Head Tube angle question...

[SoCalSteve]

Hi all,

Please bear with me here as I am not very savvy when it comes to tube angles and what they do to the handling of a bike.

My question: If a bike has a 74* head tube angle and a 74* seat tube angle, wont that make the bike quite twitchy? Can that be compensated for by changing the rake of the fork you use? if so, which direction would you go to make it less twitchy? (it was set up to use a 43 rake fork) 40 or 45?

Which brings up a second question... What does a 74* seat tube do to the handling of the bike?

I hope these questions make sense and I am not coming off as a blubberig idiot (doesnt take much for that to happen).

Thanks,

Steve

[e-RICHIE]

Originally Posted by SoCalSteve

If a bike has a 74* head tube angle and a 74* seat tube angle, wont that make the bike quite twitchy? Can that be compensated for by changing the rake of the fork you use? if so, which direction would you go to make it less twitchy? (it was set up to use a 43 rake fork) 40 or 45?

less twitchy? reduce rake.
more richie? add rake

Which brings up a second question... What does a 74* seat tube do to the handling of the bike?

nuttin' honey.

[Dave]

RAKE (offset) AND TRAIL


Rake (offset) is the perpendicular distance between two parallel lines, one through the center of the hub, and one through the center of the steering tube. Trail is the horizontal distance between the tire contact point and a line through the steering axis. The more trail, the more stable the bike (slower steering). The less trail, the quicker the steering. Both rake and head tube angle affect the amount of trail. Steepening the head tube angle or increasing rake will decrease trail, reducing stability and quickening the steering. The formula for trail is as follows, where R is the tire radius, and H is the head tube angle. Trail = (R/ tan H) – (rake/sin H). As an example if R = 33.65cm, H=73, and rake is 4.0cm, trail = 33.65/tan73 – 4.0/sin73. This calculates to 6.1cm or 2.4 inches.

http://www.kreuzotter.de/english/elenk.htm

The STA angle by itself has very little effect on the handling of a bike, because it's entirely possible to build frames with substantially different STAs that have the same front center and fit the same, with the only difference being the type of seatpost required to produce the same saddle position. I have two LOOK frames with this relationship. One has a 74.5 STA and the other has a 72.5, but the reach is within 2mm of being exactly the same. I use a straight-up (Thomson) post on the frame with the slack STA and a traditional post on the other frame.

[zank]

Originally Posted by Dave

RAKE (offset) AND TRAIL


Rake (offset) is the perpendicular distance between two parallel lines, one through the center of the hub, and one through the center of the steering tube. Trail is the horizontal distance between the tire contact point and a line through the steering axis. The more trail, the more stable the bike (slower steering). The less trail, the quicker the steering. Both rake and head tube angle affect the amount of trail. Steepening the head tube angle or increasing rake will decrease trail, reducing stability and quickening the steering. The formula for trail is as follows, where R is the tire radius, and H is the head tube angle. Trail = (R/ tan H) – (rake/sin H). As an example if R = 33.65cm, H=73, and rake is 4.0cm, trail = 33.65/tan73 – 4.0/sin73. This calculates to 6.1cm or 2.4 inches.

http://www.kreuzotter.de/english/elenk.htm

The STA angle by itself has very little effect on the handling of a bike, because it's entirely possible to build frames with substantially different STAs that have the same front center and fit the same, with the only difference being the type of seatpost required to produce the same saddle position. I have two LOOK frames with this relationship. One has a 74.5 STA and the other has a 72.5, but the reach is within 2mm of being exactly the same. I use a straight-up (Thomson) post on the frame with the slack STA and a traditional post on the other frame.

I think that is what e-Richie said...

[Dave]

Originally Posted by zank

I think that is what e-Richie said...

The answer only addressed rake (offset), not the effect of HTA. For an existing frame, that's all you can do is adjust rake if you don't like the steering.

[e-RICHIE]

Originally Posted by Dave

The answer only addressed rake (offset), not the effect of HTA. For an existing frame, that's all you can do is adjust rake if you don't like the steering.

sorry for the brevity.
i thought the question asked "...which direction"
with respect to the rake measurement. "less"
would take care of his issue. hey - i wrote that!

regarding the latter, the seat angle is a means to
an end, the end being the proper placement of
a red aspide saddle. it is easier to view the re-
lationship in linear terms because parts of a cent-
imeter are measured and transposed much more
simply than parts of an angle. ah. breathe deeply.
say s-e-t-b-a-c-k.

one of the beauties of these forums, besides
pals like Climb90210 and a few others, is that
the search engine will pull up all the past threads
that touch on related issues. i think the "angle"
thing has surfaced many times in the past and
i betchoo the archives are filled with information.
e-RICHIE©™®

[Dave]

about linear measurements, but most brands insist on quoting STA, so the user has to know how to convert this info to a linear amount. Pretty simple. Cosine of the STA times saddle rail height equals seatpost setback.

I should have read the question more carefully. Of course reducing the rake will slow the steering, but the other thing not mentioned is the frame size. A 74 degree HTA on a large frame with a long front-center and long wheelbase is not unusual and won't necessarily result in objectionable steering.

[SoCalSteve]

60 cm seat tube

62 cm top tube

19 cm headtube

74* seat tube angle

74* head tube angle

OS chainstays

43* F2 fork

What do you think???

Thanks,

Steve

[e-RICHIE]

Originally Posted by Dave

about linear measurements, but most brands insist on quoting STA, so the user has to know how to convert this info to a linear amount.

a plumbline and a 6" square is all that is needed.

[CNote]

Originally Posted by SoCalSteve

What do you think???

I think the person who rides this bike has a seatpost with a lot of setback. I also think this person should try the 40 rake fork. With a 43, the trail is around 51 mm, which is on the low end. I'd shoot for 58-60. The 40 would give this person a trail of 54 mm... better.

[e-RICHIE]

[QUOTE=CNote]I think the person who rides this bike has a seatpost with a lot of setback...QUOTE]



agreed on this.
on frames that size, that angle will
yield a seat tube setback that allows
few riders to properly attain correct
pedaling position with other resultant
compromises. scs, are you sure 74 is
the angle, or are you transcribing
catalog info?

[cpg]

Originally Posted by SoCalSteve

60 cm seat tube

62 cm top tube

19 cm headtube

74* seat tube angle

74* head tube angle

OS chainstays

43* F2 fork

What do you think???



Thanks,

Steve

It's nothing I'd build. EVER!

Curt

[SoCalSteve]

I got the specs directly from Serotta based on a serial number that was given to me.

I don't know much, but this geometry seemed a bit screwy to me. Thats why I am asking...

Learn something new everyday, huh?

Would it be better to stay away from this one?

I had to resize the photo and got ride of the ST rear end, but here is a picture of the front triangle.

Thanks,

Steve

[SoCalSteve]

Or does it not work because the angles are still funny (no wheels or tires to make it look correct)?

Thanks,

Steve

[e-RICHIE]

to be fair, the most important component
is missing........................................you .