What would cause these bikes to handle differently???

[scottcw2]

I have ridden two bikes recently that handle quite differently even though (I think) they should be very close.

Both have 73.5 HT angle.

Bike 1 - HT length is 19.2cm, fork has 40mm rake, ~55mm trail.

Bike 2 - HT length is 17.0cm, fork has 43mm rake, 56mm trail.

What other factor(s) am I missing? One of these bikes handles very stable, the other is much more twitchy. Can you tell by these numbers which is which and why? Thanks.

[Brian Smith]

If the stems and handlebars are the same, then I would say that the 2.1cm difference in handlebar height is placing more of your (the rider's) weight on one handlebar than on the other.

Depending upon the experience- and skill-levels of the rider, one of those could probably feel less controllable than the other.

[PacNW2Ford]

Something is amiss with your trail calculation. With a constant HT angle and a change in rake of 3mm, the trail will change by more than ~1mm. If all other things are relatively constant, this may cause a difference in steering response.

[Grant McLean]

What's the bottom bracket height?

Do they have the same seat tube angle?

Your weight distribution will be different if they don't have the same
front center, which changes with seat tube angle.

[scottcw2]

Something is amiss with your trail calculation. With a constant HT angle and a change in rake of 3mm, the trail will change by more than ~1mm. If all other things are relatively constant, this may cause a difference in steering response.

Not if the span is different, correct?

[scottcw2]

What's the bottom bracket height?

Do they have the same seat tube angle?

Your weight distribution will be different if they don't have the same
front center, which changes with seat tube angle.

BB drop on both is 8cm, not sure of the height.

STA on bike 1 is 72.5, bike 2 is 73.

[swoop]

I have ridden two bikes recently that handle quite differently even though (I think) they should be very close.

Both have 73.5 HT angle.

Bike 1 - HT length is 19.2cm, fork has 40mm rake, ~55mm trail.

Bike 2 - HT length is 17.0cm, fork has 43mm rake, 56mm trail.

What other factor(s) am I missing? One of these bikes handles very stable, the other is much more twitchy. Can you tell by these numbers which is which and why? Thanks.


the relationships within the geometry like bb drop and chainstay length for one. lower bb longer stays more stability... wheelbase is a factor, front center and rear center is a factor. bb drop can really affect the feel of a bike. i'm no expert.. but that's what i've noticed.

[whippettanker]

Something is amiss with your trail calculation. With a constant HT angle and a change in rake of 3mm, the trail will change by more than ~1mm. If all other things are relatively constant, this may cause a difference in steering response.

Kogswell has a nice FEG calculator here: http://kogswell.com/geo.php

You'll need Flash installed and you'll need to know your tire OD.

[Louis]

As was correctly stated above, HT length has no effect on trail.

The equation for trail is:

Trail = wheel&tire radius / tan(HT angle) - rake / sin(HT angle)

In your case:

(Rake) (HT Angle) (Wheel & Tire Radius) (Trail)
(-40-) (--73.5--) (------340----------) (59.0)
(-43-) (--73.5--) (------340----------) (55.9)

I'm not an expert on this, but a trail of 59 vs 56 is not a huge difference. Not nothing, but not huge either.

Louis

[cpg]

I have ridden two bikes recently that handle quite differently even though (I think) they should be very close.

Both have 73.5 HT angle.

Bike 1 - HT length is 19.2cm, fork has 40mm rake, ~55mm trail.

Bike 2 - HT length is 17.0cm, fork has 43mm rake, 56mm trail.

What other factor(s) am I missing? One of these bikes handles very stable, the other is much more twitchy. Can you tell by these numbers which is which and why? Thanks.

The answer to the last question is no. There are many other factors and there's no silver bullet or dimension. Stop looking for the numbers and trust your butt. The whole numbers thing is the realm of framebuilders. Once the bike gets to the consumer the numbers are constant. So there's nothing left for the consumer to do but ride it and judge for themselves. If you like it, ride it. If you don't, don't. Sorry if that sounds grumpy. It's not meant that way.

Curt