questions about recumbent design

Discussion in 'Recumbent bicycles' started by WKB, Mar 10, 2007.

  1. WKB

    WKB New Member

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    I've become slightly fascinated recently about recumbents, although I've never had the opportunity to ride one. I've noticed that a common design is to make the front wheel smaller than the back wheel, although I've never understood the reasoning behind it (at least not for long wheelbase recumbents). For a short wheelbase, where your feet have to go over the wheel, the intent of the designer is pretty obvious. This doesn't seem to apply to long wheelbase recumbnets, as far as I can tell. Any ideas from those who design recumbent frames or from those who ride long wheelbased recumbents?

    Thanks in advance, Keefe.
     
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  2. blazingpedals

    blazingpedals New Member

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    I can think of 3 reasons right off.

    A large wheel in front means the head tube must be higher, which limits how low you can put the seat (and still be able to see.)

    It also makes for an even longer wheelbase, as the front axle must be moved forward several inches for clearance to a bigger tire. Doing so has a negative impact on ease of transport, frame flex, and front tire loading.

    Finally, since a fairing on a LWB is mounted to the fork and handlebars, that means a fairing on a big-wheeled LWB will be further in front of the rider, allowing air to spill over the fairing to hit the rider on the chest - essentially negating the benefits of having the fairing in the first place.
     
  3. WKB

    WKB New Member

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    Thanks for your response. All reasons make good sense to me, but I wanted to follow up with a couple of other questions:
    --What is the issue of front tire loading with recumbents?
    --Do smaller front wheels make bumps in the road a little harder to take or does the longer wheelbase absorb these shocks more easily than a diamond frame would with a smaller wheel?
    thanks again for your insights.
    Best, Keefe.
     
  4. blazingpedals

    blazingpedals New Member

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    If the front wheel isn't loaded enough, then it can wash out easier, especially if there's a little sand or gravel on the road. I've gone around 'dirty' turns on my SWB with no problems, while a TourEasy rider behind me went skidding into the ditch, front wheel turned but to no avail. I think, but don't know for sure, that having less weight on the front tire is the difference, as our front tires were the same size.

    Generally, small wheels don't roll quite as easily as larger ones. The smaller the wheel, the steeper angle it has to follow to get itself out of small holes or over bumps. You have to run fatter tires to get the same air volume, too; so 25mm wide are almost impossible to find in the 20" size. If your roads are fairly smooth then the difference will be negligible. You're right, a long wheelbase, and to some extent a long frame, will soak up bumps regardless of the wheel size. Add to that, most bents have pretty cushy seats and you can get a very plush ride.
     
  5. Slugster438

    Slugster438 New Member

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    There is no issue with front-tire loading on recumbents.
    The reason some people don't seem to get much traction has to do with the fact that their front tires are overinflated. Many recumbents do manage near-50-50 weight distribution but some don't (LWB's often don't) and people still mount the narrowest tires they can and inflate them to the maximum pressures.

    On a LWB with a big & tall rider, nearly 70% of the weight may be riding on the rear wheel. If both tires are the same width and inflated to the same pressure but carrying much different loads, then one of them is at the incorrect pressure. A tire should be inflated proportional to the weight it is carrying, not simply inflated to the maximum pressure given on the sidewall.

    If a tire is sliding, that means it's got too much weight on it already, or it's over-inflated. And if the front tire is already carrying less weight than the rear, but the REAR doesn't have "this sliding problem", guess what explanation is left?
    ~
     
  6. sch

    sch New Member

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    Reducing the pressure in my Stelvio 406 from 100# to 50-60# might increase
    its grip on pavement in a turn, I am not so sure it will do much for a sandy or gravelly road or slick mud on the road to prevent front slide out on lightly loaded front wheels of LWB. There may be a marginal improvement, but it is not clear what would be an optimal tire pressure here. One other problem with small diameter wheels, in the rear, is that they don't climb over pavement level changes as well as larger wheels, such as repaved asphalt where the new pavement doesn't reach the width of the older pavement resulting in a stepoff !" or more high. If the bike drifts off this step the rear wheel can hang and arrest the forward motion of the bike. The front wheel usually can ride up over the step.

    Generally smaller fr wheels are used to reduce the bike height. For riders under 66-68" tall, on monotube dual 650 or 700c wheels, the seat height is
    at least 24" and this can be a stretch for the rider to reach the ground in
    stops without sitting up on the seat. Starting up is a bit harder as well.
    The 3" smaller fr wheel reduces the seat height by 1.5-2" which is enough for height challenged riders to be comfortable.
     
  7. WKB

    WKB New Member

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    This is an excellent discussion for me, as I am eventually looking to get a recumbent bicycle for long-distance loaded touring. If weight on the front wheel is an issue, why not put a set of panniers around the wheel? My guess is that a 26'' wheel would accommodate panniers more easily than a 20'' wheel, although I certainly could be wrong. I concede this causes problems with wheelbase length and possible front-end visibility if the seat is low and the front wheel is up high.

    One of the models I have looked at (at least over the internet) is the bacchetta giro 26. It has a dual 26'' wheels, but places all of the weight around the rear wheel. It is a short wheelbase recumbent and one concern is that the bottom bracket (if that is the correct term) is up very high. Longer wheelbase recumbents, such as the Easy Tour and the Slipstream don't have this, but they do use the smaller front wheel. A couple of follow-up quesitons (again):
    --Are long wheelbse recumbents better for touring?
    --Can you get a set of panniers around a 20'' wheel?
    --Do you really need a set of panniers up front?
    --Is there anything else I'm overlooking?

    Thanks in advance for all feedback, Keefe.
     
  8. PaPa

    PaPa New Member

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    steering dynamics
     
  9. Aeliel

    Aeliel New Member

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    Long wheelbase bents are generally a bit more user friendly than short wheelbase models. LWB tends to have a more upright position and while this may not be quite as as slick thru the wind it's easier to look around and you are a bit more visible. Also they are almost ideal for rear racks and have plenty of room for stuff behind the seat. Short wheelbase bikes aren't bad but they arent as good on rough pavement or rough shoulders. So while they may be faster they arent as good for touring.

    I'm not sure about panniers for the front. My personal favorite way to carry gear is a single wheeled trailer. A small amount of weight up front can help ( I have a RANS bag/windscreen thing between the bars) but honestly the light weight on the front hasnt been an issue for me.

    I did a few double centuries when I was younger (on a upright Specialized Alez) and I am convinced that if I had had a recumbent the only thing that would have stopped me was a lack of sleep...

    If I were looking for a good long distance bike today I would look at Lightfoot, if you want durability over light weight, and a Linear if weight were a primary concern. Both are among the best bikes I've ridden and if I could afford one, I'd own a Lightfoot today.
     
  10. Slugster438

    Slugster438 New Member

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    The "classic" LWB design (like the TourEasy) has the pedals set low relative to the seat, and using a small front wheel keeps the bike more compact overall. The reason a small rear wheel isn't used is because then a standard drivetrain couldn't be used.

    The reason the pedals were set low was to minimize problems with numb feet (poor foot circulation).

    You can get some bikes now with larger front wheels, RANS and Lightfoot are two companies offering them. None of these has lower-set pedals however (for this discussion, "low" pedals would be around 12 inches off the ground, "high" pedals would be 24-26" off, and medium would be somewhere in-between).

    ....Some of the Lightfoot bikes manage to have "medium-height" pedals only because compared to other brands of LWB's, they mount the seat rather high. The seat height of the World Traveler for example is up around 29 inches, where the seat height of a Cycle Genius Falcon (with small front wheel) is only about 22 inches. Lightfoot even notes on the World Traveler page that the Gazelle is a similar bike except using a 20" front wheel instead of a 26", and that the seat sits 3" lower because of this.
    ~
     
  11. PaPa

    PaPa New Member

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    The primary reasons for the 20" front on LWBs, is construction simplicity and better CoM placement. It also reduces 'tiller' by moving the steering axis closer the the rider. A smaller front wheel also 'feels' crisper and lighter during navigation, which ulitimately improves low speed balancing, without seriously compromising high speed controllability.

    The Tour Easy's low cranks was the result of using donor parts in the original design because it simplified construction. Since then, its been learned that lower cranks not only improve low speed balancing, but many claim improved climbing ability.
     
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