frame geometry or longer forks. please explain

You can get frames that are 'designed' to take a range of fork lengths and increasing the fork length obviously slackens the head angle.

And all that seams to be accepted and OK -IE the 456.

I have an old kona and when I discuss putting a slightly longer than designed for fork people say 'ooooh nooo that'll slacken the angles and it will handle like a sloppy thing'

So what is the difference – please explain.

About half a degree probably.

Your Kona was built in an age when there were perhaps just a couple of different fork lengths, those for XC and those for DH. Therefore the frame was designed with the shorter XC fork in mind and, as such, is marketed as 80mm compatible (for example). These days there are a diverse range of fork lengths on the market and the range of what is deemed acceptable for a hardtail is much wider. Therefore if you market a frame as suiting a 100mm fork you are immediately closing a huge chunk of the market out. By saying 100-140mm compatible you catch everyone and increase sales volumes.

it's what you want the bike to do and what the frame is designed for in terms of strength of the headtube, the long the fork the more force on the headtube, like using a longer spanner to get more torque to undo a stiff nut.

The geometry of the bike will also effect the ride the relationship between the length of the back end, toptube, seat tube e.t.c will have been designed for a certain length of fork and riding type. eg a long fork on an xc race hardtail would put you body in a very strange position loads of weight over the back and due to the length of the top tube would mean get you weight over the front for more grip would be awkward.
Most of the current crop hardcore or whatevere you want to call them hardtails with have significantly different geometry to you old kona with shorter top tubes generally.

cheers shepleg.

The extra force of the longer fork is a good point I had not considered.

Having ridden my Kona with a fork too long (100mm rather than 60-80mm it was designed for), the difference was noticeable but actually you could deal with it and ride fine once you were used to it. It felt heavier/slower to steer but once adjusted, I don't think I was actually riding any slower on twisty stuff.

The bigger difference I found was that it threw the seat tube angle off (slackened it) which meant having to run an inline post with the saddle fairly far forward.

Not something I'd do for a long time but it's not completely awful.

i've got an old style trailstar which is designed for a 100-120mm fork which i have 150mm on. it's fine.

The points ahve maninly been made, but…..

If you put 120mm forks on you Kona to get the same head angle as a 456 your Kona will probably have

A higher bottom bracket

Shorter effective top tube

slacker seat angle

I think Brant's plan was that the slack head angle wasn't a problem combined with a short stem, conventional bottom bracket height and steep seat angle. The steep seat angle keeps the weight on the front wheel

You may find this page useful.

http://www.on-one-shop.co.uk/?page_id=590

the front of the bike is higher, or to look at it another way, the bb is lower. Also on a lot of bikes the head angle starts off slacker so that at full compression it doesn't get too steep. You might get away with longer forks on some bikes but not by much.

Aye – seat angle is a lot to do with it. The funny floppy feeling some people feel on some bikes with "longer than they need to be" forks is that there's not enough weight on the front wheel to tame it because it's all rotated backwards.

Same reason the larger Ragley's have steeper seat angles to stop big lads sitting over the back wheel too far. Nobody else does that and I'm not sure why.

Higher BB's also lift weight backwards too. Though for the same headtube length, a higher BB has a greater saddle-to-bar drop which can pin the front end down better.

It's not just climbing though is it? In my experience you end up with diving 'oversteer' in the corners.

I snapped a Kona Caldera (when they were still skinny steel tubes) putting longer forks on it.

Given that I was a kid at the time I felt the shop could maybe have warned me that 130mm marzocchis and steel XC frames didn't mix… 🙄

adstick – Member
It's not just climbing though is it? In my experience you end up with diving 'oversteer' in the corners

don't you mean understeer?

understeer is where the front tends to go straight on despite lots of steering lock

and over steer is where the rear trys to come around ,

I meant oversteer, but it's probably a crap analogy. Too long forks seems to me to upset the self righting properties of the bike's steering. The front wheel wants to turn in on corners.

I stuck 130mm Revs on my Malt 1. I had been told that it would handle like a pig as the frame was designed to use 100mm forks. When I rode it with 100mm forks I always felt like I was gonna go over the bars on every downhill and the 130mm Revs have resolved this problem and I've found no real downside.
Basically, as someone said though ^^ it all depends on the bike and what you want out of it.