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changing fork rake for front-loading

Old 11-25-08, 03:37 PM
  #1  
themanmachine
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changing fork rake for front-loading

Hi,

I have an older Raleigh Royale that I converted to a three speed for city riding. It was originally a touring bike with drop handlebars and has lost some of its stability with upright bars installed. When the bike has a load in a front basket, the handling is very shaky.

The original fork has around 70 mm of rake, and I crudely measured this setup to have 35 mm or trail.

I want to know what effect a 50 mm rake fork will have on the handling. I have calculated that this will add 20-21 mm of trail. Should this make much of a difference and will it make the bike more stable for front loading?

Thanks
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Old 11-25-08, 04:57 PM
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Increasing trail is exactly the same as increasing caster on a car. It increases the tendency of the steering to 'center'. You might perceive it as improved stability.

Increased trail also lengthens the wheelbase. Longer wheelbase feels less twitchy around corners
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Old 11-25-08, 05:27 PM
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Originally Posted by themanmachine View Post
I want to know what effect a 50 mm rake fork will have on the handling. I have calculated that this will add 20-21 mm of trail. Should this make much of a difference and will it make the bike more stable for front loading?
Supposedly for front-loaded bikes at low speeds, less trail is better. Kogswell Cycles makes bikes based on old French newspaper carrier bikes with trail in the neighborhood of 25 mm. Some pictures are at https://www.kogswell.com/products.html and a brochure with some test reports is at https://www.kogswell.com/KogswellPR.pdf. However, if you were going to go this route, I'm not sure if you'll be able to find a fork with more than 70 mm offset.

Wider tires might help too. Is the headset in good shape?
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Old 11-25-08, 09:00 PM
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Depends upon the speed. Riding a newspaper-delivery bike in France will have you riding around old streets that aren't any wider than 10-ft wide at 10-15mph. You definitely want short trail for maneuvering around tight corners.

But if you're looking at commuting at 20-25mph and want more stability, then yeah, you'll want less rake and more trail.
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Old 11-25-08, 10:16 PM
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Originally Posted by DannoXYZ View Post
Depends upon the speed. Riding a newspaper-delivery bike in France will have you riding around old streets that aren't any wider than 10-ft wide at 10-15mph. You definitely want short trail for maneuvering around tight corners.
While I haven't looked at the original article that Kogswell references, I believe the thinking was have a low trail is not just about cornering, but is actually more stable at low speeds with a lot of front end weight. Wheel flop (how much the front end falls when it turns) is mentioned, but I didn't quite follow the discussion.
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Old 11-25-08, 11:49 PM
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Well, kind of, it's actually the reverse. When turning the bars more than 10-degrees, you can really feel the up & down motion of larger amounts of rake (small trail). This combined with the steering-head angle determines how much the front-hub moves up and down. Imagine a fork with zero rake (large trail), it would tend to have minimal flop up and down as well. More trail, less rake, gives less front-end flop AND more stability.
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Old 11-26-08, 02:25 AM
  #7  
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I once had a bike with a rack up front & milk crate on it. It was unridable without hands and as such kinda tiring to get around on, then I crashed it real good. The fork appeared unaltered but the headtube angle had changed, the bike had become a charm to ride and could be ridden no hands.
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Old 11-26-08, 10:15 AM
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Originally Posted by DannoXYZ View Post
Well, kind of, it's actually the reverse. When turning the bars more than 10-degrees, you can really feel the up & down motion of larger amounts of rake (small trail). This combined with the steering-head angle determines how much the front-hub moves up and down. Imagine a fork with zero rake (large trail), it would tend to have minimal flop up and down as well. More trail, less rake, gives less front-end flop AND more stability.
I thought about this more last night, and it still seems to me that more trail has more flop. As you rotate a zero rake fork, the contact patch moves towards the point where the steering axis intersects the wheel. On the other hand, a front end with zero trail would (I think) just rotate about the contact patch, so there would no flop.

Zinn's article about rake and trail sort of talks about this. https://www.velonews.com/media/Block40.pdf If you raise the back end of a bike so that you steepen the head angle and eliminate trail, the wheel flops less for a given lean and vice versa for raising the front end. The wheel easily falling into a turn on a high trail bike is what gives it its stability at speed since it puts the bike back under the CG.

This is all on paper, so there may be other factors that affect stability. But it's an interesting idea.
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Old 11-26-08, 12:26 PM
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It seems like there are lots of conflicting opinions on this-mostly based on anecdotal evidence.

What I really want is something that I could ride with a front end load with no hands and still be stable. I'd hope it would be stable unloaded too.

It seems there is definitely a compromise to be made. Unlike a french porteur, I don't spend most of my time with 20-30 lbs of newspapers on the front of my bike. Maybe I need to remeasure the trail or I will just have to try a new fork and see how it works. Thanks for all of the advice.
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Old 11-26-08, 12:38 PM
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Front load + No Hands !!!?!?!!?

WOW!

Hmmm, rear-steering trike recumbent mebbe??? I dunno.

EDIT: Nah, that would suck too, unless you're on a dry lake bed, or some other vast, uncrowded expanse.

Last edited by LesterOfPuppets; 11-26-08 at 04:44 PM.
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Old 11-26-08, 05:50 PM
  #11  
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?!? ya'll got me twisted.

i thought that a larger rake of the forks would increase wheelbase, high speed stability and enough of it would get you the "wheel flop" you mentioned. shorter rake would tighten up the wheelbase, reduce wheel flop and high speed stability. i think that is right, and of course always throw in there the angle of the headtube to furthur complicate things. wow, and they saw bikes are simple.
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Old 11-27-08, 07:48 PM
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Read Zinn's article above, don't get rake & trail confused. Trail is what generates a self-centering torque on the fork's steerer tube. The large the trail (smaller rake) the longer the moment arm between the contact-patch and steering-axis and the stronger teh torque. Imagine pushing sideways on a wheel. Pushing 1-lb at the tyre will generate more torque than pushing 1-lb at the hub-flange.

Now imagine a bike with zero-trail where the contact-patch is at the same spot as the steering-axis. This would the case of a 90-degree head-tube with zero-rake or a bike with tremendous amounts of rake. This bike would have no self-correcting torque, but it would have a tonne of wheel-flop.

Imagine a 0-degree head-tube with the forks pointing straight ahead. The amount of vertical flop is directly related to the amount of rake. If you've got 50mm rake, the front-hub would be 50mm above the steering-axie with the steering straight ahead. Turn the bars 90-degrees and the front-hub would move down 50mm. A fork with zero rake on the other hand, would just pivot the hub without moving it up and down at all.

Don't get rake and trail mixed up.
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Old 11-27-08, 07:58 PM
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Old 11-27-08, 10:14 PM
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Originally Posted by DannoXYZ View Post
Read Zinn's article above, don't get rake & trail confused.
I'm not. From the article:

"Using this relationship for the purposes of demonstration, you can increase trail and
stability simply by decreasing head angle. You can simulate this with your own bike by
stacking cinder blocks and phone books up under the front wheel (see Photo 40.4). Now if you lean the bike, you will see that the front wheel flops over into the turn to a much
sharper angle than before
, thus enabling the wheel contact patch to get back under the
riderís center of gravity more quickly. "

A 90 degree head tube with zero rake has no wheel flop. The hub stays in the same horizontal plane when the bars are rotated, so there is no gravitational torque on it. (In other words, leaning the bike doesn't cause the wheel to flop.)

A zero degree head tube has infinite trail, and as you point out, a lot of flop.
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Old 11-27-08, 10:24 PM
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Originally Posted by DMF View Post
Increased trail also lengthens the wheelbase.
This is true if you reduce the head tube angle, but not true if you reduce rake offset.

Al
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Old 11-27-08, 10:27 PM
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Originally Posted by stedalus View Post
I'm not. From the article:

"Using this relationship for the purposes of demonstration, you can increase trail and
stability simply by decreasing head angle." .
The OP asked about changing fork rake. Altering the head angle of frame is much more difficult.

If the bike is unridable when there's a load in the front, I'd look at the stability of the load and the condition of the headset or front wheel. Is the load tied down securely? What kind of weight are we dealing with?
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Old 11-27-08, 10:50 PM
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Originally Posted by Jeff Wills View Post
The OP asked about changing fork rake. Altering the head angle of frame is much more difficult.
DannoXYZ and I threadjacked a bit and got into a side discussion. My point is that there seems to be some evidence that decreasing trail actually helps for low speed front loaded stability.
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Old 11-28-08, 10:51 AM
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Originally Posted by LesterOfPuppets View Post
Front load + No Hands !!!?!?!!?

WOW!

Hmmm, rear-steering trike recumbent mebbe??? I dunno.

EDIT: Nah, that would suck too, unless you're on a dry lake bed, or some other vast, uncrowded expanse.
I have a randonneuring bike with 73 deg head tube angle and 45 mm trail. I can ride it no handed with a loaded front bag on a small rack just above the front wheel. No problem.
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Old 11-29-08, 08:11 AM
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I have a Trek 610 (1984) and was never happy with its handling, especially after I got a 1980 Masi. The Trek with just me on its back did not like to go in a straight line, especially at low speeds. When it was new I tried putting on a handlebar bag, and it was far worse. It had as-new trail of around 51 mm. When I took it out of mothballs I took it to a framebuilder (Ron Boi in Kenilworth, Illinois) to have the frame aligned and to discuss the handling and the trail issue. We decided to fully align and balance (make sure the wheels were parallel to and in plane with the straightened main triangle) the frame, and to reduce the rake (unbend the fork tubes) by about 8 mm. That increased the rake to around 57 mm (consideirng added bike tilt-up), compared to the Masi measured trail of around 59 mm.

The result in riding was still not perfect stability, but much improved - little sidewinds and road irregularities no longer threw the bike into the middle of the lane. No-handing became possible, but I think that's because the fork had a lateral shift in it before (from TREK!!). Still not to the standard of the Masi, but a lot better. My basic point here is that it is possible to have a 531 steel fork bent to a new value of rake, done by an expert in bike frames. BTW, there were zero cosmetic consequences to this work.

I think this was a good change, but not the direction you want. On the iBob list there has been a multi-year (often anecdotal and circular) discussion about French touring bike style versus English. The "classic" French touring and randonneuring bikes, such as Singers and Rene Herse, had medium trail numbers in the range of 40 to 50 mm, and this had been (the consensus and the conclusion of Vintage Bicycle Quarterly) evolved to give good handling with MODERATE added weight on the bars, i.e. something like a Berthoud or TA handlebar bag. Another feature of French touring was low rear panniers. English on the other hand tended to choose racing bikes for recreational and club riding, so their bikes worked out better with low front panniers and large saddle-mounted cargo bags, like a Carradice 30-liter piece. The racing bikes in question typically had trail in the 55 to 60 mm range. Many modern performance road bikes are in the 60 mm range.

So historically there is a consensus in at least one fringe of the cycling world, that touring handlebar bags work out best with trail around 45 mm rather than close to 60 mm. I don't know if any framebuilders practicing today would agree with that. There are some very good mechanical engineering papers (especially by Papadopoulous) that contain mathematical models you could use to do a theoretical assessment, but I haven't gone this far.

If you go ahead and try this, please give us a ride report!

Sorry for the long post, but I thought the OP might get something out of sharing this story and perspective.

Road Fan

Last edited by Road Fan; 11-29-08 at 08:15 AM.
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