starkmojo

So I am done looking for a bike that comes meeting all of my needs and have decided to try and build one from the bike I have (2019 quick 6 closeout sale bike... https://www.bikejunkie.com/product/c...6-277956-1.htm) I already have slated drop bars, a linger stem and different shifters but I would also like an carbon fork. The closest I could find has a 45mm offset not the 55mm on that comes with the bike. So before I go spending 100s$ on a fork I am trying to understand how such a change would affect the bike handling.

The reason for this is that it is almost impossible to get a XXL gravel type bike. My 2009 CX bike is getting pretty beat and I need to replace it or overhaul it some point soon. I found the Quick on closeout at REI for $400 and while the stem is too short for me by far (my Kona has a 160 on it) it otherwise fits pretty well. I could just stick with the stock for I suppose but while I am modding stuff might as well try and get everything I want at once right?

Jonneh

The quick answer is that for the same fork length (and therefore same headtube angle), the reduced offset will increase the trail and slow down the handling. The Quick 6 you link to already has a fairly slack headtube angle of 70 degrees, and therefore slower handling (all else being equal), so depending on your needs, you might find that slowing it down further isn't the direction you want to be going in.

Good summaries here (on trail): Bicycle Steering Geometry, and here (on fork length): https://www.sheldonbrown.com/rinard/forklengths.htm

starkmojo

So my understanding of this comes from motorcycling: shorter= faster as low speed more stable at high speed. longer = more stable at high speed, sluggish at low speed. The Quick six is currently really good at low speed handling (as I suspect one would want of an urban bike capable of comfortably pedaling at 2MPH) so some more length to the wheel base is OK. I only had a chance to use the bike a few time with kids so I really don't know what the bike is capable of at 20+ MPH. I am thinking now if doing all the other changes I want now and riding the bike like that over the summer and see how I feel about changing the fork next winter.

Thanks for the links and the information.

Jonneh

Assuming by 'shorter' you're referring to trail, it is indeed faster (more nippy) at low speeds, but also less stable at high speeds. Of course, 'good' handling is about being at the Goldilocks point (for you, although there are general rules about what tends to suit people for different cycling purposes), and any nipper or more sluggish will be sub-optimal. The fork you are considering will shorten the wheelbase, slightly decreasing stability/quickening handling, but this effect will probably be less noticable than the opposite effects caused by the increase in trail. Honestly, 10 mm is quite a big difference in offset, and you'd almost certainly notice the change, but it could well be for the better! There's only one way to find out, I'm afraid. ;-)

Incidentally, since offsets in the low-to-mid 40s are very common, perhaps you could dismantle a friend's bike and try out the fork (doing your best to match the lengths)?

DaveSSS 

A picture is worth a thousand words. Mathematically, the best formula for trail is Trail = (R/ tan H) – (offset/sin H). R is the tire radius,and H is the head tube angle. The first half of the formula is trail without the effect of fork offset. The second half is the reduction in trail due to fork offset.

https://www.rodbikes.com/articles/ph...vel-bikes.html

WizardOfBoz

See the similar recent discussion. Pictures! References to nice writeups by world experts! Check this out:
https://www.bikeforums.net/bicycle-m...fork-rake.html

WizardOfBoz

You have to be careful here. All other things being equal, a longer wheelbase is more straight line stable and comfortable, but if you are changing wheelbase by changing rake (aka offset) not all things are equal. You are shortening the distance between the axial center line that the steerer tube pivots on projects to the ground, and the point the tire touches the ground at. You are reducing what is called trail, and this makes your steering response more .... nervous. You can see the picture in the referred-to thread above, but read Dave Moulton's blog (with a good picture). https://davesbikeblog.blogspot.com/2...le-bit-of.html

Andrew R Stewart 

Not yet mentioned is whether the replacement fork (with the 45mm of rake) also has a different axle to crown seat (usually called A-C). It would be no surprise if the replacement fork sits the head tube higher (thus a slacker angle) oir lower (steeper head angle) off the ground. The next aspect to consider is the large movement of the rider's weight forward with the longer (significantly longer it's inferred) stem and the added reach of the lever hoods. Additionally the added lever/castor of the hand placement being so much further from the steering axis will skew handling perceptions.

Given all the going ons I wouldn't hazard a claim of this or that result till after it's tried out. Good luck not having to revisit some of this build up after initial use.

How will the front shifter/der/crankset compatibility be handled? Andy

HTupolev

In motorcycling, "rake" is usually used to describe the angle of the steering axis, while in cycling, it's usually used to describe the fork offset. Shortening the front-center by steepening the former has a roughly opposite effect on steering as shortening it by reducing the latter.

If you lengthen a bike's front-center by increasing fork offset, it will reduce the degree to which the bike's steering stiffens up at high speeds.

Wilfred Laurier

I think you have it backwards - going to a fork with less rake will result in more trail, meaning less nervous/more sluggish handling

Trail is the distance between the intersection of the steering axis with the ground, and the point where the tire touches the ground. The point where the steering axis intersects the ground is in front of the contact point of the tire, and the more fork rake the smaller this distance. So a fork with less rake has more trail and will be more stable or sluggish handling.

WizardOfBoz

We are violently in .... agreement. I meant to say that if you increase rake you decrease trail. And you are saying less rake means more trail. Less trail = responsive/skittish. More trail = stable/sluggish. Sorry if I wasn't clear, but we do agree (I think).

Wilfred Laurier

No! You are wrong! I completely agree with you!

Jonneh

Interesting, and good for this cyclist to know!

starkmojo

Thanks everyone for the help. I am going to stick with changing the bars and shifters this year and look into the fork more carefully now that I have a better understanding of the geometry at play. Probably next Feb. because as much as I love biking I dont do it much in Eugene OR in the depths of winter.