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passive/aggressive
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Discussion Starter · #1 ·
I have finally nailed down a bad speed shimmy in my bike. OEM fork was to blame and now with a loaner fork on my bike I felt fine at 40mph in heavily gusty conditions descending Mt Lemmon road in Tucson.

So I need to get a fork. The fork I borrowed was a Quintana Roo fork with a 47mm offset and very close A-C. Now I am not sure if the offset had something to do with the added stability or not but I am the type of guy that always has ridden a classics type of geometry.

So the question i have is am I better going with a 47mm offset fork to relax the bikes behavior a bit or the OEM 45mm offset?
 

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rockcrusher said:
I have finally nailed down a bad speed shimmy in my bike. OEM fork was to blame and now with a loaner fork on my bike I felt fine at 40mph in heavily gusty conditions descending Mt Lemmon road in Tucson.

So I need to get a fork. The fork I borrowed was a Quintana Roo fork with a 47mm offset and very close A-C. Now I am not sure if the offset had something to do with the added stability or not but I am the type of guy that always has ridden a classics type of geometry.

So the question i have is am I better going with a 47mm offset fork to relax the bikes behavior a bit or the OEM 45mm offset?
Actually, you've got it backwards. More offset lowers trail and quickens steering, less offset raises trail and slows steering. Depending on other factors (one of which being HT angle), chances are the rakes you mention will have minimal effect, but go with the 45 if slower is what you're after.
 

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eminence grease
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I'm going to guess and say it has nothing to do with the 47 offset, rather there was perhaps something wrong with the original fork or the way it was installed. Swapping it might have fixed either of those things. This is the problem with doing experiments and not holding the proper variables constant.

Higher rake numbers do not equal better stability, if anything, they might make the bike more twitchy in terms of steering. And rake is not independent of head tube angle so your bike was designed with a specific rake in mind.

If I was picking, I'd stick with the rake that came on the bike. But in reality, the true handling difference between 45 and 47 is going to be negligible.
 

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passive/aggressive
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Discussion Starter · #4 ·
Thanks for the advice. It was most certainly the OEM fork. I did the whole swap of suspect items with once change at a time.

First wheels (which I suspected as too light for my mass), then check the wheel centering and tension (just in case on both sets), then headset tension and cleanliness, then stem and bar then frame alignment (at the advice of Litespeed), and finally the fork. Unfortunately the only loaner i could get was one with the 47mm offset. The difference was night and day though. The OEM fork would get vague at 30mph and speed wobbles at 34mph.

Yesterday I easily surpassed 40mph on the descent with no feeling of vagueness and certainly no speed wobble. Terrified? I sure was but as I descended I gained some confidence that the bike wasn't going to shake me loose.

Thanks for the advice and I will get a 45mm offset fork right away that is as close as possible to my current POS OEM a-c.
 

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I thought that more rake = slower steering and more stability. Why do so many people, like myself, get so confused about this? It's greater trail that leads to more stability. Ugh. So, is there much difference between a 43mm and 45mm rake given a standard 73 degree HT?
 

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eminence grease
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rook said:
I thought that more rake = slower steering and more stability. Why do so many people, like myself, get so confused about this? It's greater trail that leads to more stability. Ugh. So, is there much difference between a 43mm and 45mm rake given a standard 73 degree HT?
It is hard to remember, maybe a simple way of thinking about it is that the higher the rake number the faster it goes, just like a speedometer.

I think many would argue that the difference between 43 and 45 with a given HTA would be hard to detect.
 

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rook said:
Why do so many people, like myself, get so confused about this?
Because one of the lines defining rake and one of the lines defining trail cross each other, reversing what is felt intuitively. Mentally, decrease the rake on the drawing below and see what happens to trail. It'll all be clear then.
 

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Steaming piles of opinion
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rook said:
I thought that more rake = slower steering and more stability. Why do so many people, like myself, get so confused about this? It's greater trail that leads to more stability. Ugh. So, is there much difference between a 43mm and 45mm rake given a standard 73 degree HT?
Probably because the terms get so fiddled, and moto uses different terms than common for cyclists. 'Rake' is often about angle, and there increasing the angle (away from perpendicular) increases trail. But when a cyclist talks 'rake', they really mean 'offset', as wim's picture below shows. What moto guys call rake, we call head tube angle, and we talk it relatively backwards at that.

But bicycles came first, so we're right.:D

The difference is about 2mm. (really, not very much at all, but generally in the right direction. 47's are usually reserved for slacker HTA's.)
 

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terry b, wim, danl1, and everyone. Thanks for the tips.

So, if trail is really the defining feature of bike handling, why do we keep talking of rake instead of trail? I know they are both related, but trail is the defining feature that directly leads to stability. So, please correct me if I'm wrong on this. Does increasing trail increase stability? Or decrease it?
 

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Visualizing

rook said:
terry b, wim, danl1, and everyone. Thanks for the tips.

So, if trail is really the defining feature of bike handling, why do we keep talking of rake instead of trail? I know they are both related, but trail is the defining feature that directly leads to stability. So, please correct me if I'm wrong on this. Does increasing trail increase stability? Or decrease it?
You can just look at the diagram that easily shows that increasing offset decreases trail, and you now know that increasing offset decreases stability, ergo increasing trail increases stability.

The way I remember this is that I used to ride with a guy who had a Fuji with a really steep head angle, and so the biggest fork offset I've ever seen on a racing bike. They needed the really big offset to give responsive steering. Any time I can't remember and don't want to think to hard, I just remember that Fuji with the "super offset" forks and I know that more offset means faster steering.
 

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Pro cyclist data offers conflicting info on stability in regards to fork rake.

OK, I just found some conflicting info here. I know that many of you guys are advocating that greater rake -> less trail = less stability. However, Cyclingnews.com is saying the opposite, that greater rake = more stability, not less.

Cyclingnews.com is reporting that the bikes designed for the Garmin-Slipstream team for use in the Paris-Roubaix race are designed with more rake so that more stability is achieved.

See Garmin-Slipstream Felt bike:
http://cyclingnews.com/photos/2009/tech/probikes/index.php?id=/photos/2009/tech/probikes/martijn_maaskant_f1_PR09/Maaskant_F1PR_front_end

Another article here:
http://cyclingnews.com/tech/2009/probikes/?id=martijn_maaskant_f1_PR09
 

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rook said:
OK, I just found some conflicting info here.
.... However, Cyclingnews.com is saying the opposite, that greater rake = more stability, not less.
the info given by the cyclingnews author(huang) in that article appears conflicting only because he is wrong.
the bikes may indeed have been outfitted with an increased amount of fork rake but it wasn't done for "stability".
 

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I recommend rockcrusher stick with the OEM 45mm rake or go LESS. I've had firsthand experience with swapping out a fork for one with 5mm MORE rake. A more rake/less trail front end works with slack head angles of say, 72 degrees, but as the head angle steepens, the steering becomes too sensitive and it becomes more apparent as the bike's speed increases. I replaced a 73 degree/45mm setup with a 50mm fork. The front end now felt like it was on caffeine. Leaning into corners, it felt like the front end was on ice and I clearly had to put more arm muscle effort into steering the bike around the turn. It's not that the bike was unrideable but the steering was not something enjoyable at speeds over say, 20mph.

To contrast, I also had the opportunity to ride an old Cannondale touring bike with a 72 degree head angle and 54mm of rake. Man, that thing was fun to steer. The slacker head angle meant steering input from turning the bars didn't cause the bike turn as quickly, but the greater rake reduced trail so you could turn the bars easily yet the bike would not respond so fast as to be nervous. I always thought that setup was exactly what I'd want on my next racing bike.

You should note that 50mm of trail on a 73 degree head angled bike does not produce the same steering feel as 50mm of trail on a 72 degree head angler. It's my observation that as head angle increases, trail has to increase to keep the bike from being too sensitive to steering input. This is especially true as your speed rises and should be readily apparent at 25mph and higher.
 

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The cyclingnews article isn't entirely incorrect, if the only change were more rake then the reduction in trail should cause quicker steering. In this case, the fork is also longer, which slackens the bike's existing angles, which if used with the same rake, would increase trail. I don't think the rake change increases stability in this case, just corrects a geometry change.

To the OP, what size/type of frame are you riding? Most bikes come with 45mm rake forks, companies that use different rakes will go with 50mm on the smallest bikes, 45mm through the middle sizes, and 40mm on bigger bikes. I ride big bikes and like 40mm forks, the bigger frames usually have steeper head angles to keep the wheelbase within the designer's ideals and since toe/wheel overlap isn't an issue, the reduced rake makes things nice and stable.
 

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eminence grease
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rook said:
why do we keep talking of rake instead of trail?
Because the Rake is the literal measured offset betweent the straight line steering axis and where the dropouts place the axle.

Trail is a calculated result that also includes the effect of the headtube angle. You can't sell a fork with a given Trail, because that value depends on what bike it goes on.

Rake is simple and measureable with a ruler. They leave the rest up to you.
 
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