5mm BB height difference

Bike geometry is a book that still has many unread pages for me.

Now, I am contemplating changing my (road bike) front fork by one with a different offset.

However, the model I have in mind also has a different, longer axle-to-crown (AtC) height. This changes the head and seat angle.
The combination of the new head angle and the different offset gives a desired trail number. And the seat tube angle I can compensate by moving the saddle height/fore-aft position. Stack and reach difference I can also compensate.

BUT, as a result of the different AtC height, the wheelbase will also grow by 5-6mm, chainstay lenght will grow a little, and the BB drop will decrease with approximately 5mm (to 65mm, which is CX territory, I read).

The latter is the reason for my post. How hard will I feel this? Does anyone have experience with this?

Just to be clear, as this is often an area where terms are used interchangeably (and erroneously), by “decrease by 5mm” you mean the BB drop is going from 70mm to 65mm?

If so, then you can expect the bike to be faster handling and less stable. Your center of gravity will be higher, leading to more responsive, but less stable, handling. In your case, this may be offset by achieving your preferred trail number, as noted above.

Personally, I prefer a lower BB.

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Yes, indeed. Less BB drop (and thus greather BB height).

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I’m personally not a fan of a low BB Drop on the road. I’ve used CX bikes as winter bikes in the past and can’t say that I ever really enjoyed the lower stability. That plus wet winter roads certainly put me on edge a little more.

Still, I know other people who ride CX bikes on the road and really don’t mind it.

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5mm can be a lot and nothing at the same time. Additionnally the end result will be the sum of all parameters. It also depends what is the bike made for. I would certainly like my BB drop much higher on a TT bike than a crit bike for example.

I don’t like talking about stability/instability. A bike doesn’t suddently feel twitchy because your BB drop is higher. Tall bikes would be impossible to ride if it was the case. BB drop affect how reactive your bike is to weight transfer when you lean your bike. That is about it.

Now how a 5mm difference will affect the handling of your bike? It is hard to judge as some other changes such as increased wheelbase might negate and trail will affect it at the same time provided your contact points are mor or less the same (saddle should be 5mm higher though).

One way to simulate if 5mm less drop makes a huge difference is to mount your saddle 5mm higher, swap your clipless pedals to flat pedals and test your bike on shoes with a thicker sole. It is not hard to reach a 5mm difference in stack.

Keep in mind that with the trend to gravel biking a lot of people are affecting the BB drop by similar or higher values on a regular basis by swapping wheel diameters and tires volumes. Even taking SAG into account, the difference in BB drop between the smallest wheel diameter in 650B and highest in 700C I have used on my gravel bike is bigger than that. While I could feel the difference, I never felt like my bike handling was compromised per se.

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You will likely notice the difference, and the bike will handle differently, but it will still be a bicycle and it will still be rideable. CX bikes were the old school gravel bikes, and plenty of people were fine with them on the road. How much it bothers you will be a personal thing, and likely come down to the whole geometry change and whether the new fork solved the previous handling issue that is leading you to the change.

My guess (very much a guess) is that if you’re changing the fork to get more stability, then you’ll probably be unhappy with the change overall. If you’re changing for a different reason and the new forked solved your issue then it might be perfectly fine once you get acquainted with it.

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roll of the dice. if you estimate 5mm at the bb, the whole bike is getting lifted by up to ~10mm at the headset. even though you can readjust and restore rider to bb center, the attitude of the frame wrt the road is changed. all in addition to whatever a different fork brings.

I know. (See my first post.) It’s mainly the BB height difference that I’m unsure about.

Yes, I see. I’ve done CX bikes w/ road tires on pavement. Didn’t find the higher BB to be important. In my case, I could tell BB was higher but it was the slacker head angle that made the ride and handling different.

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Thx for the opinion.
It’s a complex story to find a new steering balance after changing my fitting on the bike. (different handle bar width & reach, different stem)

A follow-up: in the mean time I found a fork with an intermediate lenght and offset. It was a fork that was always out of stock, until now. This fork raises the BB only by 2mm.

It is installed now, and a quick test was fine. Looking forward to longer rides with it.

EDIT 15/04/2022: It’s a keeper, this setup.

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