phatjonny

Dear Guys

I built up a frame for my girlfriend last year as her frame was just a little bit too big. I am thinking of building the old frame for myself. The original axle to crown fork (suspension) was about 47cm long. I was thinking of putting on a rigid one which is 40.5 cm long.

My question is how might this affect the geometry of the bike. Would it ride ok? The frame is a hybrid and I plan to use it as a commuter?

I appreciate the feedback

John

sooprvylyn

To answer: yes it will ride ok.

Now to answer how fit is effected:
handlebars will drop 7cm-add stack height or new stem to fix this
BB will drop 2cm give or take- not much you can do here
seatpost will move forward about 2cm-move your seat back to compensate
wheel base will be about 2-3cm shorter-nothing you can do about this


These are the main things that will change about geometry though there are a lot of little changes that you will not likely notice unless you are an avid cyclist. But if you are making a frankenbike commuter you shouldn't be too worried about performance.

Al1943

Shortening the fork will steepen the head tube angle resulting in quicker steering.

Al

krems81 

do you have a sloping top tube? if so, it will slope less or not at all. head tube angle will change, quicker steering as mentioned above, fit may be more like a road bike. You may have to play with the stem and saddle setup to get the bike to fit right again, but it is definitely worth trying.

JonathanGennick 

65mm is a big drop in axle-to-crown distance. What size fork was the frame originally designed around?

I would try to stay close to what the frame was designed for.

phatjonny

The original suspension fork had an a2c of 47, but when seated I imaging the fork compressed by about 1 cm. I am therefore lowering the fork by about 55mm. A fair bit - granted.

I will want to use it for two purposes. Riding fairly hard on gravel forest paths, and commuting in to the local town (this is a 20 minute drop from 600m to 400m altitude, and a 45 minute climb home).

So as long as the bike is only a little bit more twitchy and not actually dangerous I will give it a go...

I appreciate the tips on how to set the bike up.

Ta

John

Al1943

If possible get a fork with a lot less rake offset, this will help slow the steering down. But I think the shorter length is going to make a big difference in steering, may be too twitchy.

phatjonny

Dear All

I took my time, but am pleased to say my Franken Commuter is finished. In the end I went with this fork as it pretty much kept the original geometry.

https://www.chainreactioncycles.com/M...x?ModelID=2317

Not a lot of clearance for the front wheel (enough I hope?), so I will be staying off the forest tracks with this one.

BCRider

You may want to look at some of the 29'er rigids as a suitable option that would provide more clearance. I've also seen some carbon rigids that LOOK like suspension forks on Ebay that are light and look decent enough. No reviews of them that I could find though.

I think you were wise not to go with a 55mm drop in fork length. That would be a BIG change to the front end geometry. Not only that but it would have lowered your bars by that much for far more of a crouch in your riding position. Not only that but it would have angled your seat tube forward and reduced your pedal clearance by significant amounts. All in all fine if you wanted a triathalon time trial like geometry but not really the ideal setup for charging dirt roads or trails.

phatjonny

Not sure on how polite it is to drag up an old thread. even if it is my own...

So I have had three seasons on this bike now, and whilst I had hoped to run some wide tyres on it that was not possible due to clearance issues. I started with 32mm, and am now on 28 tyres. I might make this my winter commuter and add some wider tyres, possibly even some studded ones if I get really stubborn and HTFU as the temp drops here. So I sort of have the same question all over again. Before I ask it I just want to say that the bike rides really well, could not in any way be described as twitchy, in fact it is quite 'flat' in feel.

So If I were to replace the current 437 a2C with one at 405, I suppose the differences would be minimal? Here is the fork I am considering, any idea on what tyre clearance might be, if it is still a little tight then not really worth the change is it?

https://www.kinesisbikes.co.uk/products/forks/cross/dc37

many thanks for the input, as always!

John

phatjonny

Pretty sure I am the only one with interest in this at this stage, but FYIW I changed to the fork above and the bike is now what I originally wanted and there is good clearance for wide tyres so it is now charging through the forests!