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MountainBikeReview.com's Forum Archives - 29inch Wheeled Bikes
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Same wheelbase, less rake, slacker head angle. What happens? (18 posts)
|  Same wheelbase, less rake, slacker head angle. What happens? | Cloxxki
Jan 9, 2004 2:55 AM | | Does anyone understand how rake affects handling? Why does everyone use 43mm on all sizes bike, and which would work best in which cases?
Thanks for your help!
J |
| re: Same wheelbase, less rake, slacker head angle. What happens? | Frank Tuesday
Jan 9, 2004 5:41 AM | | That is a lot of questions in a short message.
As for your title, you'd have a very slow or stable handling bike. A slacker head angle requires MORE rake. In very simple terms, head angle+rake gives you trail. As you slacken the head angle, the prjection of the steering axis gets further away from the contact patch of the tyre. This difference is the trail. Adding more rake (or offset as it is sometimes refered) to the fork willreduce the trail by moving the contact patch of the tyre forward towards the projection of the steering axis.
The reason everyone uses 43 (or very close to that) is international standards. Nobody wants their product to be incompatible with the majority of the bikes. A fork with a significantly different rake would confuse the marketplace, which is generally quite undereducated about such things. To put in in other terms, by eliminating rake from the equation, discussing the handling characteristics of various makes comes down to head angle. 70.5-71 degrees is "normal", 72+ degrees is "east coast"(in US terms) and 69 degrees is "downhill" or "extreme".
Of course the handling issue can't be that simple. In terms of trail and handling characteristics, that really only applied to the very initial turn in. As you turn the wheel tighter a lot of other things start to happen.
Unfortunately I have to go to work now. I'll elaborate on this later.
Brian |
| re: Same wheelbase, less rake, slacker head angle. What happens? | Cloxxki
Jan 9, 2004 6:10 AM | | Thanks!
I'm about to order a custom fork and frame. I use huge toptube, so an stuck with more toe clearance I don't required. I might get an even steeper head angle than I already planned, and cut back the rake, to end up with plenty of cockit room, but a short wheelbase. The steeper head angle PLUS shorter rake might make it very flickable, which is good on Dutch singletrack. All the wheelbase is deducted from the front of the bike, putting more pressure on the front tire and shorten the minimum turning radius.
72,5º and 38mm rake, does that sound scary?
On one hand I love the way my 72º/43mm Karate Monkey handles, but might there be even more possible with 29" geometries?
My 26" 72º/72º VooDoo was often raced with a short rigid fork, making it more towards 73º/73º. The only bike that allowed me to stay with anyone on any twisty singletrack, till I got my KM. Since some people think that a 29" bike requires a steeper head angle, the 72º on the KM might not be as steep as it seems? |
| it's all about trail | 20.100 fr
Jan 9, 2004 6:18 AM | | As nicely said before, you have to calculate the trail of you bike.
This is the important factor !
There is a drawing of the trail on my site to help you see what it is:
http://velodemontagne.free.fr/fournales/i.htm
(i made these site to show how the look-fourales keeps a constant trail as oposed to a telescopic fork where the trail reduce with compression.)
Trek is the only company i know that put the values of trail of the frames on their site.
Vincent |
| "38mm rake, does that sound scary? " | Halloween
Jan 10, 2004 12:04 PM | | Maybe? All I know is this: My track bike has 38mm of rake. |
| Can you find a November?? issue of Moutain Bike Action? | fattirewilly
Jan 9, 2004 7:58 AM | | http://www.hanebrink.net/forks/ride/index.html
I think it was Novemeber. Dave Thomas (Daves Speed Dream Wheels) was using the fork above in an application which modified the rake/rail and I remember some discussion on why or why not mountain bikes still use the same specs a road bike. Might be worth a read. I quickly looked on the MBA website and couldn't come up with a link on the article.
Actually, here is a link to Dave himself
http://www.speeddream.com/thomas_offset.php |
| Another look at rake, trail and head angle... | mv
Jan 9, 2004 3:04 PM | | I think things are often simplified with just looking at rake, trail and head angle.
What does really happen? Why does a bike go staight with hands loose?
I think , (and I might be completely wrong) that it is not just trail, but that you do need rake because with that combination you will get the effect that your front end will come upwards if you turn the bars; this means that straight out is the direction your bike wants to go-because the weight pulls it down as low as it will go, and that is with bars straight-.
Make a drawing on paper and look whats happening.
I think two bikes, with the same trail but different rake and head angle combinations handle completely different. A fact often overlooked.
Any mathematicians around here?
I would not go too steep with my head angle Cloxx, definetely not steeper than 72 degrees and probably less. |
| Another look at rake, trail and head angle... | Cloxxki
Jan 10, 2004 9:23 AM | | My 71,5º head angle Fisher is great on DH's.
My 72º head angle Surly Karate Monkey is SO much better on technical stuff, the Fisher just can't compare.
Both great bikes, but on different courses. For my racing, I need something that offers the handling traits of the Karate Monkey.
A race bike is unlikely to be ridden much without hands, so if fewer rake would make it steer better, that's pure advantage. |
| Another look at rake, trail and head angle... | mv
Jan 10, 2004 12:06 PM | | I wish there was an construction like used in racing motorcycles where you can adjust the head angle through rotating exentric bushings(?) in the head tube.
Is this not used in downhill factory bikes as well?
Or you could do some testing with your current bikes by putting spacers under the headset crown. But then you alter your BB height as well.
If you like the KM so much, why dont you order a custom from that Chinese company with the same specs? $ 375! is wat they quoted me as well. If you dont like it you sell it. I think I,m about your seize so maybe we can order two.
BTW Have you ever tried riding a road bike with the fork turned 180 degrees like the stayers do? I wonder how that wouls feel.
Colnago road bikes have almost cyclocross numbers geometry on the front end and are valued high by top racers.
Doesnt make much sense all maybe, but I was almost killed once by severe, severe shimmying on my 66 cm Zullini racing frame (in the Pyrenees) which had a head angle approaching 75 degrees. Never had problems when I shifted to 73 dgree bikes. |
| MV, where do you live ? | 20.100 fr
Jan 11, 2004 8:18 AM | | |
| Another look at rake, trail and head angle... | Frank Tuesday
Jan 10, 2004 9:39 PM | | I had run out of time before I got to this. I've been looking for a link which would help to demonstrate this, unfortunately, I can't find it right now.
NOTE: Everything below is looking at changing the HA and rake to keep a constant nominal trail.
The trail mesurement that we come up with on a static bike is only valid while the fork is directly pointed forward and, in the case of a supension fork, uncompressed. So, two things can happen to the trail. As a suspension fork (telescoping) compresses, the effective head angle steepens, thereby decreasing the trail. Easy enough, and not much you can do about it other than go with a linkage type fork.
Now, as you turn the handlebars (and hence the front wheel), two things happen. The trail decreases, and the front end of the bike will change height. The amount/rate of change varies based upon which combination of HA and rake you choose for a given train measurement.
So, on a bike with a 90 degree HA and 63mm of rake, the front end height will not change as the bar is turned, and the effective trail will become 0mm at 90 degrees. Now, a bike with a 90 degree head angle will be difficult to turn. Since the rate of change in the trail is very slow initially, slight changes in the direction of the front wheel will go virtually unnoticed. If you've ever pushed a shopping cart around, you know that the front wheels tend to wander from side to side. This is exactly what would happen on a bike with a 90 degree HA.
Now on a bike with a 72 degree HA and 43mm of rake, the effective trail will become 0mm at about 60 degrees, and the front end height (at 0 effective trail)has decreased by about 8-10mm. So the rate that the rake changes has increased.
Take it out even further and your rake will change even faster. This is why a bike with a shallow HA and super raked out fork can seem floppy. At its static position, the trail is a comfortable 63mm, but as you turn the bars, its trail changes quickly, upsetting the stability, and leaving you with a bike that is difficult to handle in the turns.
The change in front end height is minimal. It does work to counteract the caster effect. IMHO, this could be seen as a good thing. The more you turn, the greater the change in height to counteract the tendency of the wheel to return to it's natural state, which will help with the predictability in a corner.
So where does this leave us. Too steep of a head angle and the front end starts to wander in a straight line, too shallow and the front end starts to wander in the turns. Right in the middle: Well the front end stays straight when you want it to, and it stays stable in the turn when you want it to. It doesn't take a lot of force to change direction, and the wheel isn't going to put up too much of a fight.
So, what is the answer. There is no single answer. There is a reason that bikes have fallen into a small range of variety. It works well for most situations, and it is a very good compromise. For a bike that would have a very specific, singulair purpose, something outside the norm may be in order. The above mentioned article about the 50/50 setup has some very valid points, but not neccessarily relative to Cloxxi's concern. I can only assume that he is getting a custom rigid fork, so you do have to look at the shock absorbtion qualities as well. When in comes to that, more rake means better shock absorbsion.
Cheers,
Frank |
| Another look at rake, trail and head angle... | mv
Jan 11, 2004 1:17 PM | | Interresting.
One remark; on a bike with a 90 degree head angle and a raked fork , the trail turns POSITIVE instead negative as normal. (shopping card with wheel forward)
It will turn 0 degree at 90 degree turn I think.
So I am wrong about the head tube rising when you turn the handle bar in a normal bike? I was told that before. I will have to think it over again.
Thanks for your response.
BTW , I live in NC but am inplanted from Cloxxki's neck of the woods. |
| Another look at rake, trail and head angle... | Frank Tuesday
Jan 11, 2004 8:34 PM | | I was mainly interested in keeping the trail the same, so on my 90 degree example, the rake would be negative.
The actual change in front end height varies quite a bit. It can actually raise in some cases, most often seen on low riders with a highly raked out fork. On "normal" bikes, it does lower the headtube ever so slightly. As you turn, the contact patch moves forward, increasing the wheelbase, and slackening the HA. Since the fork length is constant, the only way to make up that change is by lowering the front end. I checked on the bike in my living room, and it did, in fact, fall. Of course, the outer end of your bar will raise quite significantly, and the inside will fall.
Frank |
| Seperate question... | Skatepark
Jan 10, 2004 6:33 PM | | Since both frame and fork are being ordered custom, why not make it a non-sus corrected bike with a shorter fork? This will wipe the slate clean of all of the other info in this discusion but I have LOVED running 400mm forks on my Litepseed.
Besides, I know form your posts that you like a lower front end on your bikes. Also, this will make HT angle, rake and trail less of a guess as the handling is sure to quicken by virtue of the lower CG in the front and overall for tha matter.
-t |
| Thanks all for the responses! | Cloxxki
Jan 11, 2004 5:28 AM | | Oh I love a low rigid front on a bike, but at this point I like to keep the suspension option open, getting ready to try some marathon races this year, and not sure yet which frame will get the honour. Also, my custom bike will be part of me working out ideas for the ultimate box stock 29" bike, not just for geeky racers like myself, but also for Average "forrest fun ride" Joe, who heard from friends that suspension is a good thing.
From Frank's informative posts, I've learned that there's more to do wrong than right with weird rake/HA choices, and Probably Surly already early in the production life of the 29"er found a sweet spot, at least for twisty trails and all-round handling. Fisher found one for extreme more open trails. I'm affraid, to get the ultimate geometry, there's little left to do but copy the Suly's, take off some 2" of seat tube, and chaince only details, to not end up with a differently (which almost certainly equals worse a) handling bike.
Looks like I'll adjust TT to work with 105mm stems I want to try, and slacken the seat angle to shorten the wheelbase and offer me some more power while seated.
Thanks again to everyone!
J |
| Ryders bike | mv
Jan 11, 2004 2:07 PM | | Personally I like slack seatangles but not when I race on steep trails.
My climbing is so limited because my front wheel simply pops up.
Look at the bikes of Roland Green and Ryder Hesjedall (sp?) , they have their seats slammed all the way forward looking soooo ugly. They do win WC races though.
I feel that chainstays are to short for tall people , leading to this wheely effect.
Also, if you slacken seat angle, you shorten the TT and this can lead to not so good out the saddle climbing (for me).
I jammed my seat forward in last MTB race and climbed better although I didnt feel comfortable.
BTW Paula Pezzo now on specialized and not on 29" anymore I guess. |
| Ryders bike | Cloxxki
Jan 11, 2004 10:41 PM | | I agree that big riders need longer chainstays to not wheelie the smallest incline. If you encounter steeps on a regular basis, certainly worth building a bike around.
My 73º Fisher allowed me to climb grades steeper than I could ever imagine, while remaining seated. On my VooDoo 26", I am forced to stand on similar steeps. However, such steeps don't exist in races, or it must be sanddunes we're all forced to walk up.
In my local racing, it'as all about changing direction and developping speed.
I don't see how changing a seat angle makes a bike shorter, I'd rather expect longer. However, I always specify effective top tube, which seems to be a pretty bombproof method. |
| Ryders bike | Frank Tuesday
Jan 12, 2004 5:38 AM | | The seattube angle sets the relation of seated cockpit size to standing cockpit size. No matter what the angle, the seated cockpt size is determined by the TT length. With a steeper angle, your standing cockpit will be larger. |
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