#1
|
|||
|
|||
Head Tube angle question...
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 |
#2
|
||||
|
||||
Quote:
more richie? add rake Quote:
Last edited by e-RICHIE; 07-27-2005 at 06:17 PM. |
#3
|
|||
|
|||
it's all about trail...
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. |
#4
|
||||
|
||||
Quote:
|
#5
|
|||
|
|||
not really...
Quote:
|
#6
|
||||
|
||||
Quote:
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©™® Last edited by e-RICHIE; 07-27-2005 at 07:16 PM. Reason: dangled a particple |
#7
|
|||
|
|||
I agree..
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. |
#8
|
|||
|
|||
Its a really big bike, actually....
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 |
#9
|
||||
|
||||
Quote:
a plumbline and a 6" square is all that is needed. |
#10
|
||||
|
||||
Quote:
|
#11
|
||||
|
||||
[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? |
#12
|
|||
|
|||
Strange
Quote:
Curt |
#13
|
|||
|
|||
Yes, I am sure its 74...
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 |
#14
|
|||
|
|||
Does the picture help?
Or does it not work because the angles are still funny (no wheels or tires to make it look correct)?
Thanks, Steve |
#15
|
||||
|
||||
to be fair, the most important component
is missing........................................you . |
|
|