Bontrager Frame/RS Fork Geometry question

[No Longer Registered]

I wondered if anyone could clarify if i'm about to, or already have messed up the frame/fork geometry on my Bontrager.
The frame is a late one piece seat stay 96/97 race frame (still has the canti cable stop on the wishbone). I found a old 'Global Fridge' Catalog last night from 1993 and its lists the Bontrager OR being sold as a frame and fork and states that the head angle is 71' and seat tube 73' with the ORS frame having a 74' seat tube.

My question is the fork I have is a early 1994 Bontrager triple crown fork (I think) that is 390mm from head set race face to the middle of the drop outs, my frame is later and is the geometry suspention corrected for a longer fork leg? I've got a Pace RC-30 fork on the bike at the mo, which is also 390mm and the top tube is parallel to the ground on a large frame. Do they have a slight slope on them?

Sorry for all the questions, but I'd like to know if i've messed up Keith B's sums for the perfect ride!

 

[Anthony]

I don't know Bontys Steve, but Konas from that era were 'corrected' for a 41cm P2, i.e., they have a 71 degree head angle with a 41cm fork. Konas prior to 94 were not 'suspension-corrected' and had a 71 degree angle with a 39cm P2.

I wouldn't be surprised if Bontrager's idea of suspension-corrected was about the same, in which case the Pace would make the front 2cm lower than the bike was designed for and give a head angle of just over 72 degrees. This should make the handling slightly sharper and more nervous, but I wouldn't have thought you need to go as far as saying 'messed up', some people prefer it that way - anyways you're the best judge!

 

[scant]

I'm no bonty expert, but the earlier mag21 bonty (just for carl edwards crown was a shorter fork rake than normal.

 

[pete_mcc]

The frame you have is suspension corrected so the forks you have are a little too short so either:

1) get some new forks that are more modern and give me the bonty and pace ones
2) let some air out of your rear tyre and pump up the front!

Think there is a 30mm difference between your forks and what should be on there.

As Scant said the bonty specific crowns were reduced rake ones rather than a difference in length, so to get the perfect KB ride you'll need a set of black crowned KB/rock shox suspension forks and nothing else will do!

To be honest, I can't feel the difference between a sus adjusted and non adjusted fork on a suspension adjusted frame as long as it's not a real modern 80-120mm adjusted frame. Get them bonty forks on it now!



(e edwards 'bonty' count=3, annoyance level: medium high)

 

[scant]

(e edwards 'bonty' count=3, annoyance level: medium high)

LOL we love u really carl

 

[Anthony]

If Pete_mcc is right and the frame is 'corrected' for a 42cm fork, then you've currently got a head angle of almost 73 degrees with the Pace, and I suppose that is pretty steep. And if your Bonty suspension fork is the same length as the Pace static, then it'll be shorter with you on the bike and give an even steeper head angle.

Interesting idea about tyres. I guess you could run a 2.25 on the front and a 1.8 on the back and that would have the same effect as a 1cm longer fork! But it sounds as though a nice set of Judy SLs would be the right length - the ones I had were 43cm static. I didn't know that Bontrager had special reduced-trail crowns made to sharpen up the steering, but even without those I expect the bike would still handle very nicely with the standard crown.

 

[No Longer Registered]

Thanks for the enlightenment.. The obvious solution is to cut the headtube off and weld it back on at a different angle. LOL. I'll go ahead with the fitment of the forks, but may get a spacer made up like Lid has to correct the height. It rides ok, but somthing does not took right with the Pace fork at the moment.
Time to find some Bonti crowned RockShox for the Bonty frame. (2 x Slang Bontrager reference with different spelling = med high)

 

[scant]

its been a long time since i've had to think about this. but when I wanted a mag21 crown with fuquay fork blades checking the order sheet there were 3 different models, I think the numbers are correct, tho I could be slightly out:
stock 1.75 rake
1.5in rake, fat chance etc
ultra short 1.25 for bontrager, with black crown.

which reminds me i've got some 1 1/8 threaded mag10s for sale.. somewhere..

 

[Anthony]

its been a long time since i've had to think about this. but when I wanted a mag21 crown with fuquay fork blades checking the order sheet there were 3 different models, I think the numbers are correct, tho I could be slightly out:
stock 1.75 rake
1.5in rake, fat chance etc
ultra short 1.25 for bontrager, with black crown.

which reminds me i've got some 1 1/8 threaded mag10s for sale.. somewhere..

that sounds very much like Kona, who had P2s with 1.74" offset for Hahannas and Fire Mountains and 1.54" for the bikes higher in the range.

 

[Crell]

No, as your bike will almost certainly have had suspension forks (I don't know if they were made then with the Comp). You'll steepen all the angles, and the front will be pretty twitchy - they are already "light" on the std geo.
Fit your fork, check the seat angle is the easiest way to to validate if it's a Suspension adjusted frame or not.

Also answered here, useful reading (see the last comment).

http://forums.mtbr.com/archive/index.php/t-5545.html