Understanding headtube angle, fork offset, and trail in regards to handling

[blantonator]

So I'm trying to understand how heat angle, offset, and trail affect handling...

My current bike has a 72.5° heat angle and a 45mm offset fork for 59mm of trail. If all things were equal and the head angle is changed to 71.5° and the fork offset is changed to 51mm, resulting in the 59mm of trail, will the bike feel/handle exactly the same?

Thanks!

[Mark McM]

Well no, it probably won't feel exactly the same. You can't change the head angle and offset without also changing the top tube length or the front center distance. In your case, assuming the same TT length, the slacker head angle/longer offset will increase the front center. The slacker head angle will also increase the 'flop factor'. But a front center and flop factor changes will mostly affect slow speed handling - the high speed handling (with the same trail) will feel largely the same.

[charliedid]

I'm no frame builder but I think you get more wheel flop in that scenario. Add a bit more offset and you can correct the flop.

At least that is what I think. I have been known to be wrong. It would be cool if I was right but if not I will learn something.

[blantonator]

Well no, it probably won't feel exactly the same. You can't change the head angle and offset without also changing the top tube length or the front center distance. In your case, assuming the same TT length, the slacker head angle/longer offset will increase the front center. The slacker head angle will also increase the 'flop factor'. But a front center and flop factor changes will mostly affect slow speed handling - the high speed handling (with the same trail) will feel largely the same.

If you adjusted the TT length, you could make the front center the same. I agree flop will increase, in this scenario by 1mm. Would one be able to feel 1mm of more wheel flop?

[Kontact]

I ride small frames, and have had everything from 71.5° to 73° HTAs, and I would say that having the same trail largely zeros out the biggest handling differences - if the wheelbase is the same. Shorter wheelbases definitely turn quicker.

Of course, the slack angled HTA bikes came with shorter TTs, so those bikes have longer stems than the bikes with steeper HTAs. But I think the longer "tiller" on the slack angle bikes also act to zero out the effects of wheel flop, which I don't think is pronounced at all.


Overall, if I was looking for a particular kind of handling, I would look at trail and wheelbase over other considerations. And that follows what many builders seem to do as well - use HTA to control front center and TT length and rake to bring the right trail for that HTA. It's just that has gotten harder with carbon forks that aren't able to be bent for optimal rake.

[Mark McM]

If you adjusted the TT length, you could make the front center the same. I agree flop will increase, in this scenario by 1mm. Would one be able to feel 1mm of more wheel flop?

Sure, you could shorten the TT to maintain the front center. But then you'd have to lengthen the stem to retain the same handlebar reach, and the stem length change would also affect handling to some degree.

But you point out the main question: Would one be able feel the difference? I suspect that if you had two bikes with the two different setups, and you quickly switched between them, you probably could detect the small difference. But I also think that riders can quickly adapt to small changes like this, and after some time riding either bike, it would become feeling "normal" to the rider.

[blantonator]

I bring this up because I'm looking to do a custom build and want to handle similarly to my road bike, but with more clearances. I'm looking at two forks, one is a 47mm and the other is a 51. The 47mm fork is close enough I think I could keep the dimensions similar, but if I went with the 51mm fork, I'd need to slacken the bike out by 1° to get the same trail. I've have another bike with a 69° HTA and 50mm of fork and hate it.... WAY too much trail.

[rheosibal]

Are you comparing 2 different bikes?

If all the dimensions are the exactly the same except the HTA and Fork rake then trail would be the same, however, if using the same component (handlebar/stem), then you'll end up with:

about 13mm less reach and 5mm less stack on the (71.5/51mm) bike versus the (72.5/45mm) bike

https://uploads.tapatalk-cdn.com/20180309/18a5d52c7a3e2ef52e8e26858654d097.jpg

[Kontact]

I bring this up because I'm looking to do a custom build, but with more clearances and want to handle similarly to my road bike with a 72.5° HTA with a 45mm fork. I'm looking at two forks, one is a 47mm and the other is a 51. The 47mm fork is close enough I think I could keep the dimensions similar, but if I went with the 51mm fork, I'd need to slacken the bike out by 1° to get the same trail. I've have another bike with a 69° HTA and 50mm of fork and hate it.... WAY too much trail.

For the 47mm you'd want 72.2° HTA, and 71.5° for the 51. If the front center was the same, the difference in stem length between them would be just under 1cm.


I would pay close attention to wheelbase. The most difficult bikes I own to ride hands off have the shortest wheelbases, regardless of angles or trail.

[Kontact]

Are you comparing 2 different bikes?

If all the dimensions are the exactly the same except the HTA and Fork rake then trail would be the same, however, if using the same component (handlebar/stem), then you'll end up with:

about 13mm less reach and 5mm less stack on the (71.5/51mm) bike versus the (72.5/45mm) bike

https://uploads.tapatalk-cdn.com/20180309/18a5d52c7a3e2ef52e8e26858654d097.jpg

Why would you compare the reach with different stacks?

[blantonator]

For the 47mm you'd want 72.2° HTA, and 71.5° for the 51. If the front center was the same, the difference in stem length between them would be just under 1cm.


I would pay close attention to wheelbase. The most difficult bikes I own to ride hands off have the shortest wheelbases, regardless of angles or trail.

Since I'll be going custom, I could have the front center lengthened to create the same reach. I'm just trying to wrap my head around how handling would change. The wheelbase would grow if I increased front center, but would I ever feel 10mm of wheelbase vs HTA and trail.

[rheosibal]

Why would you compare the reach with different stacks?

I didn't. I took the same exact bike with the same exact components and just compared the change in HTA & Fork rake resulting in those differences = change in reach and stack

[Kontact]

Since I'll be going custom, I could have the front center lengthened to create the same reach. I'm just trying to wrap my head around how handling would change. The wheelbase would grow if I increased front center, but would I ever feel 10mm of wheelbase vs HTA and trail.
I think lengthening the wheelbase would be as noticeable as anything else you could change.

I didn't. I took the same exact bike with the same exact components and just compared the change in HTA & Fork rake resulting in those differences = change in reach and stack

I realize how you ended up there, I'm just pointing out that frame builder wouldn't make the slack HTA bike with a shorter stack, and that you can't really compare reach if you don't have the same stack as starting point. As stack increases, reach decreases in a way that doesn't actually tel you much about how far the handlebar is going to end up from the saddle. Reach is a dependent variable of stack.

[MikeD]

I think lengthening the wheelbase would be as noticeable as anything else you could change.




I doubt that.

[Kontact]

I doubt that.

Why?

[MikeD]

Why?


I doubt I'd even feel 1cm increase in front-center but I would feel a 1cm change in trail.

[Mark McM]

Why?

I attended a seminar on bicycle steering geometry design at NAHBS in Hartford, presented by Damon Rinard of Cannondale bicycles (Damon Rinard is well known in bicycle tech. circles, and had been a chief engineer at several other bicycle companies previously, including Trek and Cervelo). He said that for road bikes*, front center was not an important design parameter**, as it had little affect on bike handling.


*MTBs are different matter, and front center can affect handling on very tight single track and very steep downhills.

**The one exception is for very small bikes, where CPSC regulations on toe overlap limit how short the front center may be.

[Kontact]

I doubt I'd even feel 1cm increase in front-center but I would feel a 1cm change in trail.

What experience or data makes you doubt that?

Personally, I have bikes with trail ranging from 56 to 65mm, and they handle more similarly if their wheelbases are above 97cm. The bikes I have with the same type of trail but sub 97cm wheelbases are the "twitchy" ones.

I attended a seminar on bicycle steering geometry design at NAHBS in Hartford, presented by Damon Rinard of Cannondale bicycles (Damon Rinard is well known in bicycle tech. circles, and had been a chief engineer at several other bicycle companies previously, including Trek and Cervelo). He said that for road bikes*, front center was not an important design parameter**, as it had little affect on bike handling.


*MTBs are different matter, and front center can affect handling on very tight single track and very steep downhills.

**The one exception is for very small bikes, where CPSC regulations on toe overlap limit how short the front center may be.

Is front center and wheelbase the same? Did Damon imply that he was talking about wheelbase, or talking about front center? Obviously, two bikes can have different front centers and the same wheelbase.

[Mark McM]

Is front center and wheelbase the same? Did Damon imply that he was talking about wheelbase, or talking about front center? Obviously, two bikes can have different front centers and the same wheelbase.

He did make a few comments on chainstay length and wheel base, but the subject of his talk was on front end geometry and (road bike) handling. He specifically stated that front center was generally not considered as a design parameter (i.e. it fell where ever if fell after the other factors were considered), except in the case where front center may be too short to meet CPSC regulations.

[Kontact]

He did make a few comments on chainstay length and wheel base, but the subject of his talk was on front end geometry and (road bike) handling. He specifically stated that front center was generally not considered as a design parameter (i.e. it fell where ever if fell after the other factors were considered), except in the case where front center may be too short to meet CPSC regulations.

That doesn't sound like commentary on the handling effect of wheelbase.

[kingpin75s]

I attended a seminar on bicycle steering geometry design at NAHBS in Hartford, presented by Damon Rinard of Cannondale bicycles (Damon Rinard is well known in bicycle tech. circles, and had been a chief engineer at several other bicycle companies previously, including Trek and Cervelo). He said that for road bikes*, front center was not an important design parameter**, as it had little affect on bike handling.


*MTBs are different matter, and front center can affect handling on very tight single track and very steep downhills.

**The one exception is for very small bikes, where CPSC regulations on toe overlap limit how short the front center may be.

The qualifier about mountain bikes make sense.

Progressive builder these days often use trail and front center as their determinant factors. That should illustrate their importance if all else is resultant and fit based.

[Kontact]

The talk about front center vs wheelbase has me also thinking about the effect of weight distribution/wheel loading on handling. You could have two bike that have very similar steering and wheelbase, but if one has shorter stays and longer front center there will be less weight on the front wheel. I have no idea what that will necessarily do to the bike's ability to change direction.