1996 Kona geometry with modern 80mm forks

[kaledi]

Folks,
Recently restored a 1996 Kona Kilauea with new paint scheme and modern parts, but as yet have not got any suspension forks.
The advice I've been given to this point (by Kona and from general comments on forums) is that these frames were never designed for the sort of travel forks have today, and that a 80mm plus fork would slacken the angles too much.
I accepted this as fact until today when I started to calculate the real effects, and compare the a 1996 Kona to the angles of a modern Kona hardtail.

Ok, so question is whether or not I've missed anything obvious in my calculations/assumptions?

The 1996 Kona hardtails had the following
Head angle of 71 deg
Seat angle of 74 deg
Wheelbase of 1054.1 mm
PII forks of 410mm

Using the equation: change in angle = sin-1((old fork length - new fork length)/wheelbase)
then fitting a 85mm Magura forks of 458mm in length would produce the following angles
Head angle of 68.4 deg
Seat angle of 71.4 deg
Wheelbase of 1054.1 mm

Now clearly the changes are significant, but what I've realised is that the angles produced by fitting a modern suspension fork results in angles almost identical to a modern hardtail Kona.
e.g. 2010 Kula fitted with 100mm forks
Head angle of 68.5 deg
Seat angle of 71.5 deg
Wheelbase of 1066 mm

So have I missed something, or do we worry too much (and there is some missguided information on the internet?

 

[WD Pro]

There is the issue of additional stress in the head tube area with a longer fork :-(

 

[kaledi]

yes that part I would accept as a possible issue. However, I'm a relatively smooth, light rider, and moreover, under 'normal' use, I would expect a suspension fork to dissipate forces otherwise transferred directly to the headtube when using a rigid fork.

 

[MikeD]

The only thing I think you've missed is in your favour - that 85mm travel fork will only be 458mm long when you're not on the bike. With 25% sag it'll be 437mm or so when riding. Which means the differences are even less. You might want to drop the bars a touch to compensate for the slightly higher front end.

I used 80mm Magura Ronins on my '95 Kilauea for ages, it's absolutely fine. Wouldn't go substantially longer, though.

 

[Anthony]

I think your calculation is valid. The only aspect I'm not quite clear about is whether the quoted head angle for the 2010 Kula is a static angle, or taken with the fork sagged. I thought it was static, but I'm not certain and if it was taken sagged, then there would be more of a difference than you're thinking.

The bit you've missed out of the equation though is stem length plus bar sweep.

We know that a head angle of 71 plus a stem of 120ish and a 5 degree flat bar gives Kona-style handling. What Kona is saying now is that a head angle of 68.5 with 100mm of travel, a stem of 80ish and swept riser bars also gives the same Kona-style handling.

But leave out the stem and bar aspects and a 68.5 head angle will give you Postman Pat-style handling.

 

[Andy R]

I used 80mm Magura Ronins on my '95 Kilauea for ages, it's absolutely fine. Wouldn't go substantially longer, though.

Not like some stupid people do, who obviously don't know any better.... :roll:

 

[kaledi]

The only thing I think you've missed is in your favour - that 85mm travel fork will only be 458mm long when you're not on the bike. With 25% sag it'll be 437mm or so when riding. Which means the differences are even less. You might want to drop the bars a touch to compensate for the slightly higher front end.

I used 80mm Magura Ronins on my '95 Kilauea for ages, it's absolutely fine. Wouldn't go substantially longer, though.

Indeed, the effects are less with sag! Glad to hear I'm not missing anything obvious. It does seem strange how much of a discussion this creates on forums, and the reality is no where near as significant (provided you aren't silly with fork length).

 

[kaledi]

I think your calculation is valid. The only aspect I'm not quite clear about is whether the quoted head angle for the 2010 Kula is a static angle, or taken with the fork sagged. I thought it was static, but I'm not certain and if it was taken sagged, then there would be more of a difference than you're thinking.

The bit you've missed out of the equation though is stem length plus bar sweep.

We know that a head angle of 71 plus a stem of 120ish and a 5 degree flat bar gives Kona-style handling. What Kona is saying now is that a head angle of 68.5 with 100mm of travel, a stem of 80ish and swept riser bars also gives the same Kona-style handling.

But leave out the stem and bar aspects and a 68.5 head angle will give you Postman Pat-style handling.

Of course, I've moved with the times when it comes to bar and stem - 100 -110 mm stem with wide low risers.
I remember when I put on a set of Syncros 'downhill' riser bars back around 1998, and I couldn't believe how much more control I had of the bike compared to the narrow flat bars I had previously (they really were narrow). It was one of those moments where I realised I'd suffered for about 5 years twitchy difficult riding when I hadn't needed to.

I'm fairly certain all Kona geometry values are static.

 

[MikeD]

Not like some stupid people do, who obviously don't know any better....

Not saying it can't be done, just that it's not my personal preference

We know that a head angle of 71 plus a stem of 120ish and a 5 degree flat bar gives Kona-style handling. What Kona is saying now is that a head angle of 68.5 with 100mm of travel, a stem of 80ish and swept riser bars also gives the same Kona-style handling.

"Kona-style handling" is not what it was. A 2010 Kona doesn't ride like a 1990 Kona. Steep head angle + long stem gives different characteristics than shallow head angle + short stem. You're absolutely right that slack head angles and long stems aren't a winning combination, but you have to ride the two setups differently to get the best out od them - they're not directly equivalent.

Apologies if that's exactly the point you're making

FWIW, the "XC" Kona hardtails like the Kula have 75mm stems on the tiny ones, 90s on the middling ones and 105 on the big'uns.

 

[kaledi]

I think we're all on the same page here with respect interactions with stem/bar and angles.

The part I've learned is that actually the physical geometry of their XC hard tails hasn't really changed at all since the suspension corrected frames were introduced in the mid 90s.

That is until 2011, which does appear to be the year that some changes have been made!