[Ronson]

I'm thinking about raising the stance of my '04 Bonneville and, in addition to taller rear shocks (maybe +20mm) I'd like to know if there's a straightforward way to get a similar lift at the front end.

I'ver read other posts mentioning that Thruxton fork tubes are longer than the standard Bonneville ones. Does anyone know how much longer? Would the Thruxton tubes fit into the Bonneville fork lowers as a direct replacement (with longer springs of course). Also, any thoughts about how this increased height would affect handling?

 

[Scrambler900]

Perhaps you can approach this the way Harley people raise or lower bikes via suspension changes. They keep the stock front fork, but install different length springs. Ask a Triumph mechanic (ideally for a Truxton Cup Challange race team) if that is possible.

 

[Ronson]

Thanks Scrambler - really appreciate your thoughts. I'll investigate the implications of over-long fork springs.

 

[tcb]

If you alter ride height with different fork springs, be careful to leave at least 20-25mm sag or you'll risk losing traction in bumpy corners=crash!

 

[Scrambler900]

PS

Your bike will ride somewhat different (not necessarily bad, just different) with a change in front and/or rear suspension modifications, because the result will most likely cause a slight degree or so change in the rake and trail, and steering head geometry.

 

[Ronson]

Hmmm. With good input from you two and more research elsewhere, I'm getting increasingly nervous about messing with spring length in the existing fork tubes... tcb is right, squeezing a +20mm spring into the existing fork would be like adding an extra preload, which would stiffen up the front - possibly to the point where cornering would be dangerous. Eeek.

Since my primary goal is to get a little extra seat height, I think I'll try longer rear shocks first (BC sells them up to +30mm so presumably thats safe) and see if that does the trick. If not - or if the bike looks odd lifted up at the back, then I'll go back to the original idea of longer fork tubes and springs.

Either way, I really appreciate the replies!

 

[sweatmachine]

why do you want to raise it so much?

Why not have a taller seat made?

 

[Thrux-ton-up]

Ronson - the thruxton rear shocks will raise your ride height and quicken up the steering. You can not raise the front end by swapping longer springs into the fork tubes - no matter what spring you manage to put in there, the length of the fork tube assembly will still be the same. All you may do is eliminate the sag which as stated could lead to very dangerous handling.

I have fitted 1997 Honda F3 cartridge forks to the front of my bike. A simple conversion requiring no machining. Use the Triumph lowers, appropriate bushing, dust and fork seals and the rest of the honda guts and tubes which slide right into the stock triple clamps. Thruxtonone's album has a nice picture break down of the parts layout. I did the conversion using his album only. No Triumph or Honda manuals in hand. Traxxion Dynamics helped with the initial fork setup as they did this exact conversion on Ted Cobb's Thruxton Cup series winning bike. The conversion is somewhat tougher on a bonneville that wants to retain stock ride heigth because of interferance with the handle bars being located over the fork tubes. The forks are from a 1997 F3, and are cartridge forks as opposed to the old damping rod style of the stock forks. Cartridge style forks use a shim stack to control damping, the more fork oil pressure on the shim stack the more they flex - kind of a variable rate keeping the forks from becoming too harsh. The stock damping rod forks work by oil flowing through an orfice of fixed size - no variable rate so to speak - the only way to affect this is different fork oil weight and or different spring rates - both of which are somewhat of a compromise. Fork oil is still flowing through a single size orfice and upon a hard impact the forks will become harsh because you can only flow so much liquid through the opening.

I have also upgraded the Honda forks with Race Tech Gold Valves and Race Tech springs. The Honda forks are about 1.3 inches or so longer than the stock units. The combination of longer forks and shocks will cancell each other out keeping the rake/trail somewhat close to stock. My bike is actually lowered, Bonnie size rear shocks and lowered 1 " on the forks. Foot pegs swapped out to regain clearance lost there. Companies could make a longer set of tubes for you and the swap would require a longer spacer or longer springs with stock spacers to complete. Plenty of options for seperating you and your hard earned cash :-D .

At 29 degrees of Rake the bonneville is somewhat chopper like compared to modern day sport bikes.

Plenty of extra forks tube here.

 

[Ronson]

Thrux - thanks for such a complete and knowledgable answer; all great information. You mention that the Honda F3 cartridge forks are about 1.3" longer than stock - is that stock Bonneville or stock Thruxton? Is it true that the Thruxton fork tubes are longer than the Bonneville's to start with? If so, wouldn't my original idea of coaxingThruxton tubes into the Bonneville lowers be the easiest (and least expensive) approach? Also, you seem to think that changing out the rear shocks for a taller pair would have no detrimental effect on handling, even if the height no longer "matched" the forks... have I got that right?

Sweatmachine - the reason I want to raise the bike a little is because I'm 6'3" with a 34" inseam... The bike just about fits me in stock form, but I've always wondered what it would take to gain an inch or so on the seat. Not a dealbreaker (I wouldn't have bought the bike otherwise) but something I've thought about nonetheless. Changing the seat itself doesn't appeal (a) because I like the way the stock seat looks and (b) there seems to be some doubt from previous posters that the alternatives (such as the Scrambler gel seat) actually gain anything more than a fraction of an inch...

 

[Thrux-ton-up]

On 2007-01-09 00:19, Ronson wrote:
Thrux - thanks for such a complete and knowledgable answer; all great information. You mention that the Honda F3 cartridge forks are about 1.3" longer than stock - is that stock Bonneville or stock Thruxton? Is it true that the Thruxton fork tubes are longer than the Bonneville's to start with? If so, wouldn't my original idea of coaxingThruxton tubes into the Bonneville lowers be the easiest (and least expensive) approach? Also, you seem to think that changing out the rear shocks for a taller pair would have no detrimental effect on handling, even if the height no longer "matched" the forks... have I got that right?

Ronson - the thrux and bonnie chassis are the same - they achieved quicker steering on the thrux by using a 18" front wheel and the longer shocks in the back. Adding the same length rear shocks to the back will quicken your steering - but still not as much as the thrux due to your 19" front tire. The fork tubes for the bonnie and thrux are different numbers - but I do not think they are different lengths - I have my thrux tubes and can measure if you need the size to compare. Unless you can find the tubes at discount, buying most items from Triumph seldom ends up being the least expensive way.

John

 

[Ronson]

Got it. Thanks John for another clear and informative answer.

 

[Scrambler900]

It just occurred to me that the higher seat height and longer suspension you want is basically the set-up for a stock Scrambler. Go sit on one at a dealership and you'll see what I mean. I have both a T100 and a Scrambler, and I can compare the ergonmics between the two. The Scrambler feels about 2 inches higher and is perhaps more ideal for a taller rider. Regards.

 

[Whitehurst]

For a fact the front forks on a Bonnie and Thruxton they are the same length.
I recently ventured into lowering my thrux, decided not to - I think! But you just need to buy smaller wheels and Thruxton length Springs for the back and you will have Thruxton sized suspension...

Ter-

[ This message was edited by: Whitehurst on 2007-01-10 10:34 ]