Results 1 to 25 of 25
  1. #1
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24

    New to DH, can anyone help me with some geometry related questions?

    Hey guys, as some of you may have read in my earlier posts, I'm getting back into riding after a way too long hiatus, and am building a new bike. (Mostly from used parts though.)

    I used to do heavy XC riding, borderline light DH even, but that was when I was little enough to ride that piece of junk that passed for an okay entry level XC bike back then.
    Now that I got back into riding during the summer, I want to give some more serious DH tracks a shot too, and thus I'm trying to build a bike that is a good compromise between the two styles. Not easy apparently.

    To that end, could anyone point me to a -thorough- guide to bike geometry? Headtube angle is what I'm most interested in and how it affects the overall feel of the bike. Whatever guide I looked at simply skims over the subject saying steeper gives sharper turns, while slacker gives slower turns...

    Now I'm a bit confused. I thought a slacker angle was supposed to make a bike more stable at speed? I've got a 2004 RS Boxxer WC installed in front, which raises the nose of the bike, and gives me a pretty slack angle (I'll have to measure it tomorrow when I've access to tools). What happens, is basically that the fork itself points in front of where the wheel actually touches the ground, resulting in somewhat awkward steering at low speeds, as the steerer, once out of center, pretty much moves to the side it's moved towards on it's own. I hope what I'm saying makes sense. Are all DH setups like this, or is there something special about higher end DH frames to make low speed steering better? (Especially, if I stand up on the pedals while rollign forward, this makes steering wobble really easily. It rides fine at higher speeds, though climbing can be annoying sometimes. I've only tried it out on rough terrain in a mostly level environment so far, but there, I loved the way it handled.)

    Also, I feel that (compared to my previous bike) it's like I'm sitting further back than there. I actually had to move the seat as far front as it would go, and it's still placed lower then any of my previous bikes just so that I can reach the pedals proper. Again, is this a DH thing, or should I just start messing around with angled seatposts and whatnot? I imagine it'd work fine for an actual DH course, but feels weird on level terrain...
    I've yet to try any bikes built specifically for DH, so excuse my noobness

    On the same note, is there a limitation on head angles for given forks? I'm mostly curious how slack I can have it without damaging the fork in the long run. (Specifically 2004 RS Boxxer WC.)

    I'll snap some pics and measure the angle tomorrow. Thanks for any input regarding this!

  2. #2
    mtbr member
    Reputation:
    Join Date
    Jan 2013
    Posts
    85
    Not sure about the damage you can do to the fork steerer tube, but you can definitely damage the headtube of the bike frame by going to a slacker HA.

    What frame are you using??

    Also the handling characteristic you are describing, is because of the fork's trail, which is determined by the axle offset and head angle as you described. New to DH, can anyone help me with some geometry related questions?-traildiag2.jpg

    Usually what you do is determine the rake you need for a given frame and buy the fork accordingly. Perhaps an angleset headset can improve this.

  3. #3
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Thanks for the info! That's actually what I was looking for. Are there actually forks of a given make available with different offset variations, or is it a 'work with what you can get' situation there?

    I'm using a Canyon Cross FS frame. It's a relatively unknown alloy frame (can't even find a picture of the exact model online), but the headtube is pretty solid. It's welded on pretty heavily, so I'm not exactly worried about that cracking, I'm just note sure how I feel about low speed handling. I'm fairly certain that the current setup will work great on steep courses, but I'm also riding this thing XC on a daily basis, and am wondering if I should bother trying to improve handling.

    I'm getting a longer rear shock early next week which, based on my calculations, should raise the rear of the bike enough to get approximately the factory spec 'trail' out of the the bike by thusly changing head tube angle relative to the ground. The upside to that would be that it would also raise the pedals about 4 cms higher compared to the current position (which is a plus for me as with some turns I take regularily I often scrape the side of my shoes to the ground), but on the other hand would also raise the upper frame tube and overall raise the weight center of the bike slightly.

    I'm also thinking of just installing a smaller front wheel. Like a 24" wheel instead of the 26". Would that work proper? Though it might look a bit weird I suppose...

    Thanks for any help/insight folks!

  4. #4
    mtbr member
    Reputation:
    Join Date
    Jan 2013
    Posts
    85
    Oh this is worse than I thought.
    First, if you're scraping your foot on turns, you have a bad technique, you should always put your inside foot up and the outside foot down, avoiding scraping regardless of the degree of incline.

    Second, if you raise the bottom bracket 4cm, it will be desastrous for the bike handling. It is way too much.

    Third, 24" tire in the front?? no, it will reduce rolling ability over obstacles, it will reduce breaking traction, it will make it handle horribly.

    Fourth, what you are trying to accomplish is overly complicated. It would be best to get a used all mountain bike and leave it alone. Some of those bikes do wonders on all type of terrain. Probably even cheaper than getting all the parts for a bike that will never work as a downhill bike.

  5. #5
    Glad to Be Alive
    Reputation: SHIVER ME TIMBERS's Avatar
    Join Date
    Jan 2004
    Posts
    42,291
    Quote Originally Posted by santiagomo87 View Post
    Oh this is worse than I thought.
    First, if you're scraping your foot on turns, you have a bad technique, you should always put your inside foot up and the outside foot down, avoiding scraping regardless of the degree of incline.

    Second, if you raise the bottom bracket 4cm, it will be desastrous for the bike handling. It is way too much.

    Third, 24" tire in the front?? no, it will reduce rolling ability over obstacles, it will reduce breaking traction, it will make it handle horribly.

    Fourth, what you are trying to accomplish is overly complicated. It would be best to get a used all mountain bike and leave it alone. Some of those bikes do wonders on all type of terrain. Probably even cheaper than getting all the parts for a bike that will never work as a downhill bike.

    agree...better to buy a used all mountain bike then to keep dumping money in that bike
    the trick is ENJOYING YOUR LIFE EACH DAY, don't waste them away wishing for better days

  6. #6
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Might be, but I'm willing to re-learn hehh That was actually a facepalm moment. It's one particular place where I usually arrive at high speed and have to do an S turn thatturns into a slope midway, so I guess it just happens fast enough that I'm not reflexively raising my other foot... never even thought about that before just noted that it'd happen occasionally.

    I'm pretty sure I've seen DH frames that have the BB higher then that? Or is that just an optical thing and they're just using shorter crankarms?

    I know people who use 24" for DH both front and rear... seemed a little weird to me too, but apparently the difference isn't that big? What I actually thought about was to have both a 24" and the regular 26" ready, and using the 24 for XC/city mode, and using the 26" only for the occasional DH track.

    I can always drop in a shorter travel fork if things don't work out in the long run, and thus give the frame it's original geometry as a regular xc bike.

    I actually thought about getting an old DH specific frame and going with that, but I'm in Hungary... let's just say that even broken frames that should be tossed to the trash are sold for money... so anything usable usually costs a bundle and I don't own a bank unfortunately XD

  7. #7
    Glad to Be Alive
    Reputation: SHIVER ME TIMBERS's Avatar
    Join Date
    Jan 2004
    Posts
    42,291
    usually people put a 24 on the rear....NOT THE FRONT (unless both) ...it's not really worth spending all the money making the bike something it is not....you will be way better off just getting a USED Canfield brothers The One, or SC Nomad

    the bike will ride way better too....you gain confidence riding bikes like this instead of the patchwork bike you are trying to mickey mouse.
    the trick is ENJOYING YOUR LIFE EACH DAY, don't waste them away wishing for better days

  8. #8
    NWS
    NWS is offline
    mtbr member
    Reputation:
    Join Date
    Jun 2010
    Posts
    969
    Quote Originally Posted by Rattanee View Post
    Whatever guide I looked at simply skims over the subject saying steeper gives sharper turns, while slacker gives slower turns...

    Now I'm a bit confused. I thought a slacker angle was supposed to make a bike more stable at speed?

    Also, I feel that (compared to my previous bike) it's like I'm sitting further back than there.
    "Slower turns" and "more stable" are two ways of looking at the same thing. A bike that makes larger-radius turns for a given handlebar input is a bike that will be easier to control at high speeds. The faster you go, the more abruptly you'll change direction for a given input, so the slower steering response helps to compensate for that.

    In addition to the direct effects of a slacker steering angle, a slacker head angle also tends to lengthen the wheelbase. A long wheelbase also contributes to a slower (more stable) steering feel.

    And the additional wheelbase is all added to the front of the bike, which is probably why you now feel like you're sitting too far back. Leaning forward might help compensate for that - wider bars, lower stem, or just more bend in you elbows. And, just get used to the way it feels.

    The "sitting further back" feeling was especially pronounced when I went from my Marin AM bike to my Canfield Jedi DH bike... Longer fork (203mm travel vs 140mm) slacker HTA (63 vs 67), and shorter chainstays (405mm vs 435mm). So both wheels moved forward underneath me. At first I felt like I was always on the verge of looping out, but now it just feels right to me.

  9. #9
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Quote Originally Posted by SHIVER ME TIMBERS View Post
    usually people put a 24 on the rear...
    What's an entry level DH frame that can be had for a decent price used and is actually usable? I'll try and look around. (Actually I'm prolly going to try and get on someone's DH setup to see how riding that would feel.)

  10. #10
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Quote Originally Posted by NWS View Post
    And the additional wheelbase is all added to the front of the bike, which is probably why you now feel like you're sitting too far back. Leaning forward might help compensate for that - wider bars, lower stem, or just more bend in you elbows. And, just get used to the way it feels.

    The "sitting further back" feeling was especially pronounced when I went from my Marin AM bike to my Canfield Jedi DH bike... Longer fork (203mm travel vs 140mm) slacker HTA (63 vs 67), and shorter chainstays (405mm vs 435mm). So both wheels moved forward underneath me. At first I felt like I was always on the verge of looping out, but now it just feels right to me.
    That sounds about what I'm experiencing. Guess it might be just right then? I went from a 80mm travel -cheap- xc fork in my old frame to 176mm with the boxxer in this one. I'm going to have to try and find someone with a proper DH setup that I can try out and see what the difference is.

  11. #11
    mtbr member
    Reputation:
    Join Date
    Jan 2013
    Posts
    85
    Well in case you want to keep going with that set up, this is what worked for me:

    I turned my trail bike to something more DH oriented, and the thing that made the best change was tires. Go for a wide front tire, at least 2.3 width. And run it with low pressure. This will help with small bumps, cornering traction and gaining confidence in steep terrein.

    Short stem and wider bars. Great combo, will improve slow cornering agility while not screwing up high speed stability. Will prevent endo crashes to an extent. Try 50mm or shorter.


    Decent platform pedals, I use them even for my local XC races ( usually I finish in the back but have more fun than the rest).


    Once you feel comfortable with your bike geometry, don't mess much with it. Some guys can rip with hartails in impossible terrein, so don't put too much effort into making your bike "look" like the rest of the guys DH sleds out there. Put more effort in your abilities.

    Once you get to be fast you'll start dropping your chain every time, so invest in a chain retention system. For me a narrow wide chainring was enough.

  12. #12
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    I'm currently running Ritchey ZED 2.1 and 1.9. I'm fairly happy with'em, though I have been contemplating going wider. How low tyre pressure? Off the top of my head, I think the tyres say 65PSI max or along those lines. I'm sure it's a matter of taste in the long run, but what's a safe value to start with? I know that at 65 PSI the tyres go fat and there's less traction (experience says so hehh).

    The short stem and wide bars are in the 'to buy' list. I've got my eyes on a Blackspire Das Stem (50mm with 0 rise) direct mount stem for the boxxer. Question is, how would I go about determining how big a rise I want in the handlebar? Is that a definite trial and error sort of thing or is there some tried and true method to picking one that should be good right off the bat?

    Platform pedals are on their way. Ordered a new set of Spyral Robust Pin black pedals just two days ago. Hopefully they'll be wide enough.

    I'm pretty comfy with how my bike looks right now Actually I don't much care about looks, but I do want to make sure I've got everything I can out of the setup. The whole aim of the build is to get a decent all-rounder bike that can take the abuse of some light DH (there aren't even any too technical tracks around here... most are pretty simple) amidst daily XC through some rough terrain.

    Are there any chain tensioners out there that will work even though I want to keep my triple chainring set in the front? I have had the chain jump off from the front chainrings inside _and_ outside when jumping around to test the suspension in the nearby bike/skate park.

  13. #13
    mtbr member
    Reputation:
    Join Date
    Jan 2013
    Posts
    85
    Run tire pressures as low as you can without pinch flatting. I run my front tire at 25 PSI and my rear at 28, but as you said it has a lot to do with your preference and weight. I suggest that this weekend you take it as a testing day. Take a downhill trail that you know well and ride it taking off 5 PSI each run, until you pinch flat or feel awkward.
    When a XC ride comes, take your pressure back a little bit higher, around 30 or 35.

    Regarding stem rise, buy what is available at the moment, it won't make much difference. With a higher rise it is easier to lift the front wheel and manual. With less rise you get a better attack position and is probably faster, but it really depends on what is confortable with you. I dialed mine through trial and error.

    With chain retention there is a little DIY trick you can make which works well but not as good as real chain tensioners. DIY Bicycle Chain Tensioner - Espaņol This helped me a LOT in my first enduro races until I broke my chain due to too much tension and bad lubrication.

    What I use now is a single narrow wide 32 tooth ring. For $50 it is much cheaper than a tensioner, and clean too. You'll realize that you never needed that many gears in the first place.

  14. #14
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Alright, I'll see how that goes... but the weekend is prolly shot. I'm going to have to go buy spokes tomorrow... just snapped one in the rear wheel... already had one snap a few weeks ago, I just replaced the one... but I guess they're all tired... time to re-lace a wheel... fun times ahead...

    I like that chain tensioner idea, I actually saw something similar that's actually a readymade product. Can't recall the name off the top of my head. But that just gave me an idea on how to make a tensioner that'd work for pretty much any setup... just got to use a wider roller with end-plates... I'll see what I can dig out of some of the stuff I have laying around in my shop in the way of neoprene/rubber rollers (going to raid some dud printers/photocopiers) and see if I can actually make it work. I want to keep all the gears cause I do climb with the bike and even the occasional roll through town on asphalt...

    Also, I measured headtube angle, it's ~61-62 degrees. Maybe a little slacker then what would be ideal.

  15. #15
    mtbr member
    Reputation:
    Join Date
    Jan 2013
    Posts
    1,235
    Quote Originally Posted by SHIVER ME TIMBERS View Post
    agree...better to buy a used all mountain bike then to keep dumping money in that bike
    Second that. It's just not worth it keep spending money on that bike.

  16. #16
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Well I'm buying parts that I can keep even if I migrate to another frame so that's no big problem really. If things go too bad I'll probably end up just moving all the parts to a different frame. I've also considered taking the frame to the neighboring metalworks and having them re-weld the headtube in a slightly different angle. Maybe even reinforce it. Could possibly have it done for free.

  17. #17
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Something just occurred to me regarding the geometry. If I have custom fork clamps made, that fit just the same, but push the entire fork forward of the actual headtube, in effect, that would be the same as having more offset, and thus reducing trail right? Someone correct me if I'm wrong... I have friends with access to CNC milling equipment and could probably have them made free or around the cost of materials at most.

  18. #18
    NWS
    NWS is offline
    mtbr member
    Reputation:
    Join Date
    Jun 2010
    Posts
    969
    Quote Originally Posted by santiagomo87 View Post
    Run tire pressures as low as you can without pinch flatting. I run my front tire at 25 PSI and my rear at 28, but as you said it has a lot to do with your preference and weight. I suggest that this weekend you take it as a testing day. Take a downhill trail that you know well and ride it taking off 5 PSI each run, until you pinch flat or feel awkward.
    I run 30psi front, 40 rear. Pinch flats scare me... Seems like there's always someone hiking their bike down where I ride, and I don't want to be that guy.

  19. #19
    Professional Troll
    Reputation: Gemini2k05's Avatar
    Join Date
    Apr 2005
    Posts
    1,955
    Quote Originally Posted by Rattanee View Post
    Something just occurred to me regarding the geometry. If I have custom fork clamps made, that fit just the same, but push the entire fork forward of the actual headtube, in effect, that would be the same as having more offset, and thus reducing trail right? Someone correct me if I'm wrong... I have friends with access to CNC milling equipment and could probably have them made free or around the cost of materials at most.
    That's not just the "same effect" that IS the effect. New crowns with different offsets is a very common tuning parameter with motocross bikes. Larger offset will improve high speed stability. But in practice that will probably make the bike REALLY obnoxious to turn. The boxxers do basically one thing well, the offset. Forks like the Dorado with the larger offset turn noticeably worse than the boxxer for that reason. I'd just leave it stock. If it's free though, it could be a fun experiment.

  20. #20
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Quote Originally Posted by Gemini2k05 View Post
    That's not just the "same effect" that IS the effect. New crowns with different offsets is a very common tuning parameter with motocross bikes. Larger offset will improve high speed stability. But in practice that will probably make the bike REALLY obnoxious to turn. The boxxers do basically one thing well, the offset. Forks like the Dorado with the larger offset turn noticeably worse than the boxxer for that reason. I'd just leave it stock. If it's free though, it could be a fun experiment.
    Well my "problem" is that my head angle is perhaps a bit too slack at around 61-62 degrees, and that produces a wee bit too much trail. In effect the result is that the fork likes to turn way too easily. I've already gotten used to it to some degree, and I expect it will be less of a problem once I get my new stem and wide handlebar, but it would be interesting to see if I can completely eliminate that with new clamps.

  21. #21
    mtbr member
    Reputation:
    Join Date
    Jan 2013
    Posts
    85
    Quote Originally Posted by NWS View Post
    I run 30psi front, 40 rear. Pinch flats scare me... Seems like there's always someone hiking their bike down where I ride, and I don't want to be that guy.
    Of course pinch flats ar a scary thing. But my reason for using such low pressures are that I run tubeless and that I weight 140 pounds. Also I'm not the fastest guy out there so my lines may be too conservative.

  22. #22
    NWS
    NWS is offline
    mtbr member
    Reputation:
    Join Date
    Jun 2010
    Posts
    969
    I had a hunch those were tubeless pressures. Personally I'm 190 naked, also not the fastest, and most definitely not the skilled-est.

    Running a big front/rear difference has also saved me in quite a few corners. It's scary as hell to feel both wheels slide, but so far I've survived consistently, even though I definitely don't really have the skills to pull that off. It's gotta be the extra pressure in the rear tire that's keeping the bike controllable.

  23. #23
    mtbr member
    Reputation:
    Join Date
    Sep 2009
    Posts
    255
    putting a dh fork on a xc bike is a bad idea.

  24. #24
    mtbr member
    Reputation:
    Join Date
    Jul 2014
    Posts
    24
    Okay so I have things figured out finally. After some math, I had my pal make me a pair of crude fork crowns just to see if it'd work. Thing is, it did fix the 'turns too suddenly' feel, but I still felt like I was sitting a wee bit farther back then I liked. After some though, I reinstalled the original crowns, and quickly fabbed a "190mm rear shock" in the form of a steel rod , just to see how geometry changes. Well it turns out, that is just what I need.
    In fact, I'm now suspecting that this is the original shock size the frame is supposed to take. Why? Well aside from the bike now riding like it should, the front derailleur cage's curvature now matches the curve of the chainwheels precisely, whereas previously it most definitely did not.

    There are two questions remaining in my head I'm hoping someone here might be able to answer:
    1. Is there a shock size between 190 and 165? (There is a 'notch' in the rear swingarm's movement, as if marking a home position. Moving the swingarm there, the eye to eye of the shock mounting bolts is 177mm or so.

    2. Is there such a thing as an adjustable-travel rear shock, or is that a feature only available for forks? It would be a sweet way of instantly adjusting geometry between a setting where you can climb easily and one that's better suited to DH then climbing.

  25. #25
    > /dev/null 2&>1
    Reputation: Procter's Avatar
    Join Date
    Feb 2012
    Posts
    1,585

    Re: New to DH, can anyone help me with some geometry related questions?

    Who's sock are we listening to here?

Similar Threads

  1. A few nOOb tire related questions...
    By Oh My Sack! in forum Beginner's Corner
    Replies: 5
    Last Post: 06-08-2013, 07:18 PM
  2. Dirtbag questions related to Nomad
    By nhmtns in forum Transition Bikes
    Replies: 2
    Last Post: 06-20-2012, 04:36 PM
  3. Please help with a couple tire-related questions
    By rowotter in forum 29er Components
    Replies: 1
    Last Post: 06-14-2012, 12:40 PM
  4. 2x10s and other related questions
    By Panther Creek. in forum Drivetrain - shifters, derailleurs, cranks
    Replies: 14
    Last Post: 02-13-2012, 03:05 PM
  5. Couple Questions (helmet, and gears related)
    By naiku in forum Beginner's Corner
    Replies: 2
    Last Post: 04-14-2011, 06:34 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •