offer alternative springs as well. The Triumph Emulator is FEGV 53301, for $179; the adapters that sit on the valve and space the spring away from it are FPEVAD3301P, for $19.99 or you can substitute a length of 3/4 inch PCV pipe; the Race Tech piston ring replacement is FPPR 273022; they recommend a .8 rate spring, which is FRSP S2643 080 and is (memory correct) about $160. I have used Emulators on my race bikes and they do improve these older design forks for sure, but I have never used them on a street bike so I am wondering if anyone has experience with them on the T140 forks, on the street.
Liked Replies
by Mike Baker
Mike Baker
For what it's worth, I've quit using the rubber seal covers and gone with gaiters. And have teflon orings on the damper rods. I noticed improvement after each of these changes.
1 member likes this
by Allan G
Allan G
FWIW, heres what I have done with mine. I thought this to be a decent starting point especially when I found on my 68 A65 that by completely renewing the original damper system with progressive springs made for a real nice set of shocks over any other mod I tried in the past.

Ikon sell progressive springs, I have fitted these,

I also replaced the bottom nuts with new and bought new damper rods from SRM, I needed to anyway as the ones which came with my 71 forks for corroded.

So for me this is my starting point. As best as it can be but still factory (almost). If this is still not upto snuff then I will go the racetech route but I thought this would be a good starting point and doesnt mean im buying things I dont need anyway.
1 member likes this
by DMadigan
There are two problems with the aluminum slider forks, the damper head and no bushes between the stanchion and slider. The original damper rod had a basic problem with the two long slots along the sides, when the stanchion bush went past the top of the slot the damper head was bypassed and serves no function. The later damper rod moved the slots down to the end and served as a hydraulic bump stop and forced all the oil through the damper head.
The damper head is a simple orifice with fixed sizes for compression and rebound. For a short tube orifice the resistance to the flow is:

Rf = dP / (Cd * A) * (rho/2)^0.5
dP - delta pressure across the orifice
Cd - orifice coefficient
A - orifice area
rho - fluid density

As the forks move faster the resistance continues to increase.
Reed valve damper heads are non-linear and the resistance decreases as the reed opens further due the the pressure. By changing the dimensions and number of the reeds the damping characteristics can easily be tailored.
For very high impacts a blow-off valve is used, typically a spring against the reed stack.
This is one way to change the damper head to reed valve.
[Linked Image from]
The screw allows overall damper adjustment and the reed valves add a non-linear portion.

The sliders are not hard coated and have no bushes. When the stanchions are bent the slider is straightening the tube as it moves up. The majority of the load is concentrated at the end of the bore. Modern forks have a bush at the top of the slider and another on the end of the stanchion that allows the stanchion to flex in between. Some stiction can be reduced by boring the top of the slider bore and putting in a bronze bush. Another is to replace the slider with a hard anodized one made for a bushed stanchion.
[Linked Image from]
1 member likes this
by gavin eisler
gavin eisler
Lots of OIF fork info here.
1 member likes this
by TR7RVMan
Hi linker48x, I have no answers but a few thoughts from my experience. You probably already know all of this.

You already have "leak proof" type seals. I always use genuine leak proof brand, classic model. Both press in retainers & the later snap ring retainers. I don't know if they make the "low friction" version in our size or not. I find the stiction of Classic to be good.

I found in testing LP Williams phenolic damper rings have basically no stiction. They have worked very well for me now in 5 bikes. Huge advantage over the rubber o-ring.

I always trial fit sliders no seals. The must move zero resistance & the clearance is a lot so they really slide freely. I recently went through fork. Found very stiff slider. Like it was dented very slightly. Couldn't polish to free it. Got new genuine Triumph NOS. It fit perfectly. Was later snap ring version. The leak proof rubber is exactly the same as the earlier slider uses with press in retainer. It MUST have the 2mm or so clearance between seal & washer under snap ring. No sealant outside of seal. It must be free to float at all times.

So now I've got 2 good sliders. Installing damper rods I could feel resistance. Inspection showed upper tip of rod was bent making piston sit crooked & rubbing on inner wall of upper tube. Took pistons off & straightened rods. Trial fitted sliders with dampers, again no seals. No slides perfectly feely both sides.

Fitting seals & damper rings both sliders now still move freely with no axle, no fender. I'd like to have spare axle to trial fit, but don't so I install wheel. Then move sliders, wheel. Adjust both triple clamps as needed to get wheel to slide freely as you can with wheel off ground so you can feel the friction. No oil in fork, but the seals sliders, dampers were well lubed on assembly. A tad of moly grease in the dust wiper lip seems to help stiction & break in of seal lip. At least fork oil. Wheel should fall swiftly when pulling it up & releasing. The goal is to reduce friction as possible. Even a little extra friction will be felt on road.

Finally the fender must fit perfectly tension free. Even an 1/8" or less bind in fender mounts will be felt on road. So after fender is installed wheel should fall like it did without fender.

Moving to springs. I know you're familiar with sag. The right spring for about 2" sag is a good starting point. Factory spring will have only slight preload from new. 1/4-3/8" above top of upper tube. Just enough to make the spring nut a little hard to start.

As we speak I have two '73 T140 type bikes in my garage. One has factory original springs. Other has progressively wound springs. Both have about 2" sag for me. I'm 163# suited & ready to ride. Progressive springs have about 1/4" less preload. 1/4"difference on these long springs is not much. On road in real life if I didn't know one were progressive I'd think springs were the same. There is different opinions about progressive wound. I have 2 friends with progressives springs on Hagon rear shocks. I'm not impressed with ride. Hate to say it, but feels same as my old originals. Maybe just me...

I always use 190cc fork oil. I've not experimented with higher. When I get old leaky forks to reseal before road test they feel much smoother, but don't always damper well as most of oil has leaked out. So in real life I've not experimented with lower oil level in controlled manner.

Regarding fork oil, I've done many miles experimenting with oils. I have a short & long road test route. Do same route to evaluate oils. The long route is 70 miles & includes every possible road surface including dirt road & very rough concrete freeway joints. I cannot overstate the effect oil weight has on these forks. Even changing oil 2.5 points of weight gives a very decided change.

I did comparisons with various ATF brands versions Shell F, Pennzoil Dexron II & III, Mercedes 134A & full synthetic. All felt basically the same. I measured oil weight with timed flow test using cups with a pin hole. Results was surprising. I still had rubber damper rings, but had leak proof seals.

For flow tests I got BelRay 10 & 5w. Compared to BelRay ATF was 7.5w. It is said fork brands set their own weight, unlike a standard like for motor oils. I don't know if true or not. I choose BelRay because friend at work races motorcycles & tunes his own forks. He feels BelRay has less stiction & helps fork last logger. So I've just used BelRay

For first road tests 70 mile long route. Being retired helps. I tried all 3 versions ATF. Very harsh on small bumps & larger bumps most of the time. Bad on freeway joints.
Next I used BelRay 10w. Was simply awful. On freeway joints 65mph actually made my eyeballs shake! On the washboard curves of the canyon roads where speeds are fairly low mostly 20-45mph as curves are sharp & some steep. A few sweepers though. One washboard sweeper I came in a little hot. Just the day before I'd used ATF 143A. I was ready for worse, but with 10w the wheel simply couldn't rebound fast enough at all. The bike was skipping sideways quickly. Only God himself kept me on the road. I was utterly shocked at how bad it cornered compared to the day before.

Next day I tried BelRay 5w. On the freeway was so much improved I couldn't believe it. On sweeper where I almost died I was very careful. No problem at all. I did sweeper several times. I could easily & safely to 5-10 mph faster than with ATF. Now I know why I had such hard time keeping up on rough roads.

Going to phenolic damper rings & still using BelRay 5w the improvement was very dramatic yet again. Gave me another 3-5 mph or more on the sweeper. The difference on freeway joints was most dramatic. Huge increase in comfort. I've covered over 1500 miles on the last set up now. You get used to it real fast. Dirt roads are so much fun & smoother.

Yesterday I took the '73 Bonnie on short fork road test. Wow. It's very nice too. Just like the other one.

On both bikes I'd pull up to stop sign & fork would stay down. If I moved at all it would come up. Now on both bikes the fork compresses when I put on brakes, then comes right up when stopped like it should. The difference in feel over old patched rural roads is very dramatically improved. I like long rides. Did 200 mile ride recently. The reduction in fatigue really surprised me. Our weekly ride is generally at least 60 miles. Last week was 90. So we actually really ride these old bikes.

If you put oil in fork before springs installed & pump fork to bleed it you can very plainly feel damper working. When I went to 5w I could already feel the bid difference. We do lots of mountain bike tuning. Except the damper ring is spring loaded on Fox Fork, the low line non adjustable steel spring fox Fork is rather similar to the T140 fork. We have done lots of tuning to our mountain bike forks. Going to the adjustable Fox forks makes it easy which I've now done. Be nice to have that on Triumph.

Back to Triumph, draining fork with drain screw & pumping a few times really does get all but a few drops of oil out.

What has your experience been in trying thinner oil in your T140? The compression damping is from the holes in damper tube. Of the ground going with thicker oil really slows compression, but on road I don't feel much difference on compression as I'd thought I would. The fork dives on braking about the same, which is a lot. I don't know what the cure for that is, if there is a cure. It doesn't bother me. My bicycle fork dives greatly, about the same actually, so maybe I'm just used to it.

Please do follow up report on what you find.

1 member likes this
by Dibnah
A fork brace would probably improve the front end on my OIF TR6
1 member likes this
by TR7RVMan
Hi All, Here's a follow up on the forks I just went through. Did 100 mile road test today over my long test route & then some.

Recap. '73 T140. Installed LP Williams phenolic damper rings, genuine Leak proof fork seals, USA version bike so used ,
emgo brand dust wipers. BelRay 5w oil. Has progressive springs of unknown brand, about 3/8" shorter than factory springs. Has Avon Road rider tires. Steel rims.

This is first set I did with progressive springs. The fork responded to damper rings, seals, 5w as expected. Very good.

The spring has a very different feel. Basically the tight coils get used up mainly in sag. The coarse wound coils are stiffer than stock spring. Over all result is the fork follows road quite well. Damping feels good. The stiffer spring reduces dive during braking, but overall has much stiffer/harsher ride than stock springs. For me the fine winding feels too soft, the coarse winding feels too stiff. I'm 160# ready to ride. For rider in the neighborhood of 220# I expect spring would be just right. Has Hagon rear shocks progressive spring. Set to soft spring is too stiff for me. Damping to firm. But again 220# would be good.

This suspension set up, Road rider tires, this is the best handling Triumph I've ever ridden. Not the most comfortable, but it made even me corner better.

So end of day the $100 for parts & oil proved a good value.
1 member likes this
by Stein Roger
Stein Roger
I feel the need to qualify my inferred criticism of the Leakproof brand fork seals. I rode them for some 6000 miles and they never leaked. On occasion, after some spirited riding on rough roads, they would leave the thinnest of oil films on the stanchions, which I consider a good thing. But however hard I provoked them they never leaked as such, and I believe they would have continued to perform well almost indefinitely.
I usually prefer gaiters on my bikes, even on my 900 Daytona, almost any type of seal will last, but as my Trident is such an original looking bike I chose not to fit them. Standard seals didn't last a season, but to repeat myself, the Leakproofs were just that.

But, whether due to my fitting skills (I don't really believe that) or anything else, I felt the Leakproof seals to offer too much static friction, or stiction as some call it, which wouldn't allow the forks to respond to small bumps, undulations and ripples in the road. As I have explained to Don off-line I'm very particular about these things, and riding modern bikes have spoiled me I guess. Anyway, I found that for me and my expectations, the Ariete brand seals suits me well as they're simply slippier and provides a very nice ride indeed. I don't know much about their longevity yet, they've only been in service for a couple of thousand miles.

Many of you will no doubt have a different experience with Leakproof seals, and they do work really well in keeping the oil in, at the cost of some friction.
Remember, this is my opinion only, I can't serve up any facts at all, so take it for what it is.

1 member likes this
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