ht angle, rake, trail, steering question

[eddief]

i could not describe to a friend what the difference in handling dynamics or steering feel would be between these two bike setups:

1. 72.5 headtube angle, 43 rake, 58 trail

2. 72.5 headtube angle, 45 rake, 56 trail

for those of you who understand and can feel steering dynamics, how would you describe potential handling differences between these two geos? could the average joe even tell the difference?

[Ken Robb]

#2 would steer a little quicker but I think the difference would be imperceptible to me.

[Smiley]

DON'T fall asleep on either one, really ...I love a Trail of 6.1

[eddief]

my Roubaix, according to the Specialized published dimensions, is 72.5, 49 rake, 56 trail and it rides just right for me. i am not sure i could tell the difference in a couple of mm for trail in either direction. so i guess the suggestion here is lower trail might conjure faster or twitchier.

but it seems like that rake, ht angle, and trail number for the Roubaix don't jive with some geo configurators i have played with.

[Smiley]

my Roubaix, according to the Specialized published dimensions, is 72.5, 49 rake, 56 trail and it rides just right for me. i am not sure i could tell the difference in a couple of mm in either direction. so i guess the suggestion here is lower trail might conjure faster or twitchier. but it seems like that rake, ht angle, and trail number don't jive with some geo configurators i have played with.
YES

and there is a difference in a few mm either way, really

[Wilkinson4]

What size tires? How wide? At what speed? That all has an effect too. All things being equal, with a 23-25mm tires I would expect lower trail to handle quicker.

But, if you are running wider tires or 650b things can change. Or, if you plan on a front load it may feel quicker at low speed without the load but add a few lbs, go faster and then it will handle and carve turns beautifully.

mIKE

[Kontact]

i could not describe to a friend what the difference in handling dynamics or steering feel would be between these two bike setups:

1. 72.5 headtube angle, 43 rake, 58 trail

2. 72.5 headtube angle, 45 rake, 56 trail

for those of you who understand and can feel steering dynamics, how would you describe potential handling differences between these two geos? could the average joe even tell the difference?
The correct trail numbers are 62.1 for #1 and 60.0 for #2.
http://bikeraceinfo.com/tech/trail.html

Either of which are "high trail", or above the neutral point of 56-57mm. As trail gets higher the bike gets more stable and harder to quickly manuever at high speeds, but it also gets more difficult to ride hands off at low speeds. I would always select whatever is going to get me closer to neutral so the bike handles the same at all speeds.

Read Tom Kellogg's explanation:
http://www.spectrum-cycles.com/geometry.php

[eddief]

has one approach and Smiley has another. guess i'd go with neutral or close with 700x25 tires and no load.

[Kontact]

This article articulates the trade off very well:
http://www.dclxvi.org/chunk/tech/trail/

[tuscanyswe]

The correct trail numbers are 62.1 for #1 and 60.0 for #2.
http://bikeraceinfo.com/tech/trail.html

Either of which are "high trail", or above the neutral point of 56-57mm. As trail gets higher the bike gets more stable and harder to quickly manuever at high speeds, but it also gets more difficult to ride hands off at low speeds. I would always select whatever is going to get me closer to neutral so the bike handles the same at all speeds.

Read Tom Kellogg's explanation:
http://www.spectrum-cycles.com/geometry.php

How would one know the exact trail of the bikes in example when theres no fork lenght listed?

I cant tell a diff between a 2mm change in rake but i like quick steering!

[Peter P.]

I'm not sure whether you'd feel the difference of 2mm in trail. I know I've felt a 5mm difference.

I don't think the correct description of lower trail would be a "faster" steering bike. "Twitchier" might be better in describing it to your friend. Faster implies a steeper head angle and thus a tighter turning radius. Twitchier to me means the bike has less of a tendency to hold a straight line without more of the rider's steering correction to keep it so. That's how I view trail; the desire of the bike to hold a straight line without a lot of user intervention, and the bike's willingness to initiate a turn.


As Kontact pointed to Tom Kellogg's explanation on trail, I have to say my experience with changes in trail is the same: with low trail bikes at higher speeds, you lose the feel of the front tire contacting the pavement. My arms brace more, struggling to keep the front tire pinned to the road because it feels like I'm on ice. For a better example high trail bikes, try riding your mountain bike.

So the bike with less trail would feel more sensitive to changing a line or just maintaining a straight path while the bike with more trail would need less attention to keep straight. However, both bikes would be readily rideable as 2mm is not a significant difference and both bikes are within the range of acceptable.

You DO need to know tire size (wheel radius) when calculating trail, and fork length has no bearing whatsoever on trail.

[David Kirk]

I think it's all too tempting to try to isolate a number like trail and say that a bike with more/less trail will handle a certain way. I suspect I'll sound like a broken record here but one can't usually analyze a number like trail in isolation.

The problem is that there are any number of ways to end up with a given amount of trail - a steeper head angle with less rake can give the same trail as a shallow head angle with more rake. You have the same trail but the front center and wheelbase both change and this change effects how much weight is on the front wheel and this of course changes the handling and steering feel. Weight distribution is king.

If you are asking, as Eddie did, how the handling will change if you change the fork rake and nothing else then you can make a pretty good guess at the end result. The 2 mm of rake change will of course effect the trail by a reasonable percentage while not changing the front center or wheelbase very much as a percentage. So the difference one would feel could be attributed largely to the rake/trail change and not be too muddied by the front center/wheelbase change.

I think most of us could feel the difference if we could compare the two set ups back to back and go back and forth between the two. Even a 2 mm change in trail will have a noticeable effect on steering feel. The trouble is the difference is small and we are very adaptable and we quickly get used to the new set up and forget the feeling of the old one. This ability to adapt doesn't mean that the difference isn't there or that it doesn't matter - it just means that we can adapt. I've tested this in the past with a fork with adjustable rake (slotted dropout) so you could stop, change the rake without even getting off the bike, and take off again. When you do this you'll find that 2 mm of rake/trail change can easily be felt. Both set ups might be fine and both might be acceptable but they will feel different.

But if you are comparing two different bikes with different head angles, or stem lengths, or top tube lengths, or BB drops, or torsional stiffnesses or.......etc....... then all bets are off. This is why I raise an eyebrow a bit when I hear someone say that a given trail is best. Best when it is combined with what other numbers? Everything works with everything else and the end result is unique.

Oh...........and HAPPY NEW YEAR!

dave

[Dave]

The correct trail numbers are 62.1 for #1 and 60.0 for #2.
http://bikeraceinfo.com/tech/trail.html

Either of which are "high trail", or above the neutral point of 56-57mm. As trail gets higher the bike gets more stable and harder to quickly manuever at high speeds, but it also gets more difficult to ride hands off at low speeds. I would always select whatever is going to get me closer to neutral so the bike handles the same at all speeds.

Read Tom Kellogg's explanation:
http://www.spectrum-cycles.com/geometry.php


The trail values you've listed are only correct for one specific tire radius and it's not the value that I would use for a common 23mm road tire. I use a tire circumference of 2110mm, that yields a 336mm tire radius.

The formula for trail is (R/tanH) - (rake/sinH). Using a 336mm tire radius, I get values of 61 and 59mm. When comparing small differences in fork offset (rake), more offset reduces the trail almost 1 to 1, since the sine of 73 degrees is .96.

As for 56mm being "neutral", that's just one person's opinion. It does not make a bike steer "the same" at high and low speeds. Some brands, like Colnago and LOOK use significantly more trail on the smaller frame sizes. If you're descending around high speed corners on a small Colnago, the large trail will require more rider input to keep the bike turning. I never noticed any steering problem with low speed climbing due to the large trail. I've also ridden a first year model 51cm Cervelo R3 with 56mm of trail, combined with a very short wheelbase and didn't like it at all. Fast forward a few years and Cervelo has changed the geometry to be a lot more like a LOOK. Apparently, many customers didn't care for the 399mm chainstays and short front-center combined with a small steering trail.

As for fork length, the person asking about that is most likely thinking of the case where a bike is designed to have a specific HTA with one fork length, but the fork being used may not be that length. If you buy a new fork that's longer or shorter than the original, it will change the HTA and trail.

[eddief]

my tendency is to try to look at trail in isolation...and less obviously it works in conjunction with everything else. front center length and wheel base are two i tend to miss.

[khjr]

I I've tested this in the past with a fork with adjustable rake (slotted dropout) so you could stop, change the rake without even getting off the bike, and take off again...Oh...........and HAPPY NEW YEAR!


Be careful Dave - that's how new product offerings are born in a mature and feature-poor market! In a decade, we'll all be wondering how we ever lived with fixed rake under varying riding conditions. Inevitably there will be some ungodly expensive variant that has rake electronically adjustable on-the-fly...

Happy New Year to you too!

[Kontact]

The trail values you've listed are only correct for one specific tire radius and it's not the value that I would use for a common 23mm road tire. I use a tire circumference of 2110mm, that yields a 336mm tire radius.

The formula for trail is (R/tanH) - (rake/sinH). Using a 336mm tire radius, I get values of 61 and 59mm. When comparing small differences in fork offset (rake), more offset reduces the trail almost 1 to 1, since the sine of 73 degrees is .96.

As for 56mm being "neutral", that's just one person's opinion. It does not make a bike steer "the same" at high and low speeds. Some brands, like Colnago and LOOK use significantly more trail on the smaller frame sizes. If you're descending around high speed corners on a small Colnago, the large trail will require more rider input to keep the bike turning. I never noticed any steering problem with low speed climbing due to the large trail. I've also ridden a first year model 51cm Cervelo R3 with 56mm of trail, combined with a very short wheelbase and didn't like it at all. Fast forward a few years and Cervelo has changed the geometry to be a lot more like a LOOK. Apparently, many customers didn't care for the 399mm chainstays and short front-center combined with a small steering trail.

As for fork length, the person asking about that is most likely thinking of the case where a bike is designed to have a specific HTA with one fork length, but the fork being used may not be that length. If you buy a new fork that's longer or shorter than the original, it will change the HTA and trail.
The numbers I used seem to be the same numbers other people, like Spectrum use. Merlin's Kellogg designed geometry associates 73 degrees with 45mm of rake, 73.5 with 43mm of rake, etc. I think there is a good reason the rakes offered in forks with a choice tend to be 45, 43 and 40 - they match up pretty well to 73, 73.5 and 74 degree headtubes. I would have to wonder if there is something you aren't taking into account, like decreased wheel radius at the contact patch. This would seem more sensible than presuming that noted builders don't know their business - but I don't have any better information.

Also, 56-57mm of trail being neutral is widely held by many designers.

I don't think analyzing steering feel in a climb makes much sense. Your weight distribution is very different in a climb.

The problem is that there are any number of ways to end up with a given amount of trail - a steeper head angle with less rake can give the same trail as a shallow head angle with more rake. You have the same trail but the front center and wheelbase both change and this change effects how much weight is on the front wheel and this of course changes the handling and steering feel. Weight distribution is king.
David, I had considered the same problem before - shallower HTA's are more "self righting", but I also thought that it might be very difficult to define that effect because:
1. Stem length changes the leverage necessary to move the wheel, and the resistance. So everyone's bike is going to feel different just from their stem selection.
2. Rider weight is a big a factor as weight distribution, and small bikes for lighter riders have shallow HTAs, while heavy guys have steep HTAs, so it seems like a wash.

2mm being only .2% of wheelbase, it seems like changes in rake/trail can be made in virtual isolation from most every other geometric consideration. My understanding is that trail describes not just something we can feel, but a geometric relationship between the caster effect and lean angle: Castering's relationship changes at increased lean angles the most when trail is furthest from neutral.

But I do understand your point that anything the bike is supposed to do can be undone by weight distribution - is there a weight distribution that can turn a low trail bike into high trail behavior? Or will the low trail bike always feel vague at high speeds regardless of rider weight distribution?

[David Kirk]

....................
But I do understand your point that anything the bike is supposed to do can be undone by weight distribution - is there a weight distribution that can turn a low trail bike into high trail behavior? Or will the low trail bike always feel vague at high speeds regardless of rider weight distribution?

Good thoughts and questions - no there is nothing weight distribution can do to change the geometric trail - it is what it is. But the amount of weight acting on a given trail will make it feel different. It's the weight pressing down on the front wheel that makes the trail do what it does and keep the front wheel pointed straight ahead. For an extreme example consider riding the bike no handed and them walking along next to the bike holding the bike up by the seat. When riding the bike no handed it's still fairly stable (especially if the rider leans forward) but when walking the bike with no weight at all on it it will tend to flop to the side quickly.

So by putting more weight on the front wheel we make the bike more stable.

dave

[Kontact]

Thanks David. Sounds like we're on the same page.


Dave, here's another trail calculator where you put in tire size. Same results, though:

http://yojimg.net/bike/web_tools/trailcalc.php

According to this calculator, changing from 20 to 23 and 25c tires only changes trail between 55, 56 and 57mm.

[mister]

too many variables to only look at trail and/or head angle
i've got stable bikes at 54mm trail and 58mm and i can feel the difference going from a small tire to a larger tire...that might be from the trail difference but i'm sure the actual contact patch change makes some of that difference too.
like kirk said though, we adapt so quick that it doesn't matter that much, i like to get my bikes setup to where they feel right for me and then not change a thing, and i like to ride the same bike for extended periods...

had a crit frame that trail must've been in the high 60s on, didn't handle as weird as you'd think...maybe because the tires ended up in good spots...

i pay attention to CS length and front center more than anything, and lately i kinda realized that BB height plays a large factor in how the bike feels.

[Dave]

Weight distribution is not "very different" in a climb. Most of my long mountain climbs are in the 4-8% range. That's makes the road angle less than 5 degrees. A really steep 17% grade is only a 10 degree angle. Put a 3/8" thick board under the front tire of a bike, sitting on a level floor and it will be angled at about 5 degrees.

A really steep climb is the only time that my speed might drop into single digits for any length of time and slow speed steering become important. That's why I use it to evaluate low speed steering. How well a bike steers in a parking lot doesn't interest me.

If you want to get a significant change in the weight the front wheel, then see what happens as the torso angle is changed, from the lowest level that's comfortable to sitting more upright, with the hands on top of the bars. Place a bathroom scale under the front wheel, with the back wheel height set to the same height. Watch the weight on the front wheel increase as the torso is lowered.

[Kontact]

Weight distribution is not "very different" in a climb. Most of my long mountain climbs are in the 4-8% range. That's makes the road angle less than 5 degrees. A really steep 17% grade is only a 10 degree angle. Put a 3/8" thick board under the front tire of a bike, sitting on a level floor and it will be angled at about 5 degrees.

A really steep climb is the only time that my speed might drop into single digits for any length of time and slow speed steering become important. That's why I use it to evaluate low speed steering. How well a bike steers in a parking lot doesn't interest me.

If you want to get a significant change in the weight the front wheel, then see what happens as the torso angle is changed, from the lowest level that's comfortable to sitting more upright, with the hands on top of the bars. Place a bathroom scale under the front wheel, with the back wheel height set to the same height. Watch the weight on the front wheel increase as the torso is lowered.
I have to call you on your math again. A 3/8 board will lift the front wheel .5 degrees, not 5 degrees. 5 degrees is more like 4" up front.

Anyway, part of this equation is that people do sit up when climbing. That, combined with the slope, takes a large amount of weight off the front wheel, making comparisons to low speed handling on a level surface problematic. Which takes us back to cobblestones and Merckx, not parking lots.

[Dave]

I have to call you on your math again. A 3/8 board will lift the front wheel .5 degrees, not 5 degrees. 5 degrees is more like 4" up front.

Anyway, part of this equation is that people do sit up when climbing. That, combined with the slope, takes a large amount of weight off the front wheel, making comparisons to low speed handling on a level surface problematic. Which takes us back to cobblestones and Merckx, not parking lots.

Oops, you're right about the amount to block the wheel. I got my millimeters and centimeters mixed. The angle of the road is correct though. An 8% grade, is a rise/run (tangent) of .08. The inverse tangent of .08 is 4.6 degrees. Since it's not practical to measure the horizontal run of a road, grade then becomes the inverse sine of the grade, which is nearly identical for small angles (.3% error at 8%).

I personally don't make lengthy climbs in an upright position. I like to keep my hands on the brake hoods, but regardless, climbing is one time when it's valid to evaluate low speed steering.