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Discussion Starter #1
Slowly getting my downed 2013 Bonneville mag wheel back together. I have done the Honda F3 fork mod since the front forks were trashed. I installed the forks the other day and secured them 1" proud of the triple tree (I figured that was a good place to start.
I am now at the point where i need to fine tune things to get correct geometry going. I've searched plenty and what I typically have seen is that the Honda F3 forks are "1.3 or 1.375" longer than stock". Just about all of the posts though are comparing to Thruxton forks which I understand are longer than base model or SE mag wheel forks. I also will be installing ZRX shocks in the rear which I read are 20mm (.787") taller than stock.
After raising the front to offset rear zrx shocks I'm left with .588" or almost exactly 15mm for the front forks to stand proud. The only thing left would be to further raise the front of the bike (lowering the fork tubes in the clamps) by the amount of the distance between Thruxton fork length and mag wheel fork length.
Can someone verify that my thinking is correct here, and what is the length difference i'm looking for?
Thanks for all the help!
 

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08 Thruxton from top of fork to center of axle =30''... 08 T100 top of fork tube to center of axle =29 5/8''.... Thruxton shocks center to center = 14 1/4''. T100 shocks 13 3/8''. Are the SE shocks shorter?
What is it on the F3 forks?
 

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Discussion Starter #4
Thanks for the info. I have no idea what the stock rear shock measurements are, I bought the bike without stock rear shocks to begin with (some sort of hagons). My ZRX shocks measure at 14" eye to eye on the lightest preload setting and they look brand new. My F3 fork length is 31 3/16" from top of stanchion to center of front axle mounted on the bike and front end unloaded (wheel off the ground).
Maybe I am incorrect in thinking the front forks across the models have different length forks. As I was thinking about it I remembered reading the differences in the rake and trail between models is due to wheel diameter differences not fork length.
 

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To be clear, I am not suggesting that Triumph didn't ever, or even generally, use different length forks on Bonneville range bikes (excluding the cruisers). In my specific case, the 2008 Thruxton fork legs I used to replace my broken 2012 Bonneville SE fork legs, they were close enough to the same length that I didn't bother adjusting anything. You can read more in my blog post if you care, but it's basically this same information as I've put here.

I can't imagine that the T100 fork is the same length as the SE / mag fork. but I suppose it's possible. The wheel and tire alone add at least 1.5" or so of height to the fork length from the edge of the tire to the top of the stanction if measuring in a straight line, but this doesn't directly affect ride height because of differences in trail. If the forks are in fact the same length, then ignoring trail the front ride height would be 1.5" higher than it is on the SE/mag, a difference that is not matched at the rear with the shock length. The frame (and therefore the rake angle, assuming ride height is unchanged) appears to be the same, so this imbalance in front and rear ride height difference would slacken the rake angle on the T100 vs. the SE and also lower the ride height a smidge, which all makes sense. Anyway, if there is any difference in fork length between these models, I would guess it's in the stanction and not the fork leg. My Thruxton fork leg is exactly the same length as the SE fork leg.

Now, forget about Scramblers here, who knows. I would think they would have to be taller in general, because along with the longer rear shocks, you'd get non-Scramber geometry if it didn't have longer forks. And my suspicion is the fork difference is all in the stanction anyway. But the rest are probably all the same, even though the internet says differently, probably because people measure differently, or wrong, or with the bike loaded with different preload, etc. If you crank down the preload on a Thruxton fork then you could easily measure it on a parked bike and find it is 16mm or 30mm longer than a mag-Bonnie parked next to it. But the truth is, Triumph could have made year by year or even mid year running changes that would be seen by observers as differences attributed to the different models. Maybe they started out making two different length forks for Thruxton and Bonneville before the mag/SE/FI came out and then retooling was required so they optimized manufacturing around one fork length for everything but Scrambler. So a 2003 Bonneville fork and a 2006 Thruxton fork may be different lengths, and neither may be the same length as a 2015 T100, Thruxton, or SE fork. I wish this were better documented. But I really think nearly all of these things are just parts bin efficiencies at Triumph, not intentional design variances. If you can use two stanctions and three fork lower castings to make four different combinations in production, then you do that rather than having four variants of each.

Rear shock length is well documented and accurate on this forum and elsewhere on the internet. It's just so much easier to swap them that people remove them all the time and easily measure them off the bike. The SE rear shocks are 330mm, T100 are 340mm, Thruxton/Scrambler 360mm. Internet sez ZRX shocks are 365mm and reports are the springs are much firmer than the Bonneville springs so they will sag less on the bike.

I think given the longer stanctions, your forks are at least an inch or more longer than standard SE forks. Next time I'm in the garage I'll measure the approx distance from the bottom of my SE fork to the center of the hole so you can make a guess on how much difference, since my SE forks are very close to exactly 31" long from the bottom of the lower to the top of the stanction (my only intact fork has a slightly bent stanction so I can't make a precise measurement). Your rear ride height is going to be at least an inch or more higher than stock anyway, so besides the fact that your side stand will be too short, I would run the forks lower in the triples to try and balance it with the rear. Good news is you have some easy adjustability there.
 

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Discussion Starter #6
Mr72, thanks for all that info. It does make sense. I measured my forks again this morning and went from top of stanchion (not including for cap) to bottom of lower since you are also talking about that measurement point. Mine is a definite
32 3/8" possibly 32 3/16". My ZRX shocks are actually closer to 14 1/4 " so they have settled some but not much.
I don't know if I am splitting hairs here or not, my main point of posting is to prevent myself from (unknowingly) setting up my bike to handle in a way that is sketchy or dangerous. I do not know the finer points of suspension geometry! In a perfect world I would have been able to measure my old forks and this would all have been so much easier.
Since my rear shocks will be taller than I originally thought as compared to the 330mm (or 13") originals, I will now be compensating for 1.25" on the front. I also measured from center axle to bottom stanchion and its appx 1.125" to 1.1875". Getting to within 1/16" is difficult for me. If I subtract that from your "top to bottom" measurement we are in the ballpark of the more often quoted "top to axle center". If I average out both your fork lengths with rwebs, and subtract that from mine we are looking at 1.328" difference...aha! I now feel confident that is a "good number" to go by (the original said "extra length of F3 forks being 1.3 - 1.375" longer than stock) . I also feel good that there is no substantially shorter "SE" type fork length that I originally thought there was (and hence this thread seeking that measurement) given Mr72's explanation.
Funny thing is, because of the zrx shocks, the extra length in fork tubes will all be lowered in the clamps except maybe 1/8" or so!
I'll be back soon asking about sag and pre-load though! o_O
 
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